fix(codemode): let Headers methods report a bad receiver

docs(codemode): replace the stale Values section with Extension.make
feat(codemode): add the Headers built-in
2026-09-16 05:46:23 +00:00 · 2026-09-16 00:39:27 -05:00 · 2026-09-16 00:14:04 -05:00 · 2026-09-16 00:08:50 -05:00 · 2026-09-16 00:52:00 -04:00 · 2026-09-15 23:50:36 -05:00
70 changed files with 1399 additions and 421 deletions
@@ -32,7 +32,7 @@
    },
    "packages/ai": {
      "name": "@opencode/ai",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@aws-sdk/credential-providers": "3.1057.0",
        "@opencode/schema": "workspace:*",
@@ -54,7 +54,7 @@
    },
    "packages/app": {
      "name": "@opencode/app",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@corvu/drawer": "catalog:",
        "@dnd-kit/abstract": "0.5.0",
@@ -113,7 +113,7 @@
    },
    "packages/cli": {
      "name": "@opencode/cli",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "bin": {
        "opencode2": "./bin/opencode2.cjs",
      },
@@ -177,7 +177,7 @@
    },
    "packages/client": {
      "name": "@opencode/client",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@opencode/protocol": "workspace:*",
        "@opencode/schema": "workspace:*",
@@ -203,7 +203,7 @@
    },
    "packages/codemode": {
      "name": "@opencode/codemode",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "acorn": "8.15.0",
        "effect": "catalog:",
@@ -217,7 +217,7 @@
    },
    "packages/console/app": {
      "name": "@opencode/console-app",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@cloudflare/vite-plugin": "1.15.2",
        "@ibm/plex": "6.4.1",
@@ -253,7 +253,7 @@
    },
    "packages/console/core": {
      "name": "@opencode/console-core",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@aws-sdk/client-sts": "3.782.0",
        "@jsx-email/render": "1.1.1",
@@ -280,7 +280,7 @@
    },
    "packages/console/function": {
      "name": "@opencode/console-function",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@openauthjs/openauth": "0.0.0-20250322224806",
        "@opencode/console-core": "workspace:*",
@@ -297,7 +297,7 @@
    },
    "packages/console/mail": {
      "name": "@opencode/console-mail",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@jsx-email/all": "2.2.3",
        "@jsx-email/cli": "1.4.3",
@@ -321,7 +321,7 @@
    },
    "packages/console/support": {
      "name": "@opencode/console-support",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@cloudflare/vite-plugin": "1.15.2",
        "@opencode/console-core": "workspace:*",
@@ -341,7 +341,7 @@
    },
    "packages/core": {
      "name": "@opencode/core",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@ai-sdk/cohere": "3.0.27",
        "@ai-sdk/gateway": "3.0.104",
@@ -409,7 +409,7 @@
    },
    "packages/desktop": {
      "name": "@opencode/desktop",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@zip.js/zip.js": "2.7.62",
        "electron-context-menu": "4.1.2",
@@ -461,7 +461,7 @@
    },
    "packages/enterprise": {
      "name": "@opencode/enterprise",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@hono/standard-validator": "catalog:",
        "@opencode-ai/sdk": "1.18.21",
@@ -498,7 +498,7 @@
    },
    "packages/function": {
      "name": "@opencode/function",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@octokit/auth-app": "8.0.1",
        "@octokit/rest": "catalog:",
@@ -514,7 +514,7 @@
    },
    "packages/http-recorder": {
      "name": "@opencode/http-recorder",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@effect/platform-node-shared": "4.0.0-rc.112",
      },
@@ -533,7 +533,7 @@
    },
    "packages/httpapi-codegen": {
      "name": "@opencode/httpapi-codegen",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "effect": "catalog:",
        "prettier": "3.6.2",
@@ -546,7 +546,7 @@
    },
    "packages/latex": {
      "name": "@opencode/latex",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@opencode/plugin": "workspace:*",
        "@opentui/core": "catalog:",
@@ -560,7 +560,7 @@
    },
    "packages/merman": {
      "name": "@opencode/merman",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@opencode/plugin": "workspace:*",
        "@opentui/core": "catalog:",
@@ -575,7 +575,7 @@
    },
    "packages/plugin": {
      "name": "@opencode/plugin",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@ai-sdk/provider": "3.0.8",
        "@opencode/ai": "workspace:*",
@@ -614,7 +614,7 @@
    },
    "packages/plugin-browser": {
      "name": "@opencode/plugin-browser",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@opencode/plugin": "workspace:*",
        "@opencode/schema": "workspace:*",
@@ -644,7 +644,7 @@
    },
    "packages/protocol": {
      "name": "@opencode/protocol",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@opencode/schema": "workspace:*",
        "effect": "catalog:",
@@ -659,7 +659,7 @@
    },
    "packages/schema": {
      "name": "@opencode/schema",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@standard-schema/spec": "catalog:",
        "effect": "catalog:",
@@ -683,7 +683,7 @@
    },
    "packages/sdk": {
      "name": "@opencode/sdk",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@opencode/client": "workspace:*",
        "@opencode/core": "workspace:*",
@@ -704,7 +704,7 @@
    },
    "packages/server": {
      "name": "@opencode/server",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@effect/platform-node": "catalog:",
        "@effect/platform-node-shared": "catalog:",
@@ -726,7 +726,7 @@
    },
    "packages/session-ui": {
      "name": "@opencode/session-ui",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@kobalte/core": "catalog:",
        "@opencode/client": "workspace:*",
@@ -761,7 +761,7 @@
    },
    "packages/simulation": {
      "name": "@opencode/simulation",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@opencode/ai": "workspace:*",
        "@opencode/core": "workspace:*",
@@ -781,7 +781,7 @@
    },
    "packages/stats/app": {
      "name": "@opencode/stats-app",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@ibm/plex": "6.4.1",
        "@kobalte/core": "catalog:",
@@ -815,7 +815,7 @@
    },
    "packages/stats/core": {
      "name": "@opencode/stats-core",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@aws-sdk/client-athena": "3.933.0",
        "@planetscale/database": "1.19.0",
@@ -834,7 +834,7 @@
    },
    "packages/stats/server": {
      "name": "@opencode/stats-server",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@aws-sdk/client-firehose": "3.933.0",
        "@effect/platform-node": "catalog:",
@@ -880,7 +880,7 @@
    },
    "packages/theme": {
      "name": "@opencode/theme",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@opentui/core": "catalog:",
        "effect": "catalog:",
@@ -894,7 +894,7 @@
    },
    "packages/tui": {
      "name": "@opencode/tui",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@opencode/client": "workspace:*",
        "@opencode/core": "workspace:*",
@@ -929,7 +929,7 @@
    },
    "packages/ui": {
      "name": "@opencode/ui",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@kobalte/core": "catalog:",
        "@pierre/diffs": "catalog:",
@@ -964,7 +964,7 @@
    },
    "packages/util": {
      "name": "@opencode/util",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@effect/opentelemetry": "catalog:",
        "@effect/platform-node": "catalog:",
@@ -997,7 +997,7 @@
    },
    "packages/web": {
      "name": "@opencode/web",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "@astrojs/cloudflare": "12.6.3",
        "@astrojs/markdown-remark": "6.3.1",
@@ -1038,7 +1038,7 @@
    },
    "services/update": {
      "name": "@opencode/update",
-      "version": "2.0.3",
+      "version": "2.0.4",
      "dependencies": {
        "jose": "6.0.11",
        "semver": "catalog:",
@@ -2,7 +2,7 @@
  "$schema": "https://json.schemastore.org/package.json",
  "name": "opencode",
  "description": "AI-powered development tool",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "private": true,
  "type": "module",
  "packageManager": "bun@1.4.2",
@@ -1,6 +1,6 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "name": "@opencode/ai",
  "type": "module",
  "license": "MIT",
@@ -59,15 +59,7 @@ export const isContextOverflowFailure = (failure: unknown) =>
    : Schema.is(ProviderErrorEvent)(failure) && failure.classification === "context-overflow"

 const decodeJson = Schema.decodeUnknownOption(Schema.fromJsonString(Schema.Unknown))
-// OpenCode Zen reports account caps as typed 429/402 errors that are not throttles.
-const QUOTA_CODES = new Set([
-  "insufficient_quota",
-  "usage_not_included",
-  "billing_error",
-  "gousagelimiterror",
-  "freeusagelimiterror",
-  "creditlimitexceeded",
-])
+const QUOTA_CODES = new Set(["insufficient_quota", "usage_not_included", "billing_error"])
 const AUTH_CODES = new Set(["authentication_error", "permission_error"])
 const SERVER_CODES = new Set([
  "api_error",
@@ -95,8 +87,7 @@ const CONTENT_POLICY_CODES = new Set([
 // as a `[code]` label at the start of the rewritten message.
 const GATEWAY_CODE_LABEL = /^[^:\n]+: \[([A-Za-z0-9_.-]+)\]/
 const RATE_LIMIT_TEXT = /rate increased too quickly|rate[-_\s]?limit|too[_\s]?many[_\s]?requests/i
-// Only consulted on 429, where throttles and account caps share a status.
-const QUOTA_TEXT = /insufficient[-_\s]?quota|quota[-_\s]?exceeded|budget exceeded|usage limit/i
+const QUOTA_TEXT = /insufficient[-_\s]?quota|quota[-_\s]?exceeded/i
 // Policy rejections without a dedicated code, matched against the provider's own
 // explanation only. OpenAI reuses `invalid_prompt` for usage-policy rejections while
 // Bedrock Mantle reuses it for schema validation; Anthropic reports blocked output
@@ -152,11 +143,7 @@ export function classifyProviderFailure(input: ProviderFailure): AIError["reason
    return new InvalidRequestError({ ...details, classification: "payload-too-large" })
  if (codes.some((code) => CONTENT_POLICY_CODES.has(code)) || (clientScoped && CONTENT_POLICY_TEXT.test(input.message)))
    return new ContentPolicyError(details)
-  if (
-    input.status === 402 ||
-    codes.some((code) => QUOTA_CODES.has(code)) ||
-    (input.status === 429 && QUOTA_TEXT.test(text))
-  )
+  if (codes.some((code) => QUOTA_CODES.has(code)) || (input.status === 429 && QUOTA_TEXT.test(text)))
    return new QuotaExceededError(details)
  if (input.status === 401 || input.status === 403 || codes.some((code) => AUTH_CODES.has(code)))
    return new AuthenticationError(details)
@@ -176,12 +163,10 @@ export function classifyProviderFailure(input: ProviderFailure): AIError["reason
    input.status === 408 ||
    input.status === 409 ||
    (input.status !== undefined && input.status >= 500) ||
-    // Server codes and phrasing only decide when no HTTP status contradicts them:
-    // gateways such as OpenCode Zen substitute `server_error` for codes they do
-    // not forward, so a 4xx with a server code is still a rejected request.
    ((input.status === undefined || input.status < 400) &&
-      ((!codes.some((code) => INVALID_REQUEST_CODES.has(code)) && SERVER_ERROR_TEXT.test(text)) ||
-        codes.some((code) => SERVER_CODES.has(code) || code.includes("exhausted") || code.includes("unavailable"))))
+      !codes.some((code) => INVALID_REQUEST_CODES.has(code)) &&
+      SERVER_ERROR_TEXT.test(text)) ||
+    codes.some((code) => SERVER_CODES.has(code) || code.includes("exhausted") || code.includes("unavailable"))
  )
    return new ProviderInternalError({
      ...details,
@@ -309,7 +309,7 @@ describe("RequestExecutor", () => {
    }),
  )

-  it.effect("does not let server codes override a 4xx rejection", () =>
+  it.effect("classifies provider overloads hidden behind HTTP 400", () =>
    Effect.gen(function* () {
      const classify = (body: string) =>
        Effect.gen(function* () {
@@ -317,11 +317,11 @@ describe("RequestExecutor", () => {
          const error = yield* executor.execute(request).pipe(Effect.flip)

          expectAIError(error)
-          expect(error.reason).toMatchObject({ _tag: "InvalidRequest" })
+          expect(error.reason).toMatchObject({ _tag: "ProviderInternal" })
        }).pipe(Effect.provide(fixedResponse(body, { status: 400 })))

      yield* classify('{"code":"resource_exhausted"}')
-      yield* classify('{"error":{"type":"server_error","message":"Upstream request failed: Model is unavailable."}}')
+      yield* classify('{"code":"service_unavailable"}')
    }),
  )

@@ -249,54 +249,10 @@ describe("provider error classification", () => {

  test("classifies any remaining 4xx status as an invalid request", () => {
    expect(
-      [400, 404, 418, 422, 451].map((status) => classifyProviderFailure({ message: `HTTP ${status}`, status })._tag),
-    ).toEqual(Array(5).fill("InvalidRequest"))
-  })
-
-  test("classifies 402 as exhausted quota", () => {
-    expect(classifyProviderFailure({ message: "Payment Required", status: 402 })._tag).toBe("QuotaExceeded")
-  })
-
-  test("classifies OpenCode Zen account limits as quota rather than throttling", () => {
-    const typed = (type: string, message: string) => ({ type: "error", error: { type, message } })
-    const substituted = (message: string) => ({
-      error: { type: "server_error", message: `Upstream request failed: ${message}` },
-    })
-    const cases: ReadonlyArray<[number, { error: { message: string } }]> = [
-      [429, typed("GoUsageLimitError", "Go usage limit exceeded")],
-      [429, typed("FreeUsageLimitError", "Rate limit exceeded. Please try again later.")],
-      [402, typed("CreditLimitExceeded", "Credit limit exceeded.")],
-      [402, substituted("Insufficient account funds")],
-      [402, substituted("Account invoice is overdue")],
-      [429, substituted("Account budget exceeded")],
-    ]
-    expect(
-      cases.map(
-        ([status, body]) =>
-          classifyProviderFailure({ message: body.error.message, status, rawBody: JSON.stringify(body) })._tag,
+      [400, 402, 404, 418, 422, 451].map(
+        (status) => classifyProviderFailure({ message: `HTTP ${status}`, status })._tag,
      ),
-    ).toEqual(Array(6).fill("QuotaExceeded"))
-  })
-
-  test("does not let substituted server codes make a 4xx retryable", () => {
-    const openai = { error: { type: "server_error", message: "Upstream request failed: Model is unavailable." } }
-    const anthropic = {
-      type: "error",
-      error: { type: "api_error", message: "Upstream request failed: Model is unavailable." },
-    }
-    expect(
-      [openai, anthropic].map(
-        (body) =>
-          classifyProviderFailure({ message: body.error.message, status: 400, rawBody: JSON.stringify(body) })._tag,
-      ),
-    ).toEqual(["InvalidRequest", "InvalidRequest"])
-    // Without a contradicting status the same codes still mark provider trouble.
-    expect(classifyProviderFailure({ message: openai.error.message, rawBody: JSON.stringify(openai) })._tag).toBe(
-      "ProviderInternal",
-    )
-    expect(
-      classifyProviderFailure({ message: openai.error.message, status: 200, rawBody: JSON.stringify(openai) })._tag,
-    ).toBe("ProviderInternal")
+    ).toEqual(Array(6).fill("InvalidRequest"))
  })

  test("classifies nested provider codes when a top-level code is also present", () => {
@@ -1,6 +1,6 @@
 {
  "name": "@opencode/app",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "description": "",
  "type": "module",
  "exports": {
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/cli",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "type": "module",
  "license": "MIT",
  "bin": {
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/client",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "type": "module",
  "license": "MIT",
  "repository": {
@@ -13,8 +13,8 @@ The idea of code mode was originally introduced by Cloudflare. See

 ## How it differs from JavaScript

- **Only supported APIs are available.** Programs can use the provided tools and supported JavaScript built-ins. APIs
-  such as `fetch`, timers, `process`, filesystem access, imports, and modules are unavailable.
+- **Only supported APIs are available.** Programs can use the provided tools, supported JavaScript built-ins, and the
+  globals the host adds through extensions. Timers, `process`, filesystem access, imports, and modules are unavailable.
 - **Unfinished work is interrupted.** Tool calls and async functions start when called. When the program finishes,
  anything still running is interrupted. Unhandled rejections from un-awaited promises are returned as warnings.
 - **REPL-style results.** Without an explicit `return`, the final top-level expression becomes the result. `undefined`
@@ -88,14 +88,25 @@ runtime.catalog() // structured tool descriptions
 runtime.execute(source) // Effect<CodeMode.Result, never, ToolServices>
 ```

-The Effect environment is inferred from the supplied tools. `onToolCallStart` observes admitted calls with decoded
-input; `onToolCallEnd` observes settled outcomes and duration. Both hooks return Effects and must not fail.
+The Effect environment is inferred from the supplied tools. `hooks` surround every call the program makes into the
+host: `tool.before`/`tool.after` receive `{ name, input }` with decoded input, `extension.before`/`extension.after`
+receive `{ extension, name, args }`. An `after` hook also receives how the call ended (`success` with its value,
+`failure` with its error, or `interrupted`). A failing `before` hook denies the call, and the program catches the
+failure as a thrown error.

-### `Values`
+### `Extension.make`

-`Values` exports the runtime's non-JSON value classes: `Values.URL`, `Values.URLSearchParams`, `Values.Date`,
-`Values.RegExp`, `Values.Map`, `Values.Set`, and `Values.Promise`. The interpreter recognizes these by class; a
-program's `new URL(...)` is a `Values.URL` wrapping the host `URL`. `Values.isValue` narrows to the data-like kinds.
+Extensions are host functions a program calls directly as globals, such as `fetch`. Unlike tools they are not in the
+catalog, not counted against `maxToolCalls`, and not described to the model; the host decides what they mean.
+
+```ts
+const web = Extension.make({ name: "web", globals: { fetch: (url: string) => globalThis.fetch(url) } })
+const runtime = CodeMode.make({ tools, extensions: [web] })
+```
+
+Every value crossing in either direction is converted, never shared: arguments come in as copies, results go out as
+copies, and a function inside a result is callable the same way. A global that shadows a built-in or another
+extension throws at `CodeMode.make`.

 ### OpenAPI tools

@@ -29,7 +29,7 @@ ultimate source of truth. Upstream test262 files run verbatim from `test/test262
      Uint8Array is rejected with a hint to encode as text, and own `__proto__` keys are dropped so merging tool
      inputs or results cannot replace a prototype. In-program `JSON.stringify` keeps JS behavior except for the
      Error form and a promise, which is a `TypeError` with an await hint rather than a silent `{}`.
- [x] Live Date, RegExp, Map, Set, URL, URLSearchParams, and Uint8Array values inside CodeMode.
+- [x] Live Date, RegExp, Map, Set, URL, URLSearchParams, Headers, and Uint8Array values inside CodeMode.
 - [x] Tool calls through the host-provided `tools` tree only.
 - [x] The global `search(...)` built-in: synchronous tool discovery that counts as an admitted tool call and is
      shadowable by program declarations like other globals.
@@ -47,8 +47,8 @@ ultimate source of truth. Upstream test262 files run verbatim from `test/test262
 ## Values and literals

 - [x] `null`, `undefined`, booleans, finite and non-finite numbers, and strings.
- [x] Array literals, including holes and spread from arrays, strings, Maps, Sets, URLSearchParams, custom synchronous
-      iterators, and synchronous generators.
+- [x] Array literals, including holes and spread from arrays, strings, Maps, Sets, URLSearchParams, Headers, custom
+      synchronous iterators, and synchronous generators.
 - [x] Object literals with shorthand, computed string/number keys, and spread following ToObject: data objects and
      arrays copy own enumerable keys, strings copy index keys, and other values contribute nothing.
 - [x] Template literals with interpolation.
@@ -95,8 +95,8 @@ ultimate source of truth. Upstream test262 files run verbatim from `test/test262
 - [x] `if`/`else` and conditional expressions.
 - [x] `switch`, including default clauses and fallthrough.
 - [x] `for`, `while`, and `do...while`.
- [x] `for...of` over arrays, strings, Maps, Sets, URLSearchParams, custom synchronous iterators, and confined
-      synchronous generators. Abrupt completion invokes the iterator's optional `return()`.
+- [x] `for...of` over arrays, strings, Maps, Sets, URLSearchParams, Headers, custom synchronous iterators, and
+      confined synchronous generators. Abrupt completion invokes the iterator's optional `return()`.
 - [x] `for...in` over own keys of plain objects, arrays, strings, and tool references; other values iterate nothing.
 - [x] Unlabeled `break` and `continue`.
 - [x] `try`, `catch`, optional catch bindings, and `finally`.
@@ -127,7 +127,7 @@ ultimate source of truth. Upstream test262 files run verbatim from `test/test262
      string). A detached method loses its receiver, as in JS: `values.filter("abc".includes)` is a `TypeError`
      because `includes` is called without a string `this`.
 - [x] Constructors work as callbacks with JS call semantics: `Error` types construct (`messages.map(Error)`),
-      and new-requiring constructors (`Map`, `Set`, `URL`, `URLSearchParams`, `Promise`) throw a `TypeError`,
+      and new-requiring constructors (`Map`, `Set`, `URL`, `URLSearchParams`, `Headers`, `Promise`) throw a `TypeError`,
      like JS.
 - [x] Tool references and detached `Promise` statics are rejected as callbacks with a hint to wrap them in an
      arrow function.
@@ -179,10 +179,10 @@ ultimate source of truth. Upstream test262 files run verbatim from `test/test262
 - [x] Sequence expressions (the comma operator).
 - [x] `await` for CodeMode promises and callable thenables; a plain value passes through unchanged, though every
      `await` still defers its continuation one reaction turn.
- [x] `new` for Array, Object, Error types, Date, RegExp, Map, Set, URL, URLSearchParams, and Promise. `new` on any
-      other value throws a catchable `TypeError` naming the callee: other built-in functions such as `Number` say
-      `new` is unsupported and point at the plain call, user-defined functions report the constructor gap below, and
-      non-callable values are not constructors.
+- [x] `new` for Array, Object, Error types, Date, RegExp, Map, Set, URL, URLSearchParams, Headers, and Promise. `new`
+      on any other value throws a catchable `TypeError` naming the callee: other built-in functions such as `Number`
+      say `new` is unsupported and point at the plain call, user-defined functions report the constructor gap below,
+      and non-callable values are not constructors.
 - [x] Arithmetic operators: `+`, `-`, `*`, `/`, `%`, and `**`.
 - [x] Equality and ordering: `==`, `!=`, `===`, `!==`, `<`, `<=`, `>`, and `>=`.
 - [x] Bitwise operators: `&`, `|`, `^`, `~`, `<<`, `>>`, and `>>>`.
@@ -450,7 +450,13 @@ with a hint to encode as text first (`TextDecoder`, `toBase64`, `toHex`).
 - [x] `crypto.randomUUID()` and `crypto.getRandomValues(uint8Array)`.
 - [x] `TextEncoder` and `TextDecoder` for UTF-8 only: any other label is a `RangeError`. `TextDecoder` accepts the
      `fatal` and `ignoreBOM` options; `decode` takes a Uint8Array or nothing.
- [ ] `crypto.subtle`, `Blob`, and `TextDecoder` streaming or non-UTF-8 encodings.
+- [x] `new Headers()` from records, synchronous iterables of pairs, and Headers, wrapping the host's `Headers`: names
+      fold to lowercase, values are normalized and combined, and invalid names or values throw a `TypeError`.
+- [x] Headers `append`, `delete`, `get`, `getSetCookie`, `has`, `set`, `forEach`, `keys`, `values`, and `entries`;
+      iteration is live and sorted by name, with `set-cookie` values kept apart.
+- [x] Headers serialize to a `{ name: value }` object in JSON, in results, and in tool arguments.
+- [ ] `Request`, `Response`, and `Blob`.
+- [ ] `crypto.subtle` and `TextDecoder` streaming or non-UTF-8 encodings.

 ## Extensions

@@ -460,13 +466,15 @@ Nothing is exposed unless a host provides it; extension calls are not tool calls
 - [x] Each global is a function, callable but not constructible, run with `this` undefined. A global that shadows
      a built-in or another extension throws at `make`.
 - [x] Every value crossing in either direction is converted, never shared: plain objects and arrays are copied,
-      `Date`, `RegExp`, `URL`, `URLSearchParams`, `Map`, `Set`, and `Uint8Array` become fresh copies with their
-      contents converted (a host `ArrayBuffer` comes in as a `Uint8Array`; other typed arrays cannot come out),
+      `Date`, `RegExp`, `URL`, `URLSearchParams`, `Headers`, `Map`, `Set`, and `Uint8Array` become fresh copies with
+      their contents converted (a host `ArrayBuffer` comes in as a `Uint8Array`; other typed arrays cannot come out),
      errors cross as errors with their name and message, and a `__proto__` key is dropped. Functions, generators,
      un-awaited promises, and symbols cannot be passed in; a class instance, a symbol, or a BigInt cannot come out.
 - [x] A host function inside a result becomes a program function whose calls cross the same way, so a result can
      carry methods (`res.json()`) whose host closures keep the host state. Diagnostics name it by its path
      (`fetch.json`). Like any program function it vanishes at the data boundary.
+- [x] Each call to an extension global runs inside the host's `extension.before`/`extension.after` hooks as
+      `{ extension, name, args }`, with the host's own error on failure; calls to functions inside results do not.
 - [x] A host `Promise` becomes a program promise. Whatever host code returns, resolves, throws, or rejects with
      crosses the same way, so `catch (e)` receives a copy of the thrown value (an `Error` of the matching type, or
      plain data).
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/codemode",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "description": "Effect-native confined code execution over schema-described tools",
  "type": "module",
  "license": "MIT",
@@ -7,7 +7,14 @@ import { type Services, type ToolDescription, ToolRuntime } from "./tool-runtime
 import type { Tools } from "./tools.js"

 /** A tool call admitted during an execution. */
-export type { ToolCall, ToolCallEnded, ToolCallHooks, ToolCallStarted, ToolDescription } from "./tool-runtime.js"
+export type {
+  CallResult,
+  ExtensionInvocation,
+  Hooks,
+  ToolCall,
+  ToolDescription,
+  ToolInvocation,
+} from "./tool-runtime.js"
 /** Signature-construction helpers for host-owned catalog instructions. */
 export { searchSignature, toolExpression } from "./tool-runtime.js"

@@ -34,9 +41,11 @@ export type ResolvedExecutionLimits = {
 }

 /** Configuration shared by `CodeMode.make` and `CodeMode.execute`. */
-export type Options<Provided extends Record<string, unknown> = {}> = ToolRuntime.ToolCallHooks<Services<Provided>> & {
+export type Options<Provided extends Record<string, unknown> = {}> = {
  /** Explicit tools exposed to the program as `tools`. */
  tools?: Provided & Tools<Services<Provided>>
+  /** Hooks around every tool and extension call the program makes; see `Hooks`. */
+  hooks?: ToolRuntime.Hooks<Services<Provided>>
  /** Host functions exposed as globals; see `Extension.make`. */
  extensions?: ReadonlyArray<Extension>
  /** Resource limits enforced on each execution. */
@@ -147,7 +156,10 @@ export const make = <const Provided extends Record<string, unknown> = {}>(
    }
  }
  return {
-    catalog: prepared.catalog,
-    execute: (code) => executeProgram(code, prepared, limits, options, (ctx) => extensionGlobals(ctx, extensions)),
+    get catalog() {
+      return prepared.catalog
+    },
+    execute: (code) =>
+      executeProgram(code, prepared, limits, options.hooks ?? {}, (ctx) => extensionGlobals(ctx, extensions)),
  }
 }
@@ -17,6 +17,7 @@ import {
  record,
  SetObj,
  URLSearchParamsObj,
+  HeadersObj,
 } from "./interpreter/objects.js"
 import { typeofValue } from "./interpreter/references.js"

@@ -69,6 +70,7 @@ const walk = <R>(
        )
      }
      if (boundary && value instanceof URLSearchParamsObj) return value.params.toString()
+      if (value instanceof HeadersObj) return Object.fromEntries(value.headers)
      const target = boundary && value instanceof SetObj ? new Arr(ctx.builtins.Array, [...value.set]) : value
      if (stack.has(target)) throw typeError("Converting circular structure to JSON.")
      stack.add(target)
@@ -16,7 +16,7 @@ export const executeProgram = <R>(
  code: string,
  prepared: ToolRuntime.Prepared<R>,
  limits: ResolvedExecutionLimits,
-  hooks: ToolRuntime.ToolCallHooks<R>,
+  hooks: ToolRuntime.Hooks<R>,
  globals?: (ctx: Interpreter<R>) => ReadonlyArray<readonly [string, unknown]>,
 ): Effect.Effect<Result, never, R> => {
  if (code.trim().length === 0) {
@@ -1,6 +1,7 @@
 import { Effect } from "effect"
 import type { Extension } from "../extension.js"
 import { coerceToString } from "../stdlib/value.js"
+import { type ExtensionInvocation, hooked } from "../tool-runtime.js"
 import type { Interpreter } from "./interpreter.js"
 import { createErrorValue, isErrorType } from "./intrinsics.js"
 import { MAX_VALUE_DEPTH } from "./limits.js"
@@ -11,7 +12,6 @@ import {
  define,
  entries,
  get,
-  type Native,
  Arr,
  Bytes,
  DateObj,
@@ -24,6 +24,7 @@ import {
  SetObj,
  URLObj,
  URLSearchParamsObj,
+  HeadersObj,
 } from "./objects.js"
 import { describeValue } from "./references.js"

@@ -49,6 +50,7 @@ export const extensionGlobals = <R>(
    if (value instanceof RegExpObj) return new RegExp(value.regex.source, value.regex.flags)
    if (value instanceof URLObj) return new URL(value.url.href)
    if (value instanceof URLSearchParamsObj) return new URLSearchParams(value.params)
+    if (value instanceof HeadersObj) return new Headers(value.headers)
    const next = (item: unknown) => toHost(item, label, depth + 1, seen)
    if (value instanceof MapObj) return new Map([...value.map].map(([key, item]) => [next(key), next(item)]))
    if (value instanceof SetObj) return new Set([...value.set].map(next))
@@ -96,6 +98,7 @@ export const extensionGlobals = <R>(
      if (value instanceof URLSearchParams) {
        return new URLSearchParamsObj(builtins.URLSearchParams, new URLSearchParams(value))
      }
+      if (value instanceof Headers) return new HeadersObj(builtins.Headers, new Headers(value))
      const next = (item: unknown, path: string) => fromHost(item, path, depth + 1, seen)
      if (value instanceof Map) {
        const wrapped = new MapObj(builtins.Map)
@@ -128,26 +131,35 @@ export const extensionGlobals = <R>(
    throw typeError(`${label} produced ${describeHost(value)}, which the program cannot hold.`)
  }

-  // A host function as a program function: arguments cross in, and whatever it returns, resolves, throws, or
-  // rejects with crosses out, so the program catches what the author threw.
-  const wrap = (value: Function, label: string): Native<R> =>
+  // A host function as a program function. Arguments cross in; a global's call runs inside the host's extension
+  // hooks, which see the host's own error on failure; then whatever came back, or was thrown, crosses out so the
+  // program catches a copy. Functions inside results are part of a value's API and skip the hooks.
+  const wrap = (value: Function, label: string, describe?: (args: ReadonlyArray<unknown>) => ExtensionInvocation) =>
    fn<R>(builtins, value.name, value.length, (_, values) => {
-      const converted = values.map((item, index) => toHost(item, `Argument ${index + 1} to ${label}`))
-      const thrown = (reason: unknown) => new Throw(fromHost(reason, label))
+      const args = values.map((item, index) => toHost(item, `Argument ${index + 1} to ${label}`))
+      const hooks = ctx.tools.hooks
+      const settle = (run: Effect.Effect<unknown, unknown, R>) =>
+        (describe === undefined
+          ? run
+          : hooked(describe(args), hooks["extension.before"], hooks["extension.after"], run)
+        ).pipe(
+          Effect.mapError((reason) => new Throw(fromHost(reason, label))),
+          Effect.map((settled) => fromHost(settled, label)),
+        )
      let result: unknown
      try {
-        result = value.apply(undefined, converted)
+        result = value.apply(undefined, args)
      } catch (reason) {
-        return Effect.fail(thrown(reason))
+        return settle(Effect.fail(reason))
      }
-      if (!(result instanceof Promise)) return fromHost(result, label)
-      return ctx.pending.create(
-        Effect.map(Effect.tryPromise({ try: () => result, catch: thrown }), (settled) => fromHost(settled, label)),
-      )
+      if (!(result instanceof Promise)) return settle(Effect.succeed(result))
+      return ctx.pending.create(settle(Effect.tryPromise({ try: () => result, catch: (reason) => reason })))
    })

  return extensions.flatMap((extension) =>
-    Object.entries(extension.globals).map(([name, value]) => [name, wrap(value, name)] as const),
+    Object.entries(extension.globals).map(
+      ([name, value]) => [name, wrap(value, name, (args) => ({ extension: extension.name, name, args }))] as const,
+    ),
  )
 }

@@ -11,6 +11,7 @@ import { objectGlobal } from "../stdlib/object.js"
 import { regexpGlobal } from "../stdlib/regexp.js"
 import { stringGlobal } from "../stdlib/string.js"
 import { uriGlobal, urlGlobal, urlSearchParamsGlobal } from "../stdlib/url.js"
+import { headersGlobal } from "../stdlib/headers.js"
 import { coercion } from "../stdlib/value.js"
 import { base64Global, cryptoGlobal } from "../stdlib/web.js"
 import { ToolReference } from "../tool-runtime.js"
@@ -80,6 +81,7 @@ const table: Record<string, Factory> = {
  Set: (ctx) => setGlobal(ctx),
  URL: (ctx) => urlGlobal(ctx),
  URLSearchParams: (ctx) => urlSearchParamsGlobal(ctx),
+  Headers: (ctx) => headersGlobal(ctx),
  Uint8Array: (ctx) => uint8ArrayGlobal(ctx),
  TextEncoder: (ctx) => textEncoderGlobal(ctx),
  TextDecoder: (ctx) => textDecoderGlobal(ctx),
@@ -84,6 +84,7 @@ import {
  PromiseObj,
  SetObj,
  URLSearchParamsObj,
+  HeadersObj,
  record,
  remove,
  set,
@@ -653,7 +654,7 @@ class Frame<R> {
      const cursor = iterator === undefined ? yield* self.iterate(right, node) : undefined
      if (iterator === undefined && cursor === undefined) {
        throw invalidData(
-          `${awaiting ? "for await...of" : "for...of"} requires an array, string, Map, Set, or URLSearchParams, or custom iterator value.`,
+          `${awaiting ? "for await...of" : "for...of"} requires an array, string, Map, Set, URLSearchParams, or Headers, or custom iterator value.`,
          node,
        )
      }
@@ -756,9 +757,11 @@ class Frame<R> {
              ? value.set.values()
              : value instanceof URLSearchParamsObj
                ? value.params.entries()
-                : value instanceof Bytes
-                  ? value.bytes.values()
-                  : undefined
+                : value instanceof HeadersObj
+                  ? value.headers.entries()
+                  : value instanceof Bytes
+                    ? value.bytes.values()
+                    : undefined
    if (iterator !== undefined) {
      const proto = this.ctx.builtins.Array
      return Effect.succeed({
@@ -1848,6 +1851,7 @@ class Frame<R> {
        value instanceof MapObj ||
        value instanceof SetObj ||
        value instanceof URLSearchParamsObj ||
+        value instanceof HeadersObj ||
        value instanceof Bytes
      ) {
        const cursor = yield* self.iterate(value, node)
@@ -29,6 +29,7 @@ const builtins = [
  "Set",
  "URL",
  "URLSearchParams",
+  "Headers",
  "Uint8Array",
  "TextEncoder",
  "TextDecoder",
@@ -80,6 +81,7 @@ export const createBuiltins = (): Builtins => {
    Set: plain(),
    URL: plain(),
    URLSearchParams: plain(),
+    Headers: plain(),
    Uint8Array: plain(),
    TextEncoder: plain(),
    TextDecoder: plain(),
@@ -156,6 +156,15 @@ export class URLSearchParamsObj extends Obj {
  }
 }

+export class HeadersObj extends Obj {
+  constructor(
+    proto: Obj,
+    readonly headers: Headers,
+  ) {
+    super(proto)
+  }
+}
+
 export class URLObj extends Obj {
  readonly searchParams: URLSearchParamsObj
  constructor(
@@ -181,13 +190,14 @@ export class Bytes extends Obj {
 /** Built-in objects that wrap a host value; data-like, but never plain data. */
 export const isWrapper = (
  value: unknown,
-): value is DateObj | RegExpObj | MapObj | SetObj | URLObj | URLSearchParamsObj | Bytes =>
+): value is DateObj | RegExpObj | MapObj | SetObj | URLObj | URLSearchParamsObj | HeadersObj | Bytes =>
  value instanceof DateObj ||
  value instanceof RegExpObj ||
  value instanceof MapObj ||
  value instanceof SetObj ||
  value instanceof URLObj ||
  value instanceof URLSearchParamsObj ||
+  value instanceof HeadersObj ||
  value instanceof Bytes

 const MAX_ARRAY_INDEX = 4_294_967_295
@@ -16,6 +16,7 @@ import {
  SetObj,
  URLObj,
  URLSearchParamsObj,
+  HeadersObj,
 } from "./objects.js"

 /** Values that cannot cross the data boundary. */
@@ -85,6 +86,7 @@ export const describeValue = (value: unknown): string => {
  if (value instanceof SetObj) return "a Set"
  if (value instanceof URLObj) return "a URL"
  if (value instanceof URLSearchParamsObj) return "a URLSearchParams"
+  if (value instanceof HeadersObj) return "a Headers"
  if (value instanceof Bytes) return "a Uint8Array"
  if (value instanceof GeneratorObj) return "a generator"
  if (isRuntimeReference(value)) return "a function"
@@ -12,6 +12,7 @@ import {
  SetObj,
  URLObj,
  URLSearchParamsObj,
+  HeadersObj,
 } from "../interpreter/objects.js"
 import { containsOpaqueReference, isRuntimeReference } from "../interpreter/references.js"
 import type { Interpreter } from "../interpreter/interpreter.js"
@@ -66,6 +67,7 @@ const formatConsoleValue = (value: unknown, seen: Set<object>, depth: number): s
  if (value instanceof RegExpObj) return coerceToString(value)
  if (value instanceof URLObj) return coerceToString(value)
  if (value instanceof URLSearchParamsObj) return coerceToString(value)
+  if (value instanceof HeadersObj) return `Headers ${JSON.stringify(Object.fromEntries(value.headers))}`
  if (value instanceof Bytes) return `Uint8Array(${value.bytes.length}) [${value.bytes.join(",")}]`
  if (depth > MAX_CONSOLE_DEPTH) return "..."
  if (seen.has(value)) return "[Circular]"
@@ -0,0 +1,119 @@
+import { Effect } from "effect"
+import { constructor, methods, prototypeFrom, receiver, requiresNew } from "../interpreter/native.js"
+import { PendingThrow, typeError } from "../interpreter/model.js"
+import { entries, Arr, HeadersObj, Obj } from "../interpreter/objects.js"
+import { applyCollectionCallback } from "../interpreter/callback.js"
+import { isRuntimeReference } from "../interpreter/references.js"
+import type { Interpreter } from "../interpreter/interpreter.js"
+import { coerceToString } from "./value.js"
+import { readPairs } from "./url.js"
+
+// The host validates header names and values and throws its own TypeError; the program gets one of its own.
+const attempt = <T>(run: () => T): T => {
+  try {
+    return run()
+  } catch (error) {
+    if (error instanceof PendingThrow) throw error
+    throw typeError(error instanceof Error ? error.message : String(error))
+  }
+}
+
+const constructHeaders = <R>(ctx: Interpreter<R>, init: unknown, proto: Obj): Effect.Effect<HeadersObj, unknown, R> => {
+  const wrap = (headers: Headers) => new HeadersObj(proto, headers)
+  if (init === undefined) return Effect.succeed(wrap(new Headers()))
+  if (init instanceof HeadersObj) return Effect.succeed(wrap(new Headers(init.headers)))
+  return Effect.gen(function* () {
+    const pairs = init instanceof Obj ? yield* readPairs(ctx, init, "new Headers(...)") : undefined
+    if (pairs !== undefined) return wrap(attempt(() => new Headers(pairs)))
+    if (!(init instanceof Obj) || isRuntimeReference(init)) {
+      throw typeError("new Headers(...) expects a record of names to values, iterable [name, value] pairs, or Headers.")
+    }
+    return wrap(
+      attempt(() => new Headers(Object.fromEntries(entries(init).map(([key, value]) => [key, coerceToString(value)])))),
+    )
+  })
+}
+
+export const headersGlobal = <R>(ctx: Interpreter<R>) => {
+  const builtins = ctx.builtins
+  const proto = builtins.Headers
+  const headers = constructor<R>(builtins, proto, {
+    name: "Headers",
+    call: requiresNew("Headers"),
+    construct: (args, newTarget) => constructHeaders(ctx, args[0], prototypeFrom(newTarget, proto)),
+  })
+  const self = (thisValue: unknown, name: string) => receiver(HeadersObj, thisValue, `Headers.prototype.${name}`)
+  const wrap = (items: Array<unknown>) => new Arr(builtins.Array, items)
+  const arg = (args: Array<unknown>, index: number): string => coerceToString(args[index])
+  const requireArgs = (name: string, args: Array<unknown>, count: number): void => {
+    if (args.length < count) throw typeError(`Headers.${name} requires ${count} argument${count === 1 ? "" : "s"}.`)
+  }
+  methods(builtins, proto, [
+    [
+      "append",
+      2,
+      (thisValue, args) => {
+        requireArgs("append", args, 2)
+        attempt(() => self(thisValue, "append").headers.append(arg(args, 0), arg(args, 1)))
+        return undefined
+      },
+    ],
+    [
+      "delete",
+      1,
+      (thisValue, args) => {
+        requireArgs("delete", args, 1)
+        attempt(() => self(thisValue, "delete").headers.delete(arg(args, 0)))
+        return undefined
+      },
+    ],
+    [
+      "get",
+      1,
+      (thisValue, args) => {
+        requireArgs("get", args, 1)
+        return attempt(() => self(thisValue, "get").headers.get(arg(args, 0)))
+      },
+    ],
+    ["getSetCookie", 0, (thisValue) => wrap(self(thisValue, "getSetCookie").headers.getSetCookie())],
+    [
+      "has",
+      1,
+      (thisValue, args) => {
+        requireArgs("has", args, 1)
+        return attempt(() => self(thisValue, "has").headers.has(arg(args, 0)))
+      },
+    ],
+    [
+      "set",
+      2,
+      (thisValue, args) => {
+        requireArgs("set", args, 2)
+        attempt(() => self(thisValue, "set").headers.set(arg(args, 0), arg(args, 1)))
+        return undefined
+      },
+    ],
+    ["keys", 0, (thisValue) => wrap(Array.from(self(thisValue, "keys").headers.keys()))],
+    ["values", 0, (thisValue) => wrap(Array.from(self(thisValue, "values").headers.values()))],
+    [
+      "entries",
+      0,
+      (thisValue) =>
+        wrap(Array.from(self(thisValue, "entries").headers.entries(), ([key, value]) => wrap([key, value]))),
+    ],
+    [
+      "forEach",
+      1,
+      (thisValue, args) => {
+        requireArgs("forEach", args, 1)
+        const target = self(thisValue, "forEach")
+        const apply = applyCollectionCallback(ctx, args[0], "Headers.forEach")
+        return Effect.gen(function* () {
+          for (const [key, value] of Array.from(target.headers.entries())) yield* apply([value, key, target])
+          return undefined
+        })
+      },
+    ],
+  ])
+  return headers
+}
@@ -107,12 +107,10 @@ export const urlGlobal = <R>(ctx: Interpreter<R>) => {
  return url
 }

-const readPair = <R>(ctx: Interpreter<R>, value: unknown): Effect.Effect<Array<string>, unknown, R> =>
+const readPair = <R>(ctx: Interpreter<R>, value: unknown, label: string): Effect.Effect<Array<string>, unknown, R> =>
  Effect.gen(function* () {
    const cursor = yield* ctx.iterate(value)
-    if (cursor === undefined) {
-      throw typeError("new URLSearchParams(...) expects iterable [name, value] pairs.")
-    }
+    if (cursor === undefined) throw typeError(`${label} expects iterable [name, value] pairs.`)
    const items: Array<string> = []
    while (true) {
      const step = yield* cursor.next
@@ -126,6 +124,26 @@ const readPair = <R>(ctx: Interpreter<R>, value: unknown): Effect.Effect<Array<s
    }
  })

+/** Reads a synchronous iterable of `[name, value]` pairs as strings; `undefined` when `init` is not iterable. */
+export const readPairs = <R>(
+  ctx: Interpreter<R>,
+  init: unknown,
+  label: string,
+): Effect.Effect<Array<[string, string]> | undefined, unknown, R> =>
+  Effect.gen(function* () {
+    const cursor = yield* ctx.iterate(init)
+    if (cursor === undefined) return undefined
+    const pairs: Array<Array<string>> = []
+    while (true) {
+      const step = yield* cursor.next
+      if (step.done) {
+        if (pairs.some((entry) => entry.length !== 2)) throw typeError(`${label} expects iterable [name, value] pairs.`)
+        return pairs.map((entry): [string, string] => [entry[0] ?? "", entry[1] ?? ""])
+      }
+      pairs.push(yield* preserveConsumerError(cursor, readPair(ctx, step.value, label)))
+    }
+  })
+
 const constructURLSearchParams = <R>(
  ctx: Interpreter<R>,
  init: unknown,
@@ -139,20 +157,8 @@ const constructURLSearchParams = <R>(
    return Effect.succeed(wrap(new URLSearchParams(coerceToString(init))))
  }
  return Effect.gen(function* () {
-    const cursor = yield* ctx.iterate(init)
-    if (cursor !== undefined) {
-      const pairs: Array<Array<string>> = []
-      while (true) {
-        const step = yield* cursor.next
-        if (step.done) {
-          if (pairs.some((entry) => entry.length !== 2)) {
-            throw typeError("new URLSearchParams(...) expects iterable [name, value] pairs.")
-          }
-          return wrap(new URLSearchParams(pairs.map((entry): [string, string] => [entry[0] ?? "", entry[1] ?? ""])))
-        }
-        pairs.push(yield* preserveConsumerError(cursor, readPair(ctx, step.value)))
-      }
-    }
+    const pairs = yield* readPairs(ctx, init, "new URLSearchParams(...)")
+    if (pairs !== undefined) return wrap(new URLSearchParams(pairs))
    if (isRuntimeReference(init)) {
      throw typeError("new URLSearchParams(...) expects a query string, data object, or synchronous iterable pairs.")
    }
@@ -13,6 +13,7 @@ import {
  SetObj,
  URLObj,
  URLSearchParamsObj,
+  HeadersObj,
 } from "../interpreter/objects.js"
 import type { Interpreter } from "../interpreter/interpreter.js"

@@ -28,6 +29,7 @@ export const coerceToString = (value: unknown): string => {
  if (value instanceof SetObj) return "[object Set]"
  if (value instanceof URLObj) return value.url.href
  if (value instanceof URLSearchParamsObj) return value.params.toString()
+  if (value instanceof HeadersObj) return "[object Headers]"
  if (value instanceof Bytes) return value.bytes.join(",")
  if (value instanceof ErrorObj) {
    // Match Error.prototype.toString: "name: message", or just one when the other is empty.
@@ -36,26 +36,48 @@ export type ToolCall = {
  readonly name: string
 }

-export type ToolCallStarted = {
-  readonly index: number
+/** A tool call the program is making, with its decoded input. */
+export type ToolInvocation = { readonly name: string; readonly input: unknown }
+
+/** A call the program is making to an extension global, with its arguments. */
+export type ExtensionInvocation = {
+  readonly extension: string
  readonly name: string
-  readonly input: unknown
+  readonly args: ReadonlyArray<unknown>
 }

-export type ToolCallEnded = {
-  readonly index: number
-  readonly name: string
-  readonly input: unknown
-  readonly durationMs: number
-  readonly outcome: "success" | "failure" | "interrupted"
-  readonly message?: string
+/** How a call ended; `after` hooks observe it and cannot change it. */
+export type CallResult =
+  | { readonly status: "success"; readonly value: unknown }
+  | { readonly status: "failure"; readonly error: unknown }
+  | { readonly status: "interrupted" }
+
+/** Hooks around every call the program makes into the host. A failing `before` denies the call. */
+export type Hooks<R = never> = {
+  readonly "tool.before"?: ((call: ToolInvocation) => Effect.Effect<void, unknown, R>) | undefined
+  readonly "tool.after"?: ((call: ToolInvocation, result: CallResult) => Effect.Effect<void, never, R>) | undefined
+  readonly "extension.before"?: ((call: ExtensionInvocation) => Effect.Effect<void, unknown, R>) | undefined
+  readonly "extension.after"?:
+    | ((call: ExtensionInvocation, result: CallResult) => Effect.Effect<void, never, R>)
+    | undefined
 }

-export type ToolCallHooks<R = never> = {
-  /** Observes decoded tool input immediately before tool execution. */
-  readonly onToolCallStart?: ((call: ToolCallStarted) => Effect.Effect<void, never, R>) | undefined
-  /** Observes each admitted tool call as it succeeds, fails, or is interrupted. */
-  readonly onToolCallEnd?: ((call: ToolCallEnded) => Effect.Effect<void, never, R>) | undefined
+/** Runs `before`, then `run`, then `after` with how it ended, including when interrupted. */
+export const hooked = <Call, A, R>(
+  call: Call,
+  before: ((call: Call) => Effect.Effect<void, unknown, R>) | undefined,
+  after: ((call: Call, result: CallResult) => Effect.Effect<void, never, R>) | undefined,
+  run: Effect.Effect<A, unknown, R>,
+): Effect.Effect<A, unknown, R> => {
+  const observed =
+    after === undefined
+      ? run
+      : Effect.onExit(run, (exit) => {
+          if (Exit.isSuccess(exit)) return after(call, { status: "success", value: exit.value })
+          if (Cause.hasInterruptsOnly(exit.cause)) return after(call, { status: "interrupted" })
+          return after(call, { status: "failure", error: Cause.squash(exit.cause) })
+        })
+  return before === undefined ? observed : Effect.andThen(before(call), observed)
 }

 export type ToolDescription = {
@@ -144,15 +166,24 @@ const flattenTools = <R>(
  ]
 }

-const describeTool = <R>(visible: VisibleTool<R>): ToolDescription => ({
-  path: visible.path,
-  description: visible.tool.description,
-  signature: isEmptyInput(visible.tool)
-    ? `${toolExpression(visible.path)}(): Promise<${outputTypeScript(visible.tool, true)}>`
-    : `${toolExpression(visible.path)}(input: ${inputTypeScript(visible.tool, true)}): Promise<${outputTypeScript(visible.tool, true)}>`,
-})
+const describeTool = <R>(visible: VisibleTool<R>): ToolDescription => {
+  let signature: string | undefined
+  return {
+    path: visible.path,
+    description: visible.tool.description,
+    get signature() {
+      // Search ranks paths and descriptions first; only returned matches need their schemas rendered.
+      // Joining the final fragments avoids retaining the rendering's intermediate string ropes in JSC.
+      return (signature ??= [
+        toolExpression(visible.path),
+        isEmptyInput(visible.tool) ? "()" : `(input: ${inputTypeScript(visible.tool, true)})`,
+        `: Promise<${outputTypeScript(visible.tool, true)}>`,
+      ].join(""))
+    },
+  }
+}

-/** Tools indexed once per runtime: the lookup trie plus the model-facing catalog and search index. */
+/** Tools indexed once per runtime, with discovery materialized on demand. */
 export type Prepared<R = never> = {
  readonly root: ToolNode<R>
  readonly catalog: ReadonlyArray<ToolDescription>
@@ -264,12 +295,20 @@ const toSearchEntry = <R>(visible: VisibleTool<R>): SearchEntry => ({

 export const prepare = <R>(tools: Tools<R>): Prepared<R> => {
  const root = toolTrie(tools)
-  // Discovery bytes are durable instructions, so order only after canonical-path collisions settle.
-  const visible = flattenTools(root).sort((left, right) => compareText(left.path, right.path))
+  let searchIndex: ReadonlyArray<SearchEntry> | undefined
+  let catalog: ReadonlyArray<ToolDescription> | undefined
  return {
    root,
-    catalog: visible.map(describeTool),
-    searchIndex: visible.map(toSearchEntry),
+    get catalog() {
+      return (catalog ??= this.searchIndex.map((entry) => entry.description))
+    },
+    get searchIndex() {
+      // Executing known tools only needs the trie. Render discovery when it is actually read,
+      // ordering after canonical-path collisions settle so instruction bytes stay deterministic.
+      return (searchIndex ??= flattenTools(root)
+        .sort((left, right) => compareText(left.path, right.path))
+        .map(toSearchEntry))
+    },
  }
 }

@@ -316,6 +355,7 @@ export class ToolRuntimeError extends Error {
 /** The tool bridge of one execution. Arguments arrive and results leave as JSON; program values never enter. */
 export type ToolRuntime<R = never> = {
  readonly calls: Array<ToolCall>
+  readonly hooks: Hooks<R>
  readonly execute: (
    path: ReadonlyArray<string>,
    args: Array<Json | undefined>,
@@ -328,27 +368,10 @@ export type ToolRuntime<R = never> = {
 export const make = <R>(
  prepared: Prepared<R>,
  maxToolCalls: number | undefined,
-  hooks?: ToolCallHooks<R>,
+  hooks: Hooks<R> = {},
 ): ToolRuntime<R> => {
  const calls: Array<ToolCall> = []
  const root = prepared.root
-  const searchTool = makeSearchTool(prepared.searchIndex)
-
-  const observeEnd = <A, E>(effect: Effect.Effect<A, E, R>, call: ToolCallStarted): Effect.Effect<A, E, R> => {
-    const onEnd = hooks?.onToolCallEnd
-    if (onEnd === undefined) return effect
-    const startedAt = Date.now()
-    return effect.pipe(
-      Effect.onExit((exit) => {
-        const durationMs = Date.now() - startedAt
-        if (Exit.isSuccess(exit)) return onEnd({ ...call, durationMs, outcome: "success" })
-        if (Cause.hasInterruptsOnly(exit.cause)) return onEnd({ ...call, durationMs, outcome: "interrupted" })
-        const error = Cause.squash(exit.cause)
-        const message = error instanceof Error ? error.message : Cause.pretty(exit.cause)
-        return onEnd({ ...call, durationMs, outcome: "failure", message })
-      }),
-    )
-  }

  const recordCall = (call: ToolCall): void => {
    if (maxToolCalls !== undefined && calls.length >= maxToolCalls) {
@@ -371,14 +394,12 @@ export const make = <R>(
            name === "search" ? [] : ["The signature may have changed. Use search to get the current signature."],
          ),
      })
-      const index = yield* Effect.sync(() => {
-        recordCall({ name })
-        return calls.length - 1
-      })
-      const call = { index, name, input }
-      return yield* observeEnd(
+      yield* Effect.sync(() => recordCall({ name }))
+      return yield* hooked(
+        { name, input },
+        hooks["tool.before"],
+        hooks["tool.after"],
        Effect.gen(function* () {
-          if (hooks?.onToolCallStart !== undefined) yield* hooks.onToolCallStart(call)
          const raw = yield* Effect.suspend(() => tool.execute(input)).pipe(
            Effect.catchCause((cause) => {
              if (Cause.hasInterruptsOnly(cause)) return Effect.interrupt
@@ -400,20 +421,20 @@ export const make = <R>(
            catch: (cause) => new ToolRuntimeError("InvalidToolOutput", `Invalid output from tool '${name}': ${cause}`),
          })
        }),
-        call,
      )
    })

  return {
    calls,
+    hooks,
    keys: (path) => namespaceKeys(root, path),
-    search: (args) => Effect.suspend(() => executeTool("search", searchTool, args)),
+    search: (args) => Effect.suspend(() => executeTool("search", makeSearchTool(prepared.searchIndex), args)),
    execute: (path, args) =>
      Effect.suspend(() => {
        const segments = canonicalSegments(path)
        // Models often write `tools.search(...)` for the bare `search(...)`; honor it unless a tool owns that path.
        if (segments.length === 1 && segments[0] === "search" && lookup(root, segments) === undefined)
-          return executeTool("search", searchTool, args)
+          return executeTool("search", makeSearchTool(prepared.searchIndex), args)
        return executeTool(segments.join("."), resolve(root, path), args)
      }),
  }
@@ -1,6 +1,6 @@
 import { describe, expect, test } from "bun:test"
 import { Cause, Effect, Schema } from "effect"
-import { CodeMode, Tool, toolError } from "../src/index.js"
+import { CodeMode, Extension, Tool, toolError } from "../src/index.js"

 const run = (tool: Tool.Tool<never>) =>
  Effect.runPromise(CodeMode.make({ tools: { host: { call: tool } } }).execute("return await tools.host.call({})"))
@@ -160,77 +160,154 @@ describe("CodeMode host failure boundary", () => {
  })
 })

-describe("CodeMode tool-call observation", () => {
-  test("reports the tools actually invoked with decoded input", async () => {
-    const calls: Array<unknown> = []
+describe("CodeMode call hooks", () => {
+  const ended = (result: CodeMode.CallResult) => {
+    if (result.status !== "failure") return result.status
+    return `failure:${result.error instanceof Error ? result.error.message : String(result.error)}`
+  }
+  // Every hook logs its event, so the order across tools and extensions is visible.
+  const observe = (events: Array<string>): CodeMode.Hooks => ({
+    "tool.before": (call) => Effect.sync(() => void events.push(`tool.before ${JSON.stringify(call)}`)),
+    "tool.after": (call, result) => Effect.sync(() => void events.push(`tool.after ${call.name} ${ended(result)}`)),
+    "extension.before": (call) => Effect.sync(() => void events.push(`extension.before ${JSON.stringify(call)}`)),
+    "extension.after": (call, result) =>
+      Effect.sync(() => void events.push(`extension.after ${call.name} ${ended(result)}`)),
+  })
+
+  test("surround the tools actually invoked, with decoded input", async () => {
+    const events: Array<string> = []
    const lookup = Tool.make({
      description: "Look up a value",
      input: Schema.Struct({ query: Schema.String }),
      output: Schema.String,
-      execute: ({ query }) => Effect.succeed(query),
+      execute: ({ query }) => (query === "boom" ? Effect.fail(toolError("Lookup refused")) : Effect.succeed(query)),
    })
+    const runtime = CodeMode.make({ tools: { context: { lookup } }, hooks: observe(events) })

-    const result = await Effect.runPromise(
-      CodeMode.make({
-        tools: { context: { lookup } },
-        onToolCallStart: (call) => Effect.sync(() => calls.push(call)),
-      }).execute(`
+    const success = await Effect.runPromise(
+      runtime.execute(`
        if (false) await tools.context.lookup({ query: "not called" })
        return await tools.context.lookup({ query: "deployment failure" })
      `),
    )
-
-    expect(result.ok).toBe(true)
-    expect(calls).toStrictEqual([{ index: 0, name: "context.lookup", input: { query: "deployment failure" } }])
-  })
-
-  test("observes settled calls with outcome and duration", async () => {
-    const events: Array<{ phase: string; index: number; name: string; outcome?: string; message?: string }> = []
-    const lookup = Tool.make({
-      description: "Look up a value",
-      input: Schema.Struct({ query: Schema.String }),
-      output: Schema.String,
-      execute: ({ query }) =>
-        query === "boom"
-          ? Effect.fail(toolError("Lookup refused"))
-          : query === "defect"
-            ? Effect.die("broken")
-            : Effect.succeed(query),
-    })
-
-    const runtime = CodeMode.make({
-      tools: { context: { lookup } },
-      onToolCallStart: (call) =>
-        Effect.sync(() => {
-          events.push({ phase: "start", index: call.index, name: call.name })
-        }),
-      onToolCallEnd: (call) =>
-        Effect.sync(() => {
-          expect(call.durationMs).toBeGreaterThanOrEqual(0)
-          events.push({
-            phase: "end",
-            index: call.index,
-            name: call.name,
-            outcome: call.outcome,
-            ...(call.message === undefined ? {} : { message: call.message }),
-          })
-        }),
-    })
-
-    const success = await Effect.runPromise(runtime.execute(`return await tools.context.lookup({ query: "ok" })`))
-    expect(success.ok).toBe(true)
+    expect(success).toMatchObject({ ok: true, value: "deployment failure" })
    const failure = await Effect.runPromise(runtime.execute(`return await tools.context.lookup({ query: "boom" })`))
    expect(failure.ok).toBe(false)
-    const defect = await Effect.runPromise(runtime.execute(`return await tools.context.lookup({ query: "defect" })`))
-    expect(defect.ok).toBe(false)
+    expect(events).toEqual([
+      'tool.before {"name":"context.lookup","input":{"query":"deployment failure"}}',
+      "tool.after context.lookup success",
+      'tool.before {"name":"context.lookup","input":{"query":"boom"}}',
+      "tool.after context.lookup failure:Lookup refused",
+    ])
+  })

-    expect(events).toStrictEqual([
-      { phase: "start", index: 0, name: "context.lookup" },
-      { phase: "end", index: 0, name: "context.lookup", outcome: "success" },
-      { phase: "start", index: 0, name: "context.lookup" },
-      { phase: "end", index: 0, name: "context.lookup", outcome: "failure", message: "Lookup refused" },
-      { phase: "start", index: 0, name: "context.lookup" },
-      { phase: "end", index: 0, name: "context.lookup", outcome: "failure", message: "broken" },
+  test("surround each call to an extension global, with its arguments, but not functions inside results", async () => {
+    const events: Array<string> = []
+    const runtime = CodeMode.make({
+      extensions: [
+        Extension.make({
+          name: "web",
+          globals: {
+            fetch: async (url: string, init?: { method?: string }) => ({
+              url,
+              method: init?.method ?? "GET",
+              json: () => 1,
+            }),
+            fail: () => {
+              throw new RangeError("nope")
+            },
+          },
+        }),
+      ],
+      hooks: observe(events),
+    })
+    const result = await Effect.runPromise(
+      runtime.execute(`
+        const res = await fetch("https://a.test/", { method: "POST" })
+        res.json()
+        try { fail() } catch {}
+        return res.url
+      `),
+    )
+    expect(result).toMatchObject({ ok: true, value: "https://a.test/", toolCalls: [] })
+    expect(events).toEqual([
+      'extension.before {"extension":"web","name":"fetch","args":["https://a.test/",{"method":"POST"}]}',
+      "extension.after fetch success",
+      'extension.before {"extension":"web","name":"fail","args":[]}',
+      "extension.after fail failure:nope",
+    ])
+  })
+
+  test("a failing before hook denies the call; the program catches the host's error", async () => {
+    const runtime = CodeMode.make({
+      tools: {
+        lookup: Tool.make({
+          description: "Look up",
+          input: Schema.Struct({}),
+          output: Schema.String,
+          execute: () => Effect.succeed("ok"),
+        }),
+      },
+      extensions: [Extension.make({ name: "web", globals: { fetch: async (url: string) => url } })],
+      hooks: { "extension.before": (call) => Effect.fail(new Error(`${call.name} is not allowed`)) },
+    })
+    expect(
+      await Effect.runPromise(
+        runtime.execute(`
+          const denied = await fetch("https://a.test/").catch((e) => [e instanceof Error, e.message])
+          return [denied, await tools.lookup({})]
+        `),
+      ),
+    ).toMatchObject({ ok: true, value: [[true, "fetch is not allowed"], "ok"] })
+    expect(await Effect.runPromise(runtime.execute(`await fetch("https://a.test/")`))).toMatchObject({
+      ok: false,
+      error: { kind: "ExecutionFailure", message: "Error: fetch is not allowed" },
+    })
+    const refused = CodeMode.make({
+      tools: {
+        lookup: Tool.make({
+          description: "Look up",
+          input: Schema.Struct({}),
+          output: Schema.String,
+          execute: () => Effect.succeed("ok"),
+        }),
+      },
+      hooks: { "tool.before": () => Effect.fail(toolError("lookup is not allowed")) },
+    })
+    expect(
+      await Effect.runPromise(
+        refused.execute(`try { await tools.lookup({}) } catch (e) { return [e.name, e.message] }`),
+      ),
+    ).toMatchObject({ ok: true, value: ["Error", "lookup is not allowed"], toolCalls: [{ name: "lookup" }] })
+    expect(await Effect.runPromise(refused.execute(`await tools.lookup({})`))).toMatchObject({
+      ok: false,
+      error: { kind: "ToolFailure", message: "lookup is not allowed" },
+    })
+  })
+
+  test("see a tool defect as the failure the program gets, and an interrupted extension call", async () => {
+    const events: Array<string> = []
+    const broken = Tool.make({
+      description: "Broken",
+      input: Schema.Struct({}),
+      output: Schema.String,
+      execute: () => Effect.die("broken"),
+    })
+    const runtime = CodeMode.make({
+      tools: { broken },
+      extensions: [Extension.make({ name: "slow", globals: { forever: () => new Promise(() => {}) } })],
+      hooks: observe(events),
+    })
+    expect(await Effect.runPromise(runtime.execute(`await tools.broken({})`))).toMatchObject({ ok: false })
+    expect(await Effect.runPromise(runtime.execute(`forever(); return "done"`))).toMatchObject({
+      ok: true,
+      value: "done",
+    })
+    expect(events).toEqual([
+      'tool.before {"name":"broken","input":{}}',
+      "tool.after broken failure:broken",
+      'extension.before {"extension":"slow","name":"forever","args":[]}',
+      "extension.after forever interrupted",
    ])
  })

@@ -243,15 +320,11 @@ describe("CodeMode tool-call observation", () => {
      execute: () => Effect.interrupt,
    })
    const exit = await Effect.runPromiseExit(
-      CodeMode.make({
-        tools: { host: { call } },
-        onToolCallStart: () => Effect.sync(() => events.push("start")),
-        onToolCallEnd: (call) => Effect.sync(() => events.push(`end:${call.outcome}`)),
-      }).execute("return await tools.host.call({})"),
+      CodeMode.make({ tools: { host: { call } }, hooks: observe(events) }).execute("return await tools.host.call({})"),
    )

    expect(exit._tag).toBe("Failure")
-    expect(events).toEqual(["start", "end:interrupted"])
+    expect(events).toEqual(['tool.before {"name":"host.call","input":{}}', "tool.after host.call interrupted"])
  })

  test("observes running calls interrupted during completion", async () => {
@@ -263,39 +336,17 @@ describe("CodeMode tool-call observation", () => {
      execute: () => Effect.never,
    })
    const result = await Effect.runPromise(
-      CodeMode.make({
-        tools: { host: { call } },
-        onToolCallStart: () => Effect.sync(() => events.push("start")),
-        onToolCallEnd: (call) => Effect.sync(() => events.push(`end:${call.outcome}`)),
-      }).execute('tools.host.call({}); return "done"'),
+      CodeMode.make({ tools: { host: { call } }, hooks: observe(events) }).execute(
+        'tools.host.call({}); return "done"',
+      ),
    )

    expect(result).toMatchObject({ ok: true, value: "done" })
-    expect(events).toEqual(["start", "end:interrupted"])
-  })
-
-  test("ends calls interrupted during start observation", async () => {
-    const events: Array<string> = []
-    const call = Tool.make({
-      description: "Unused",
-      input: Schema.Struct({}),
-      output: Schema.String,
-      execute: () => Effect.succeed("unused"),
-    })
-    const exit = await Effect.runPromiseExit(
-      CodeMode.make({
-        tools: { host: { call } },
-        onToolCallStart: () => Effect.interrupt,
-        onToolCallEnd: (call) => Effect.sync(() => events.push(call.outcome)),
-      }).execute("return await tools.host.call({})"),
-    )
-
-    expect(exit._tag).toBe("Failure")
-    expect(events).toEqual(["interrupted"])
+    expect(events).toEqual(['tool.before {"name":"host.call","input":{}}', "tool.after host.call interrupted"])
  })

  test("observes calls interrupted by the execution timeout", async () => {
-    const outcomes: Array<string> = []
+    const events: Array<string> = []
    const call = Tool.make({
      description: "Pending",
      input: Schema.Struct({}),
@@ -303,15 +354,13 @@ describe("CodeMode tool-call observation", () => {
      execute: () => Effect.never,
    })
    const result = await Effect.runPromise(
-      CodeMode.make({
-        tools: { host: { call } },
-        limits: { timeoutMs: 10 },
-        onToolCallEnd: (call) => Effect.sync(() => outcomes.push(call.outcome)),
-      }).execute("return await tools.host.call({})"),
+      CodeMode.make({ tools: { host: { call } }, limits: { timeoutMs: 10 }, hooks: observe(events) }).execute(
+        "return await tools.host.call({})",
+      ),
    )

    expect(result).toMatchObject({ ok: false, error: { kind: "TimeoutExceeded" } })
-    expect(outcomes).toEqual(["interrupted"])
+    expect(events).toEqual(['tool.before {"name":"host.call","input":{}}', "tool.after host.call interrupted"])
  })
 })

@@ -840,20 +889,17 @@ describe("CodeMode public contract", () => {
    }
  })

-  test("search is a counted tool call: it burns maxToolCalls and fires the hooks", async () => {
-    const started: Array<string> = []
-    const ended: Array<string> = []
+  test("search is a counted tool call: it burns maxToolCalls and is observed", async () => {
+    const observed: Array<CodeMode.ToolInvocation> = []
    const limited = CodeMode.make({
      tools,
      limits: { maxToolCalls: 1 },
-      onToolCallStart: (call) => Effect.sync(() => void started.push(call.name)),
-      onToolCallEnd: (call) => Effect.sync(() => void ended.push(`${call.name}:${call.outcome}`)),
+      hooks: { "tool.before": (call) => Effect.sync(() => void observed.push(call)) },
    })
    const result = await Effect.runPromise(limited.execute(`search({}); return search({})`))
    expect(result.ok).toBe(false)
    if (!result.ok) expect(result.error.kind).toBe("ToolCallLimitExceeded")
-    expect(started).toEqual(["search"])
-    expect(ended).toEqual(["search:success"])
+    expect(observed).toEqual([{ name: "search", input: {} }])
  })

  test("search is an opaque, shadowable global like other built-ins", async () => {
@@ -1089,7 +1135,7 @@ describe("CodeMode public contract", () => {
    })
    const runtime = CodeMode.make({
      tools: { math: { double: transformed } },
-      onToolCallStart: (call) => Effect.sync(() => observed.push(call.input)),
+      hooks: { "tool.before": (call) => Effect.sync(() => void observed.push(call.input)) },
    })

    const success = await Effect.runPromise(runtime.execute(`return await tools.math.double({ value: "21" })`))
@@ -128,6 +128,34 @@ describe("values are converted at the boundary, never shared", () => {
    expect([...(held[0] as Set<{ z: number }>)][0]).toEqual({ z: 1 })
  })

+  test("Headers cross as copies in both directions", async () => {
+    const stored = new Headers({ "X-A": "1" })
+    const target = CodeMode.make({
+      extensions: [
+        Extension.make({
+          name: "http",
+          globals: {
+            headers: () => stored,
+            keep: (value: Headers) => {
+              held.push(value)
+              return value
+            },
+          },
+        }),
+      ],
+    })
+    held.length = 0
+    expect(
+      await value(
+        `const h = headers(); h.set("x-a", "2"); const back = keep(h); back.set("x-a", "3"); return [h instanceof Headers, h.get("x-a"), back === h, back.get("x-a"), [...back]]`,
+        target,
+      ),
+    ).toEqual([true, "2", false, "3", [["x-a", "3"]]])
+    expect(stored.get("x-a")).toBe("1")
+    expect(held[0]).toBeInstanceOf(Headers)
+    expect((held[0] as Headers).get("x-a")).toBe("2")
+  })
+
  test("bytes cross as copies in both directions; ArrayBuffer comes in as Uint8Array", async () => {
    const stored = new Uint8Array([1, 2, 3])
    const target = CodeMode.make({
@@ -192,6 +220,16 @@ describe("values are converted at the boundary, never shared", () => {
    expect((await failure(`big()`, target)).message).toContain("big produced a bigint")
  })

+  test("a value that cannot come out carries a location like any built-in error, sync or async", async () => {
+    const target = CodeMode.make({
+      extensions: [
+        Extension.make({ name: "odd", globals: { sym: () => Symbol("s"), later: async () => Symbol("s") } }),
+      ],
+    })
+    expect((await failure(`sym()`, target)).location).toEqual((await failure(`JSON.parse("{")`, target)).location)
+    expect((await failure(`await later()`, target)).location).toBeDefined()
+  })
+
  test("a class instance cannot come out", async () => {
    class Other {}
    const target = CodeMode.make({
@@ -635,6 +635,154 @@ describe("URL and URI helpers", () => {
  })
 })

+describe("Headers", () => {
+  test("constructs from records, pairs, Maps, and Headers; names fold to lowercase and values combine", async () => {
+    expect(
+      await value(`
+        const headers = new Headers({ "Content-Type": "text/plain", "X-Count": 1, "X-Null": null })
+        headers.append("Accept", "text/html")
+        headers.append("accept", "application/json")
+        headers.set("x-count", "2")
+        headers.delete("x-null")
+        const copy = new Headers(headers)
+        copy.set("content-type", "text/html")
+        return {
+          get: headers.get("content-type"),
+          missing: headers.get("x-missing"),
+          combined: headers.get("ACCEPT"),
+          has: [headers.has("Accept"), headers.has("x-null")],
+          count: headers.get("x-count"),
+          copied: [headers.get("content-type"), copy.get("content-type")],
+          pairs: [...new Headers([["b", "2"], ["A", "1"]])],
+          map: [...new Headers(new Map([["k", "v"]]))],
+          keys: headers.keys(),
+          values: headers.values(),
+          entries: headers.entries(),
+        }
+      `),
+    ).toEqual({
+      get: "text/plain",
+      missing: null,
+      combined: "text/html, application/json",
+      has: [true, false],
+      count: "2",
+      copied: ["text/plain", "text/html"],
+      pairs: [
+        ["a", "1"],
+        ["b", "2"],
+      ],
+      map: [["k", "v"]],
+      keys: ["accept", "content-type", "x-count"],
+      values: ["text/html, application/json", "text/plain", "2"],
+      entries: [
+        ["accept", "text/html, application/json"],
+        ["content-type", "text/plain"],
+        ["x-count", "2"],
+      ],
+    })
+  })
+
+  test("iterates in sorted order everywhere iteration is allowed, and getSetCookie keeps cookies apart", async () => {
+    expect(
+      await value(`
+        const headers = new Headers({ b: "2", a: "1" })
+        headers.append("Set-Cookie", "x=1")
+        headers.append("set-cookie", "y=2")
+        const seen = []
+        headers.forEach((value, name, self) => seen.push(name + "=" + value + ":" + (self === headers)))
+        const [first] = headers
+        function* pairs() { yield* headers }
+        return {
+          seen,
+          first,
+          spread: [...headers],
+          from: Array.from(headers).length,
+          generator: [...pairs()].length,
+          object: Object.fromEntries(headers),
+          cookies: headers.getSetCookie(),
+        }
+      `),
+    ).toEqual({
+      seen: ["a=1:true", "b=2:true", "set-cookie=x=1:true", "set-cookie=y=2:true"],
+      first: ["a", "1"],
+      spread: [
+        ["a", "1"],
+        ["b", "2"],
+        ["set-cookie", "x=1"],
+        ["set-cookie", "y=2"],
+      ],
+      from: 4,
+      generator: 4,
+      object: { a: "1", b: "2", "set-cookie": "y=2" },
+      cookies: ["x=1", "y=2"],
+    })
+  })
+
+  test("serializes as a name-to-value object at the boundary and in JSON; prints for console", async () => {
+    const result = await run(`
+      const headers = new Headers({ "X-A": "1", b: "2" })
+      console.log(headers)
+      return { headers, json: JSON.stringify({ headers }), text: String(headers), type: typeof headers, is: headers instanceof Headers }
+    `)
+    expect(result.ok && result.value).toEqual({
+      headers: { b: "2", "x-a": "1" },
+      json: '{"headers":{"b":"2","x-a":"1"}}',
+      text: "[object Headers]",
+      type: "object",
+      is: true,
+    })
+    expect(result.ok && result.logs?.[0]).toBe('Headers {"b":"2","x-a":"1"}')
+  })
+
+  test("rejects what it cannot build from, and invalid names and values, with TypeErrors the program can catch", async () => {
+    expect(
+      await value(`
+        function message(run) {
+          try { run(); return null } catch (error) { return error instanceof TypeError ? error.message : error }
+        }
+        const headers = new Headers()
+        return [
+          message(() => Headers()),
+          message(() => new Headers(null)),
+          message(() => new Headers(1)),
+          message(() => new Headers("a=1")),
+          message(() => new Headers(new Date())),
+          message(() => new Headers(() => 1)),
+          message(() => new Headers([["name"]])),
+          message(() => new Headers([["a", "b", "c"]])),
+          message(() => new Headers({ "bad name": "x" })),
+          message(() => new Headers({ name: "bad\u0000value" })),
+          message(() => headers.get("invalid\u0100")),
+          message(() => headers.has({})),
+          message(() => headers.set("a", "invalid\u0100")),
+          message(() => headers.append("a")),
+          message(() => headers.forEach()),
+          message(() => headers.forEach(1)),
+          message(() => { const get = headers.get; return get("a") }),
+        ]
+      `),
+    ).toEqual([
+      "Constructor Headers requires 'new'.",
+      "new Headers(...) expects a record of names to values, iterable [name, value] pairs, or Headers.",
+      "new Headers(...) expects a record of names to values, iterable [name, value] pairs, or Headers.",
+      "new Headers(...) expects a record of names to values, iterable [name, value] pairs, or Headers.",
+      "new Headers(...) expects a record of names to values, iterable [name, value] pairs, or Headers.",
+      "new Headers(...) expects a record of names to values, iterable [name, value] pairs, or Headers.",
+      "new Headers(...) expects iterable [name, value] pairs.",
+      "new Headers(...) expects iterable [name, value] pairs.",
+      expect.stringContaining("bad name"),
+      expect.stringContaining("invalid value"),
+      expect.stringContaining("Invalid header name"),
+      expect.stringContaining("[object Object]"),
+      expect.stringContaining("invalid value"),
+      "Headers.append requires 2 arguments.",
+      "Headers.forEach requires 1 argument.",
+      "Headers.forEach expects a function callback.",
+      "Headers.prototype.get called on incompatible receiver undefined.",
+    ])
+  })
+})
+
 describe("Map", () => {
  test("get/set/has/size with chaining", async () => {
    expect(
@@ -3,10 +3,14 @@
 * - html/webappapis/atob/base64.any.js (btoa reference encoder, input list, and atob WebIDL cases)
 * - fetch/data-urls/resources/base64.json (copied to fixtures/wpt-base64.json)
 * - WebCryptoAPI/randomUUID.https.any.js
+ * - fetch/api/headers/{headers-basic,headers-casing,headers-combine,headers-errors,headers-normalize,header-setcookie}.any.js
 *
 * Copyright © web-platform-tests contributors. Governed by the 3-Clause BSD license in LICENSE.wpt.
 *
 * `assert_throws_dom("InvalidCharacterError", …)` becomes a check for a TypeError: CodeMode has no DOMException.
+ * Headers cases that need `Symbol.iterator`, iterator objects from `keys()`/`values()`/`entries()` (CodeMode returns
+ * arrays), or a custom iterator on a Headers instance are left out, as are two set-cookie cases that Bun's own Headers
+ * fails by sorting `set-cookie2` ahead of `set-cookie`.
 */
 import { describe, expect, test } from "bun:test"
 import { Effect } from "effect"
@@ -166,3 +170,411 @@ describe("crypto.randomUUID WPT parity (WebCryptoAPI/randomUUID.https.any.js)",
    ).toEqual([true, true, true, 768])
  })
 })
+
+// Enough of testharness.js to run the Headers files close to verbatim; each `test` records its failure, if any.
+const testharness = `
+  const failures = []
+  function test(run, name) { try { run() } catch (error) { failures.push(name + ": " + (error && error.message ? error.message : error)) } }
+  function assert_equals(actual, expected, message) { if (actual !== expected) throw new Error((message || "") + " expected " + JSON.stringify(expected) + " got " + JSON.stringify(actual)) }
+  function assert_true(actual, message) { assert_equals(actual, true, message) }
+  function assert_false(actual, message) { assert_equals(actual, false, message) }
+  function assert_array_equals(actual, expected, message) { assert_equals(JSON.stringify(actual), JSON.stringify(expected), message) }
+  function assert_nested_array_equals(actual, expected) { assert_array_equals(actual, expected) }
+  function assert_throws_js(type, run) { try { run() } catch (error) { if (error instanceof type) return; throw new Error("threw " + error.name) } throw new Error("did not throw") }
+  function assert_unreached() { throw new Error("unreachable") }
+`
+
+describe("Headers WPT parity (fetch/api/headers)", () => {
+  test("headers-basic.any.js", async () => {
+    expect(
+      await value(`
+        ${testharness}
+        test(function() { new Headers() }, "Create headers from no parameter")
+        test(function() { new Headers(undefined) }, "Create headers from undefined parameter")
+        test(function() { new Headers({}) }, "Create headers from empty object")
+        var parameters = [null, 1]
+        parameters.forEach(function(parameter) {
+          test(function() { assert_throws_js(TypeError, function() { new Headers(parameter) }) }, "Create headers with " + parameter + " should throw")
+        })
+        var headerDict = {"name1": "value1", "name2": "value2", "name3": "value3", "name4": null, "name5": undefined, "name6": 1, "Content-Type": "value4"}
+        var headerSeq = []
+        for (var name in headerDict) headerSeq.push([name, headerDict[name]])
+        test(function() {
+          var headers = new Headers(headerSeq)
+          for (name in headerDict) assert_equals(headers.get(name), String(headerDict[name]), "name: " + name + " has value: " + headerDict[name])
+          assert_equals(headers.get("length"), null, "init should be treated as a sequence, not as a dictionary")
+        }, "Create headers with sequence")
+        test(function() {
+          var headers = new Headers(headerDict)
+          for (name in headerDict) assert_equals(headers.get(name), String(headerDict[name]), "name: " + name + " has value: " + headerDict[name])
+        }, "Create headers with record")
+        test(function() {
+          var headers = new Headers(headerDict)
+          var headers2 = new Headers(headers)
+          for (name in headerDict) assert_equals(headers2.get(name), String(headerDict[name]), "name: " + name + " has value: " + headerDict[name])
+        }, "Create headers with existing headers")
+        test(function() {
+          var headers = new Headers()
+          for (name in headerDict) {
+            headers.append(name, headerDict[name])
+            assert_equals(headers.get(name), String(headerDict[name]), "name: " + name + " has value: " + headerDict[name])
+          }
+        }, "Check append method")
+        test(function() {
+          var headers = new Headers()
+          for (name in headerDict) {
+            headers.set(name, headerDict[name])
+            assert_equals(headers.get(name), String(headerDict[name]), "name: " + name + " has value: " + headerDict[name])
+          }
+        }, "Check set method")
+        test(function() {
+          var headers = new Headers(headerDict)
+          for (name in headerDict) assert_true(headers.has(name), "headers has name " + name)
+          assert_false(headers.has("nameNotInHeaders"), "headers do not have header: nameNotInHeaders")
+        }, "Check has method")
+        test(function() {
+          var headers = new Headers(headerDict)
+          for (name in headerDict) {
+            assert_true(headers.has(name), "headers have a header: " + name)
+            headers.delete(name)
+            assert_true(!headers.has(name), "headers do not have anymore a header: " + name)
+          }
+        }, "Check delete method")
+        test(function() {
+          var headers = new Headers(headerDict)
+          for (name in headerDict) assert_equals(headers.get(name), String(headerDict[name]), "name: " + name + " has value: " + headerDict[name])
+          assert_equals(headers.get("nameNotInHeaders"), null, "header: nameNotInHeaders has no value")
+        }, "Check get method")
+        var headerEntriesDict = {"name1": "value1", "Name2": "value2", "name": "value3", "content-Type": "value4", "Content-Typ": "value5", "Content-Types": "value6"}
+        var sortedHeaderDict = {}
+        var headerValues = []
+        var sortedHeaderKeys = Object.keys(headerEntriesDict).map(function(value) {
+          sortedHeaderDict[value.toLowerCase()] = headerEntriesDict[value]
+          headerValues.push(headerEntriesDict[value])
+          return value.toLowerCase()
+        }).sort()
+        test(function() {
+          var headers = new Headers(headerEntriesDict)
+          assert_array_equals(headers.keys(), sortedHeaderKeys)
+          for (const key of headers.keys()) assert_true(sortedHeaderKeys.indexOf(key) != -1)
+        }, "Check keys method")
+        test(function() {
+          var headers = new Headers(headerEntriesDict)
+          assert_array_equals(headers.values(), sortedHeaderKeys.map((key) => sortedHeaderDict[key]))
+          for (const value of headers.values()) assert_true(headerValues.indexOf(value) != -1)
+        }, "Check values method")
+        test(function() {
+          var headers = new Headers(headerEntriesDict)
+          assert_array_equals(headers.entries(), sortedHeaderKeys.map((key) => [key, sortedHeaderDict[key]]))
+          for (const entry of headers.entries()) assert_equals(entry[1], sortedHeaderDict[entry[0]])
+        }, "Check entries method")
+        test(function() {
+          var headers = new Headers(headerEntriesDict)
+          assert_array_equals([...headers], sortedHeaderKeys.map((key) => [key, sortedHeaderDict[key]]))
+        }, "Check Symbol.iterator method")
+        test(function() {
+          var headers = new Headers(headerEntriesDict)
+          var index = 0
+          headers.forEach(function(value, key, container) {
+            assert_equals(headers, container)
+            assert_equals(key, sortedHeaderKeys[index])
+            assert_equals(value, sortedHeaderDict[sortedHeaderKeys[index]])
+            index++
+          })
+          assert_equals(index, sortedHeaderKeys.length)
+        }, "Check forEach method")
+        test(() => {
+          const headers = new Headers({"foo": "2", "baz": "1", "BAR": "0"})
+          const actualKeys = []
+          const actualValues = []
+          for (const [header, value] of headers) {
+            actualKeys.push(header)
+            actualValues.push(value)
+            headers.delete("foo")
+          }
+          assert_array_equals(actualKeys, ["bar", "baz"])
+          assert_array_equals(actualValues, ["0", "1"])
+        }, "Iteration skips elements removed while iterating")
+        test(() => {
+          const headers = new Headers({"foo": "2", "baz": "1", "BAR": "0", "quux": "3"})
+          const actualKeys = []
+          const actualValues = []
+          for (const [header, value] of headers) {
+            actualKeys.push(header)
+            actualValues.push(value)
+            if (header === "baz") headers.delete("bar")
+          }
+          assert_array_equals(actualKeys, ["bar", "baz", "quux"])
+          assert_array_equals(actualValues, ["0", "1", "3"])
+        }, "Removing elements already iterated over causes an element to be skipped during iteration")
+        test(() => {
+          const headers = new Headers({"foo": "2", "baz": "1", "BAR": "0", "quux": "3"})
+          const actualKeys = []
+          const actualValues = []
+          for (const [header, value] of headers) {
+            actualKeys.push(header)
+            actualValues.push(value)
+            if (header === "baz") headers.append("X-yZ", "4")
+          }
+          assert_array_equals(actualKeys, ["bar", "baz", "foo", "quux", "x-yz"])
+          assert_array_equals(actualValues, ["0", "1", "2", "3", "4"])
+        }, "Appending a value pair during iteration causes it to be reached during iteration")
+        test(() => {
+          const headers = new Headers({"foo": "2", "baz": "1", "BAR": "0", "quux": "3"})
+          const actualKeys = []
+          const actualValues = []
+          for (const [header, value] of headers) {
+            actualKeys.push(header)
+            actualValues.push(value)
+            if (header === "baz") headers.append("abc", "-1")
+          }
+          assert_array_equals(actualKeys, ["bar", "baz", "baz", "foo", "quux"])
+          assert_array_equals(actualValues, ["0", "1", "1", "2", "3"])
+        }, "Prepending a value pair before the current element position causes it to be skipped during iteration and adds the current element a second time")
+        return failures
+      `),
+    ).toEqual([])
+  })
+
+  test("headers-casing.any.js", async () => {
+    expect(
+      await value(`
+        ${testharness}
+        var headerDictCase = {"UPPERCASE": "value1", "lowercase": "value2", "mixedCase": "value3", "Content-TYPE": "value4"}
+        function checkHeadersCase(originalName, headersToCheck, expectedDict) {
+          var lowCaseName = originalName.toLowerCase()
+          var upCaseName = originalName.toUpperCase()
+          var expectedValue = expectedDict[originalName]
+          assert_equals(headersToCheck.get(originalName), expectedValue, "name: " + originalName + " has value: " + expectedValue)
+          assert_equals(headersToCheck.get(lowCaseName), expectedValue, "name: " + lowCaseName + " has value: " + expectedValue)
+          assert_equals(headersToCheck.get(upCaseName), expectedValue, "name: " + upCaseName + " has value: " + expectedValue)
+        }
+        test(function() {
+          var headers = new Headers(headerDictCase)
+          for (const name in headerDictCase) checkHeadersCase(name, headers, headerDictCase)
+        }, "Create headers, names use characters with different case")
+        test(function() {
+          var headers = new Headers()
+          for (const name in headerDictCase) {
+            headers.append(name, headerDictCase[name])
+            checkHeadersCase(name, headers, headerDictCase)
+          }
+        }, "Check append method, names use characters with different case")
+        test(function() {
+          var headers = new Headers()
+          for (const name in headerDictCase) {
+            headers.set(name, headerDictCase[name])
+            checkHeadersCase(name, headers, headerDictCase)
+          }
+        }, "Check set method, names use characters with different case")
+        test(function() {
+          var headers = new Headers()
+          for (const name in headerDictCase) headers.set(name, headerDictCase[name])
+          for (const name in headerDictCase) headers.delete(name.toLowerCase())
+          for (const name in headerDictCase) assert_false(headers.has(name), "header " + name + " should have been deleted")
+        }, "Check delete method, names use characters with different case")
+        return failures
+      `),
+    ).toEqual([])
+  })
+
+  test("headers-combine.any.js", async () => {
+    expect(
+      await value(`
+        ${testharness}
+        var headerSeqCombine = [["single", "singleValue"], ["double", "doubleValue1"], ["double", "doubleValue2"], ["triple", "tripleValue1"], ["triple", "tripleValue2"], ["triple", "tripleValue3"]]
+        var expectedDict = {"single": "singleValue", "double": "doubleValue1, doubleValue2", "triple": "tripleValue1, tripleValue2, tripleValue3"}
+        test(function() {
+          var headers = new Headers(headerSeqCombine)
+          for (const name in expectedDict) assert_equals(headers.get(name), expectedDict[name])
+        }, "Create headers using same name for different values")
+        test(function() {
+          var headers = new Headers(headerSeqCombine)
+          for (const name in expectedDict) {
+            assert_true(headers.has(name), "name: " + name + " has value(s)")
+            headers.delete(name)
+            assert_false(headers.has(name), "name: " + name + " has no value(s) anymore")
+          }
+        }, "Check delete and has methods when using same name for different values")
+        test(function() {
+          var headers = new Headers(headerSeqCombine)
+          for (const name in expectedDict) {
+            headers.set(name, "newSingleValue")
+            assert_equals(headers.get(name), "newSingleValue", "name: " + name + " has value: newSingleValue")
+          }
+        }, "Check set methods when called with already used name")
+        test(function() {
+          var headers = new Headers(headerSeqCombine)
+          for (const name in expectedDict) {
+            var value = headers.get(name)
+            headers.append(name, "newSingleValue")
+            assert_equals(headers.get(name), (value + ", " + "newSingleValue"))
+          }
+        }, "Check append methods when called with already used name")
+        test(() => {
+          const headers = new Headers([["1", "a"], ["1", "b"]])
+          for (let header of headers) assert_array_equals(header, ["1", "a, b"])
+        }, "Iterate combined values")
+        test(() => {
+          const headers = new Headers([["2", "a"], ["1", "b"], ["2", "b"]]), expected = [["1", "b"], ["2", "a, b"]]
+          let i = 0
+          for (let header of headers) {
+            assert_array_equals(header, expected[i])
+            i++
+          }
+          assert_equals(i, 2)
+        }, "Iterate combined values in sorted order")
+        return failures
+      `),
+    ).toEqual([])
+  })
+
+  test("headers-errors.any.js", async () => {
+    expect(
+      await value(`
+        ${testharness}
+        test(function() { assert_throws_js(TypeError, function() { new Headers([["name"]]) }) }, "Create headers giving an array having one string as init argument")
+        test(function() { assert_throws_js(TypeError, function() { new Headers([["invalid", "invalidValue1", "invalidValue2"]]) }) }, "Create headers giving an array having three strings as init argument")
+        test(function() { assert_throws_js(TypeError, function() { new Headers([["invalid\u0100", "Value1"]]) }) }, "Create headers giving bad header name as init argument")
+        test(function() { assert_throws_js(TypeError, function() { new Headers([["name", "invalidValue\u0100"]]) }) }, "Create headers giving bad header value as init argument")
+        var badNames = ["invalid\u0100", {}]
+        var badValues = ["invalid\u0100"]
+        badNames.forEach(function(name) {
+          test(function() { var headers = new Headers(); assert_throws_js(TypeError, function() { headers.get(name) }) }, "Check headers get with an invalid name " + name)
+        })
+        badNames.forEach(function(name) {
+          test(function() { var headers = new Headers(); assert_throws_js(TypeError, function() { headers.delete(name) }) }, "Check headers delete with an invalid name " + name)
+        })
+        badNames.forEach(function(name) {
+          test(function() { var headers = new Headers(); assert_throws_js(TypeError, function() { headers.has(name) }) }, "Check headers has with an invalid name " + name)
+        })
+        badNames.forEach(function(name) {
+          test(function() { var headers = new Headers(); assert_throws_js(TypeError, function() { headers.set(name, "Value1") }) }, "Check headers set with an invalid name " + name)
+        })
+        badValues.forEach(function(value) {
+          test(function() { var headers = new Headers(); assert_throws_js(TypeError, function() { headers.set("name", value) }) }, "Check headers set with an invalid value " + value)
+        })
+        badNames.forEach(function(name) {
+          test(function() { var headers = new Headers(); assert_throws_js(TypeError, function() { headers.append("invalid\u0100", "Value1") }) }, "Check headers append with an invalid name " + name)
+        })
+        badValues.forEach(function(value) {
+          test(function() { var headers = new Headers(); assert_throws_js(TypeError, function() { headers.append("name", value) }) }, "Check headers append with an invalid value " + value)
+        })
+        test(function() {
+          var headers = new Headers([["name", "value"]])
+          assert_throws_js(TypeError, function() { headers.forEach() })
+          assert_throws_js(TypeError, function() { headers.forEach(undefined) })
+          assert_throws_js(TypeError, function() { headers.forEach(1) })
+        }, "Headers forEach throws if argument is not callable")
+        test(function() {
+          var headers = new Headers([["name1", "value1"], ["name2", "value2"], ["name3", "value3"]])
+          var counter = 0
+          try {
+            headers.forEach(function(value, name) {
+              counter++
+              if (name == "name2") throw "error"
+            })
+          } catch (e) {
+            assert_equals(counter, 2)
+            assert_equals(e, "error")
+            return
+          }
+          assert_unreached()
+        }, "Headers forEach loop should stop if callback is throwing exception")
+        return failures
+      `),
+    ).toEqual([])
+  })
+
+  test("headers-normalize.any.js", async () => {
+    expect(
+      await value(`
+        ${testharness}
+        const expectations = {
+          "name1": [" space ", "space"],
+          "name2": ["\\ttab\\t", "tab"],
+          "name3": [" spaceAndTab\\t", "spaceAndTab"],
+          "name4": ["\\r\\n newLine", "newLine"],
+          "name5": ["newLine\\r\\n ", "newLine"],
+          "name6": ["\\r\\n\\tnewLine", "newLine"],
+          "name7": ["\\t\\f\\tnewLine\\n", "\\f\\tnewLine"],
+          "name8": ["newLine\\xa0", "newLine\\xa0"],
+        }
+        test(function () {
+          const headerDict = Object.fromEntries(Object.entries(expectations).map(([name, [actual]]) => [name, actual]))
+          var headers = new Headers(headerDict)
+          for (const name in expectations) {
+            const expected = expectations[name][1]
+            assert_equals(headers.get(name), expected, "name: " + name + " has normalized value: " + expected)
+          }
+        }, "Create headers with not normalized values")
+        test(function () {
+          var headers = new Headers()
+          for (const name in expectations) {
+            headers.append(name, expectations[name][0])
+            const expected = expectations[name][1]
+            assert_equals(headers.get(name), expected, "name: " + name + " has value: " + expected)
+          }
+        }, "Check append method with not normalized values")
+        test(function () {
+          var headers = new Headers()
+          for (const name in expectations) {
+            headers.set(name, expectations[name][0])
+            const expected = expectations[name][1]
+            assert_equals(headers.get(name), expected, "name: " + name + " has value: " + expected)
+          }
+        }, "Check set method with not normalized values")
+        return failures
+      `),
+    ).toEqual([])
+  })
+
+  test("header-setcookie.any.js", async () => {
+    expect(
+      await value(`
+        ${testharness}
+        const headerList = [["set-cookie", "foo=bar"], ["Set-Cookie", "fizz=buzz; domain=example.com"]]
+        const setCookie2HeaderList = [["set-cookie2", "foo2=bar2"], ["Set-Cookie2", "fizz2=buzz2; domain=example2.com"]]
+        test(function () {
+          const headers = new Headers(headerList)
+          assert_equals(headers.get("set-cookie"), "foo=bar, fizz=buzz; domain=example.com")
+        }, "Headers.prototype.get combines set-cookie headers in order")
+        test(function () {
+          const headers = new Headers(headerList)
+          assert_nested_array_equals([...headers], [["set-cookie", "foo=bar"], ["set-cookie", "fizz=buzz; domain=example.com"]])
+        }, "Headers iterator does not combine set-cookie headers")
+        test(function () {
+          const headers = new Headers(setCookie2HeaderList)
+          assert_nested_array_equals([...headers], [["set-cookie2", "foo2=bar2, fizz2=buzz2; domain=example2.com"]])
+        }, "Headers iterator does not special case set-cookie2 headers")
+        test(function () {
+          const headers = new Headers([["set-cookie", "z=z"], ["set-cookie", "a=a"], ["set-cookie", "n=n"]])
+          assert_nested_array_equals([...headers], [["set-cookie", "z=z"], ["set-cookie", "a=a"], ["set-cookie", "n=n"]])
+        }, "Headers iterator preserves set-cookie ordering")
+        test(function () {
+          const headers = new Headers(headerList)
+          assert_true(headers.has("sEt-cOoKiE"))
+        }, "Headers.prototype.has works for set-cookie")
+        test(function () {
+          const headers = new Headers(headerList)
+          headers.set("set-cookie", "foo2=bar2")
+          assert_nested_array_equals([...headers], [["set-cookie", "foo2=bar2"]])
+        }, "Headers.prototype.set works for set-cookie")
+        test(function () {
+          const headers = new Headers(headerList)
+          headers.delete("set-Cookie")
+          assert_nested_array_equals([...headers], [])
+        }, "Headers.prototype.delete works for set-cookie")
+        test(function () { assert_array_equals(new Headers().getSetCookie(), []) }, "Headers.prototype.getSetCookie with no headers present")
+        test(function () { assert_array_equals(new Headers([headerList[0]]).getSetCookie(), ["foo=bar"]) }, "Headers.prototype.getSetCookie with one header")
+        test(function () { assert_array_equals(new Headers({ "Set-Cookie": "foo=bar" }).getSetCookie(), ["foo=bar"]) }, "Headers.prototype.getSetCookie with one header created from an object")
+        test(function () { assert_array_equals(new Headers(headerList).getSetCookie(), ["foo=bar", "fizz=buzz; domain=example.com"]) }, "Headers.prototype.getSetCookie with multiple headers")
+        test(function () { assert_array_equals(new Headers([["set-cookie", ""]]).getSetCookie(), [""]) }, "Headers.prototype.getSetCookie with an empty header")
+        test(function () { assert_array_equals(new Headers([["set-cookie", "x"], ["set-cookie", "x"]]).getSetCookie(), ["x", "x"]) }, "Headers.prototype.getSetCookie with two equal headers")
+        test(function () { assert_array_equals(new Headers([["set-cookie2", "x"], ["set-cookie", "y"], ["set-cookie2", "z"]]).getSetCookie(), ["y"]) }, "Headers.prototype.getSetCookie ignores set-cookie2 headers")
+        test(function () { assert_array_equals(new Headers([["set-cookie", "z=z"], ["set-cookie", "a=a"], ["set-cookie", "n=n"]]).getSetCookie(), ["z=z", "a=a", "n=n"]) }, "Headers.prototype.getSetCookie preserves header ordering")
+        return failures
+      `),
+    ).toEqual([])
+  })
+})
@@ -1,6 +1,6 @@
 {
  "name": "@opencode/console-app",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "type": "module",
  "license": "MIT",
  "scripts": {
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/console-core",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "private": true,
  "type": "module",
  "license": "MIT",
@@ -1,6 +1,6 @@
 {
  "name": "@opencode/console-function",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "$schema": "https://json.schemastore.org/package.json",
  "private": true,
  "type": "module",
@@ -1,6 +1,6 @@
 {
  "name": "@opencode/console-mail",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "dependencies": {
    "@jsx-email/all": "2.2.3",
    "@jsx-email/cli": "1.4.3",
@@ -1,6 +1,6 @@
 {
  "name": "@opencode/console-support",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "type": "module",
  "license": "MIT",
  "scripts": {
@@ -1,6 +1,6 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "name": "@opencode/core",
  "type": "module",
  "license": "MIT",
@@ -13,6 +13,7 @@ import type {
 import { Effect, Ref, Schema, Semaphore } from "effect"
 import { definition, normalizedName } from "../tool/runtime.js"
 import { CodeModeCatalog } from "./catalog.js"
+import { CodeModeWeb } from "./web.js"

 const ExecuteFile = Schema.Struct({
  data: Schema.String,
@@ -59,8 +60,8 @@ export type Inventory = {

 // Invariant model-facing guidance; the changing tool catalog is delivered through Instructions.
 const description = [
-  "Run JavaScript in a confined Code Mode runtime to orchestrate tool calls and compose their results.",
-  "Imports, direct filesystem access, and timers are unavailable. Do not use `fetch`; all external access goes through `tools`.",
+  "Run JavaScript in a confined Code Mode runtime to script tool calls and HTTP requests and compose their results.",
+  "`fetch` is available for HTTP requests. Imports, direct filesystem access, and timers are unavailable; all other external access goes through `tools`.",
  "Within `{ code }`, the only callable tools are those explicitly listed in the Code Mode catalog instructions or returned by the `search` function. Inside `{ code }`, ignore tools shown outside the Code Mode catalog. They are not available in the Code Mode runtime.",
  'Call tools through `tools` using only exact paths and signatures from the catalog. Do not infer or normalize tool names; preserve bracket notation such as `tools.<namespace>["tool-name"](input)`.',
  "Prefer an explicit `return`; if omitted, the final top-level expression becomes the result.",
@@ -82,9 +83,9 @@ export const create = (
        const files = yield* Ref.make<Array<CollectedFiles>>([])
        const calls = yield* Ref.make<Array<ExecuteCall>>([])
        const lock = Semaphore.makeUnsafe(1)
-        const updateCalls = (update: (items: Array<ExecuteCall>) => Array<ExecuteCall>) =>
+        const record = (update: (items: Array<ExecuteCall>) => Array<ExecuteCall>) =>
          lock.withPermit(
-            Ref.updateAndGet(calls, update).pipe(Effect.flatMap((toolCalls) => context.progress({ toolCalls }))),
+            Ref.updateAndGet(calls, update).pipe(Effect.tap((toolCalls) => context.progress({ toolCalls }))),
          )
        const result = yield* runtime(
          inventory,
@@ -103,27 +104,7 @@ export const create = (
              const text = content.flatMap((part) => (part.type === "text" ? [part.text] : [])).join("\n")
              return text === "" ? null : text
            }),
-          {
-            onToolCallStart: ({ index, name, input }) => {
-              const shown = displayInput(input)
-              return updateCalls((items) => {
-                const next = [...items]
-                next[index] = { tool: name, status: "running", ...(shown ? { input: shown } : {}) }
-                return next
-              })
-            },
-            onToolCallEnd: ({ index, name, input, outcome }) => {
-              const shown = displayInput(input)
-              return updateCalls((items) => {
-                const next = [...items]
-                next[index] = {
-                  ...(items[index] ?? { tool: name, ...(shown ? { input: shown } : {}) }),
-                  status: outcome === "success" ? "completed" : "error",
-                }
-                return next
-              })
-            },
-          },
+          progressHooks(record),
        ).execute(code)
        const toolCalls = yield* Ref.get(calls)
        const collected = (yield* Ref.get(files))
@@ -158,6 +139,38 @@ export const create = (
  } satisfies Info
 }

+// Rows appear in start order; the same call object arrives at both hooks, so a call finds its row again.
+function progressHooks(record: (update: (items: Array<ExecuteCall>) => Array<ExecuteCall>) => Effect.Effect<unknown>) {
+  const rows = new WeakMap<object, number>()
+  const start = (call: object, entry: ExecuteCall) =>
+    record((items) => {
+      rows.set(call, items.length)
+      return [...items, entry]
+    })
+  const settle = (call: object, result: CodeMode.CallResult) =>
+    record((items) => {
+      const index = rows.get(call)
+      if (index === undefined) return items
+      const next = [...items]
+      next[index] = { ...items[index], status: result.status === "success" ? "completed" : "error" }
+      return next
+    })
+  return {
+    "tool.before": (call) => {
+      const shown = displayInput(call.input)
+      return start(call, { tool: call.name, status: "running", ...(shown ? { input: shown } : {}) })
+    },
+    "tool.after": settle,
+    "extension.before": (call) =>
+      start(call, {
+        tool: call.name,
+        status: "running",
+        ...(call.name === "fetch" ? { input: CodeModeWeb.display(call.args) } : {}),
+      }),
+    "extension.after": settle,
+  } satisfies CodeMode.Hooks
+}
+
 export const catalog = (inventory: Inventory) => {
  const pinned = new Set(
    Array.from(inventory.tools.values())
@@ -204,7 +217,7 @@ function renderCatalog(root: CatalogNode): ReadonlyArray<CodeModeCatalog.Tool |
 function runtime(
  inventory: Inventory,
  executeTool: (name: string, tool: Info, input: unknown) => Effect.Effect<unknown, unknown>,
-  hooks?: CodeMode.ToolCallHooks,
+  hooks?: CodeMode.Hooks,
 ) {
  // A path may carry namespace metadata, a callable tool, child tools, or all three.
  const root: ToolNode = { children: new Map() }
@@ -219,7 +232,7 @@ function runtime(
    })
  }
  const tools = renderTools(root)
-  return CodeMode.make<typeof tools>({ tools, ...hooks })
+  return CodeMode.make<typeof tools>({ tools, extensions: [CodeModeWeb.extension], hooks })
 }

 function getNode<T>(root: Node<T>, path: string) {
@@ -0,0 +1,46 @@
+export * as CodeModeWeb from "./web.js"
+
+import { Extension } from "@opencode/codemode"
+
+const TIMEOUT_MS = 30_000
+
+type Init = {
+  readonly method?: string
+  readonly headers?: Record<string, string> | Array<[string, string]>
+  readonly body?: string | Uint8Array<ArrayBuffer> | URLSearchParams
+}
+
+const fetch = async (input: string | URL, init: Init = {}) => {
+  const response = await globalThis.fetch(input, {
+    method: init.method,
+    headers: init.headers,
+    body: init.body,
+    signal: AbortSignal.timeout(TIMEOUT_MS),
+  })
+  const bytes = await response.bytes()
+  const headers = Object.fromEntries(response.headers)
+  const text = () => new TextDecoder().decode(bytes)
+  return {
+    url: response.url,
+    status: response.status,
+    statusText: response.statusText,
+    ok: response.ok,
+    redirected: response.redirected,
+    headers: {
+      get: (name: string) => headers[name.toLowerCase()] ?? null,
+      has: (name: string) => name.toLowerCase() in headers,
+      entries: () => Object.entries(headers),
+    },
+    text: async () => text(),
+    json: async () => JSON.parse(text()) as unknown,
+    bytes: async () => bytes,
+  }
+}
+
+export const extension = Extension.make({ name: "web", globals: { fetch } })
+
+/** What to show for a fetch call: its method and URL. */
+export const display = (args: ReadonlyArray<unknown>) => {
+  const [input, init] = args as [string | URL, Init | undefined]
+  return { method: init?.method?.toUpperCase() ?? "GET", url: String(input) }
+}
@@ -168,7 +168,12 @@ export const provider = (options: Options): OAuthClientProvider => {
      if (!resource) return identity
      if (!checkResourceAllowed({ requestedResource: identity, configuredResource: resource }))
        throw new Error(`Protected resource ${resource} does not cover ${identity}`)
-      return new URL(resource)
+      const canonical = new URL(resource)
+      // The transport dials the configured URL with extra query parameters and some servers echo
+      // that back as the resource. Query is transport detail, not identity: the token stays bound
+      // to the configured URL so a later refresh names the same resource the login did.
+      if (canonical.origin === identity.origin && canonical.pathname === identity.pathname) return identity
+      return canonical
    },
    ...(options.clientMetadataUrl ? { clientMetadataUrl: options.clientMetadataUrl } : {}),
    ...(redirect ? { state: () => redirect.state } : {}),
@@ -154,6 +154,7 @@ const layer = Layer.effect(
      }
    })

+    let catalog: { data: Data; names: string; value: CodeModeCatalog.Inventory } | undefined
    const state = State.create<Data, Editor>({
      name: "tool",
      initial: () => ({
@@ -201,8 +202,9 @@ const layer = Layer.effect(
          editor.tools.delete(id)
        },
      }),
-      notify: (value) =>
-        Effect.forEach(
+      notify: (value) => {
+        catalog = undefined
+        return Effect.forEach(
          value.errors,
          ({ kind, name, namespace, error }) =>
            Effect.logError(`Skipping invalid ${kind} registration`, {
@@ -211,7 +213,8 @@ const layer = Layer.effect(
              error: error.message,
            }),
          { discard: true },
-        ),
+        )
+      },
    })

    return Service.of({
@@ -219,15 +222,16 @@ const layer = Layer.effect(
      reload: state.reload,
      snapshot: Effect.fn("Tool.snapshot")((permissions) =>
        Effect.sync(() => {
+          const data = state.get()
          const active = new Map<string, Tool.Info>()
          const rules = permissions ?? []
-          for (const [name, tool] of state.get().tools) {
+          for (const [name, tool] of data.tools) {
            if (whollyDisabled(tool.options?.permission ?? name, rules)) continue
            active.set(name, tool)
          }
          const direct = new Map(Array.from(active).filter(([, tool]) => tool.options?.codemode === false))
          const codeModeTools = new Map(Array.from(active).filter(([, tool]) => tool.options?.codemode !== false))
-          const namespaces = state.get().namespaces
+          const namespaces = data.namespaces
          const codeModeInventory = { tools: codeModeTools, namespaces }
          const codeModeEnabled = !whollyDisabled("execute", rules)
          const codeModeTool = codeModeEnabled
@@ -237,7 +241,15 @@ const layer = Layer.effect(
                ),
              )
            : undefined
-          const codeModeCatalog = codeModeEnabled ? CodeModeTool.catalog(codeModeInventory) : undefined
+          const names = Array.from(codeModeTools.keys()).join("\0")
+          // Discovery is immutable for a registry revision and visible tool set. Keep request
+          // definitions/executors fresh, but share the much larger rendered catalog across steps.
+          const codeModeCatalog = !codeModeEnabled
+            ? undefined
+            : catalog?.data === data && catalog.names === names
+              ? catalog.value
+              : CodeModeTool.catalog(codeModeInventory)
+          if (codeModeCatalog) catalog = { data, names, value: codeModeCatalog }
          return {
            ...(codeModeCatalog === undefined ? {} : { codeModeCatalog }),
            definitions: [
@@ -804,12 +804,12 @@ it.effect("classifies retryable AI SDK failures with retry-after details", () =>
 it.effect("classifies data-only AI SDK provider codes", () =>
  Effect.gen(function* () {
    const data = {
-      error: { code: "rate_limit_error", metadata: { requestId: "data-request", retryable: true } },
+      error: { code: "api_error", metadata: { requestId: "data-request", retryable: true } },
      trace: { region: "test-region" },
    }
    const cause = apiCallError({ statusCode: 400, data })
    const error = yield* streamFailure(cause)
-    expect(error.reason).toMatchObject({ _tag: "RateLimit" })
+    expect(error.reason).toMatchObject({ _tag: "ProviderInternal" })
    expect(error.reason.http?.status).toBe(400)
    expect(SessionRunnerRetry.isRetryable(error)).toBeTrue()
    expect(error.reason.body).toBe(JSON.stringify(data))
@@ -324,6 +324,44 @@ describe("MCP OAuth", () => {
    expect(probes.some((probe) => probe.includes("codemode"))).toBe(false)
  })

+  test("keeps the configured URL as the resource when metadata echoes the dialed query", async () => {
+    const tokenRequests: URLSearchParams[] = []
+    const server = Bun.serve({
+      port: 0,
+      async fetch(request) {
+        const url = new URL(request.url)
+        if (url.pathname === "/.well-known/oauth-authorization-server")
+          return Response.json({
+            issuer: url.origin,
+            authorization_endpoint: `${url.origin}/authorize`,
+            token_endpoint: `${url.origin}/token`,
+            response_types_supported: ["code"],
+          })
+        if (url.pathname === "/.well-known/oauth-protected-resource/mcp")
+          return Response.json({ resource: `${url.origin}/mcp${url.search}`, authorization_servers: [url.origin] })
+        if (request.method === "POST" && url.pathname === "/token") {
+          tokenRequests.push(new URLSearchParams(await request.text()))
+          return Response.json({ access_token: "next", token_type: "Bearer" })
+        }
+        return new Response(null, { status: 404 })
+      },
+    })
+    const url = `${server.url.origin}/mcp`
+    const oauthProvider = await connectProvider(
+      remote(url),
+      memoryCredentials([credential({ access: "expired", refresh: "refresh", url })]),
+    )
+
+    // The transport dials with ?codemode=false and follows the 401 challenge to metadata that echoes it.
+    await auth(oauthProvider, {
+      serverUrl: `${url}?codemode=false`,
+      resourceMetadataUrl: new URL(`${server.url.origin}/.well-known/oauth-protected-resource/mcp?codemode=false`),
+    }).finally(() => server.stop(true))
+
+    expect(tokenRequests[0]?.get("grant_type")).toBe("refresh_token")
+    expect(tokenRequests[0]?.get("resource")).toBe(url)
+  })
+
  test("finds resource metadata through the 401 header when the well-known path is not served", async () => {
    const server = Bun.serve({
      port: 0,
@@ -22,8 +22,8 @@ const createCodeMode = (tools: ReadonlyMap<string, Info>) =>
 test("execute describes invariant Code Mode behavior", () => {
  expect(createCodeMode(new Map()).description).toBe(
    [
-      "Run JavaScript in a confined Code Mode runtime to orchestrate tool calls and compose their results.",
-      "Imports, direct filesystem access, and timers are unavailable. Do not use `fetch`; all external access goes through `tools`.",
+      "Run JavaScript in a confined Code Mode runtime to script tool calls and HTTP requests and compose their results.",
+      "`fetch` is available for HTTP requests. Imports, direct filesystem access, and timers are unavailable; all other external access goes through `tools`.",
      "Within `{ code }`, the only callable tools are those explicitly listed in the Code Mode catalog instructions or returned by the `search` function. Inside `{ code }`, ignore tools shown outside the Code Mode catalog. They are not available in the Code Mode runtime.",
      'Call tools through `tools` using only exact paths and signatures from the catalog. Do not infer or normalize tool names; preserve bracket notation such as `tools.<namespace>["tool-name"](input)`.',
      "Prefer an explicit `return`; if omitted, the final top-level expression becomes the result.",
@@ -1,7 +1,7 @@
 {
  "name": "@opencode/desktop",
  "private": true,
-  "version": "2.0.3",
+  "version": "2.0.4",
  "type": "module",
  "license": "MIT",
  "homepage": "https://opencode.ai",
@@ -1,6 +1,6 @@
 {
  "name": "@opencode/enterprise",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "private": true,
  "type": "module",
  "license": "MIT",
@@ -1,6 +1,6 @@
 {
  "name": "@opencode/function",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "$schema": "https://json.schemastore.org/package.json",
  "private": true,
  "type": "module",
@@ -1,6 +1,6 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "name": "@opencode/http-recorder",
  "description": "Record and replay Effect HTTP and WebSocket traffic with deterministic cassettes",
  "type": "module",
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/httpapi-codegen",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "private": true,
  "type": "module",
  "exports": {
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/latex",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "private": true,
  "type": "module",
  "exports": {
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/merman",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "private": true,
  "type": "module",
  "exports": {
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/plugin-browser",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "description": "OpenCode's desktop browser plugin",
  "type": "module",
  "license": "MIT",
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/plugin",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "type": "module",
  "license": "MIT",
  "scripts": {
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/protocol",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "type": "module",
  "license": "MIT",
  "repository": {
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/schema",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "type": "module",
  "license": "MIT",
  "repository": {
@@ -1,6 +1,6 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "name": "@opencode/sdk",
  "type": "module",
  "license": "MIT",
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/server",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "type": "module",
  "license": "MIT",
  "repository": {
@@ -1,6 +1,6 @@
 {
  "name": "@opencode/session-ui",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "private": true,
  "type": "module",
  "license": "MIT",
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/simulation",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "type": "module",
  "license": "MIT",
  "repository": {
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/stats-app",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "private": true,
  "type": "module",
  "license": "MIT",
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/stats-core",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "private": true,
  "type": "module",
  "license": "MIT",
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/stats-server",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "private": true,
  "type": "module",
  "license": "MIT",
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/theme",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "type": "module",
  "license": "MIT",
  "repository": {
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/tui",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "private": true,
  "type": "module",
  "license": "MIT",
@@ -2965,7 +2965,7 @@ function ShellDisplay(props: {
    return stripAnsi(props.output?.trim() ?? "")
  })
  const maxLines = 10
-  const maxChars = createMemo(() => maxLines * Math.max(20, ctx.width - 6))
+  const maxChars = createMemo(() => maxLines * Math.max(20, ctx.width - 6 - (isRunning() ? 2 : 0)))
  const prefix = createMemo(() => (workdir() && workdir() !== "." ? `cd ${workdir()} && ` : ""))
  const input = createMemo(() => (props.command ? `${isRunning() ? "" : "$ "}${prefix()}${props.command}` : ""))
  const collapsed = createMemo(() => collapseShellOutput(input(), output(), maxLines, maxChars()))
@@ -2991,8 +2991,30 @@ function ShellDisplay(props: {
            )
          }
        >
-          <Show when={isRunning()} fallback={<text fg={theme.text.default}>{limitedInput()}</text>}>
-            <Spinner color={color()}>{limitedInput()}</Spinner>
+          <Show
+            when={isRunning()}
+            fallback={
+              <text
+                fg={theme.text.default}
+                wrapMode={expanded() ? "word" : "char"}
+                maxHeight={expanded() ? undefined : 2}
+              >
+                {limitedInput()}
+              </text>
+            }
+          >
+            <box flexDirection="row" gap={1}>
+              <Spinner color={color()} />
+              <text
+                fg={color()}
+                wrapMode={expanded() ? "word" : "char"}
+                maxHeight={expanded() ? undefined : 2}
+                flexGrow={1}
+                minWidth={0}
+              >
+                {limitedInput()}
+              </text>
+            </box>
          </Show>
          <Show when={limitedOutput()}>
            <text fg={theme.text.subdued}>{limitedOutput()}</text>
@@ -1,3 +1,6 @@
+import { Locale } from "./locale"
+import { stringWidth } from "./string-width"
+
 export function collapseToolOutput(output: string, maxLines: number, maxChars: number) {
  const lines = output.split("\n")
  if (lines.length <= maxLines && Array.from(output).length <= maxChars) {
@@ -21,21 +24,60 @@ export function collapseToolOutput(output: string, maxLines: number, maxChars: n
 }

 export function collapseShellOutput(input: string, output: string, maxLines: number, maxChars: number) {
-  const content = [input, output].filter(Boolean).join("\n\n")
-  const collapsed = collapseToolOutput(content, maxLines, maxChars)
-  if (!collapsed.overflow) return { input, output, overflow: false }
-  if (!input) return { input, output: collapseTail(output, maxLines, maxChars), overflow: true }
-  if (!output) return { input: collapsed.output, output, overflow: true }
+  if (!input) {
+    const collapsed = collapseToolOutput(output, maxLines, maxChars)
+    return {
+      input,
+      output: collapsed.overflow ? collapseTail(output, maxLines, maxChars) : output,
+      overflow: collapsed.overflow,
+    }
+  }

-  const reserved = Math.max(1, Math.floor(maxChars / maxLines))
-  const command = collapseToolOutput(
-    input,
-    Math.max(1, maxLines - 2),
-    Math.max(1, maxChars - reserved - 2),
-  ).output
-  const lines = Math.max(1, maxLines - command.split("\n").length - 1)
-  const chars = Math.max(1, maxChars - Array.from(command).length - 2)
-  return { input: command, output: collapseTail(output, lines, chars), overflow: true }
+  const commandLines = Math.min(2, maxLines)
+  const lineChars = Math.max(1, Math.floor(maxChars / Math.max(1, maxLines)))
+  const command = collapseShellCommand(input, commandLines, lineChars)
+  if (!output) return { input: command.output, output, overflow: command.overflow }
+
+  const lines = Math.max(1, maxLines - command.lines - 1)
+  const chars = Math.max(1, maxChars - Array.from(command.output).length - 2)
+  const collapsed = collapseToolOutput(output, lines, chars)
+  return {
+    input: command.output,
+    output: collapsed.overflow ? collapseTail(output, lines, chars) : output,
+    overflow: command.overflow || collapsed.overflow,
+  }
+}
+
+function collapseShellCommand(input: string, maxLines: number, lineWidth: number) {
+  const visible: string[] = []
+  let lines = 1
+  let width = 0
+  const overflow = Locale.graphemes(input).some((segment) => {
+    if (segment === "\n") {
+      if (lines >= maxLines) return true
+      visible.push(segment)
+      lines++
+      width = 0
+      return false
+    }
+
+    const next = stringWidth(segment)
+    if (width + next > lineWidth) {
+      if (lines >= maxLines) return true
+      lines++
+      width = 0
+    }
+    visible.push(segment)
+    width += next
+    return false
+  })
+  if (!overflow) return { output: input, overflow, lines }
+
+  if (width >= lineWidth) {
+    const removed = visible.pop()
+    if (removed !== undefined && removed !== "\n") width -= stringWidth(removed)
+  }
+  return { output: visible.join("") + "…", overflow, lines }
 }

 function collapseTail(output: string, maxLines: number, maxChars: number) {
@@ -1,6 +1,6 @@
 {
  "name": "@opencode/ui",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "type": "module",
  "license": "MIT",
  "repository": {
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/util",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "type": "module",
  "license": "MIT",
  "repository": {
@@ -2,7 +2,7 @@
  "name": "@opencode/web",
  "type": "module",
  "license": "MIT",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "scripts": {
    "dev": "astro dev",
    "dev:remote": "VITE_API_URL=https://api.opencode.ai astro dev",
@@ -2,7 +2,7 @@
  "name": "opencode",
  "displayName": "opencode",
  "description": "opencode for VS Code",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "publisher": "sst-dev",
  "repository": {
    "type": "git",
@@ -1,7 +1,7 @@
 {
  "$schema": "https://json.schemastore.org/package.json",
  "name": "@opencode/update",
-  "version": "2.0.3",
+  "version": "2.0.4",
  "private": true,
  "type": "module",
  "scripts": {
Author	SHA1	Message	Date
Aiden Cline	9569ddc6a0	fix(codemode): let Headers methods report a bad receiver	2026-09-16 00:39:27 -05:00
Aiden Cline	32ae1062f5	docs(codemode): replace the stale Values section with Extension.make	2026-09-16 00:14:04 -05:00
Aiden Cline	1594bdab43	feat(codemode): add the Headers built-in	2026-09-16 00:08:50 -05:00
Dax	5379a2dfe0	fix(codemode): reuse tool discovery catalogs from OpenCode	2026-09-16 00:52:00 -04:00
Aiden Cline	6ee156e0b9	fix(core): keep the configured MCP URL as the OAuth resource (#49241 )	2026-09-15 23:50:36 -05:00
opencode	7c3c50bac2	sync release versions for v2.0.4	2026-09-16 04:41:12 +00:00
Aiden Cline	194a006932	feat(core): expose fetch to code mode scripts (#49235 )	2026-09-15 23:29:22 -05:00
Dax Raad	3c1f87d2d4	fix(tui): limit shell command previews	2026-09-16 00:21:05 -04:00
Aiden Cline	c57733a647	refactor(codemode): before/after hooks for tool and extension calls (#49242 )	2026-09-15 23:05:06 -05:00