refactor(acp): split the promise turn from the session lifecycle

packages/cli/src/acp/agent.ts

@@ -8,22 +8,34 @@ import {
   type AgentRequestMethod,
   type Stream,
 } from "@agentclientprotocol/sdk"
-import { ClientError, type OpenCodeClient } from "@opencode/client/promise"
-import { Cause, Effect, type Scope } from "effect"
+import type { OpenCodeClient } from "@opencode/client/promise"
+import { Cause, Deferred, Effect, Ref, type Scope } from "effect"
 import { ACPCatalog } from "./catalog"
 import { ACPConnection } from "./connection"
 import { ACPError } from "./error"
+import { ACPPromise } from "./promise"
 import { ACPService } from "./service"
+import { ACPSessions } from "./sessions"
+import { ACPTurn } from "./turn"
 
 // Untraced so request spans parent to the caller's span instead of a setup span that has already ended.
 export const connect = Effect.fnUntraced(function* (client: OpenCodeClient, stream: Stream) {
   const run = Effect.runPromiseWith(yield* Effect.context<Scope.Scope>())
   const catalog = yield* ACPCatalog.make(client)
+  // Requests can dispatch once the stream's read loop yields, which may be before the service below is built.
+  const ready = yield* Deferred.make<ACPService.Interface>()
   const handle =
-    <Params, A>(call: (ctx: AgentHandlerContext<Params>) => Effect.Effect<A, ACPError.Error | RequestError>) =>
+    <Params, A>(
+      call: (service: ACPService.Interface, ctx: AgentHandlerContext<Params>) => Effect.Effect<A, ACPService.Failure>,
+    ) =>
     (name: string) => {
       const handler = Effect.fn(name)(
-        call,
+        (ctx: AgentHandlerContext<Params>) =>
+          Deferred.await(ready).pipe(Effect.flatMap((service) => call(service, ctx))),
+        Effect.catchTags({
+          ACPCatalogLoadError: (error) => ACPPromise.classify(error.cause),
+          ACPCatalogNotReadyError: (error) => Effect.die(error),
+        }),
         Effect.mapError((error) => (error instanceof RequestError ? error : ACPError.toRequestError(error))),
         Effect.tapCauseIf(Cause.hasDies, (cause) => Effect.logError("ACP request failed", cause)),
         Effect.catchDefect((defect) => Effect.fail(ACPError.toRequestError(ACPError.fromUnknown(defect)))),
@@ -42,78 +54,70 @@ export const connect = Effect.fnUntraced(function* (client: OpenCodeClient, stre
 
   request(
     "initialize",
-    handle((ctx) => promise(() => service.initialize(ctx.params))),
+    handle((service, ctx) => service.initialize(ctx.params)),
   )
   request(
     "authenticate",
-    handle((ctx) => promise(() => service.authenticate(ctx.params))),
+    handle((service, ctx) => service.authenticate(ctx.params)),
   )
   request(
     "session/new",
-    handle((ctx) => promise(() => service.newSession(ctx.params))),
+    handle((service, ctx) => service.newSession(ctx.params)),
   )
   request(
     "session/load",
-    handle((ctx) => promise(() => service.loadSession(ctx.params))),
+    handle((service, ctx) => service.loadSession(ctx.params)),
   )
   request(
     "session/list",
-    handle((ctx) => promise(() => service.listSessions(ctx.params))),
+    handle((service, ctx) => service.listSessions(ctx.params)),
   )
   request(
     "session/delete",
-    handle((ctx) => promise(() => service.deleteSession(ctx.params))),
+    handle((service, ctx) => service.deleteSession(ctx.params)),
   )
   request(
     "session/resume",
-    handle((ctx) => promise(() => service.resumeSession(ctx.params))),
+    handle((service, ctx) => service.resumeSession(ctx.params)),
   )
   request(
     "session/close",
-    handle((ctx) => promise(() => service.closeSession(ctx.params))),
+    handle((service, ctx) => service.closeSession(ctx.params)),
   )
   request(
     "session/fork",
-    handle((ctx) => promise(() => service.forkSession(ctx.params))),
+    handle((service, ctx) => service.forkSession(ctx.params)),
   )
   request(
     "session/set_config_option",
-    handle((ctx) => promise(() => service.setSessionConfigOption(ctx.params))),
+    handle((service, ctx) => service.setSessionConfigOption(ctx.params)),
   )
   request(
     "session/set_mode",
-    handle((ctx) => promise(() => service.setSessionMode(ctx.params))),
+    handle((service, ctx) => service.setSessionMode(ctx.params)),
   )
   // The SDK signal is passed through rather than interrupting the fiber: a cancelled turn still resolves with
   // `stopReason: "cancelled"`.
   request(
     "session/prompt",
-    handle((ctx) => promise(() => service.prompt(ctx.params, ctx.signal))),
+    handle((service, ctx) => ACPPromise.promise(() => service.prompt(ctx.params, ctx.signal))),
   )
   notification(
     "session/cancel",
-    handle((ctx) => promise(() => service.cancel(ctx.params))),
+    handle((service, ctx) => ACPPromise.promise(() => service.cancel(ctx.params))),
   )
   const connection = app.connect(stream)
-  // Inbound dispatch starts after the stream's async read loop yields, so handlers never observe this before assignment.
-  const service = ACPService.make({ client, connection: ACPConnection.make(connection), catalog, run })
+  const promiseConnection = ACPConnection.promise(connection)
+  const sessions = yield* ACPSessions.make({ client, connection: ACPConnection.make(connection), catalog })
+  const capabilities = yield* Ref.make({ childSessionUpdates: false })
+  const turn = ACPTurn.make({ client, connection: promiseConnection, sessions, catalog, capabilities, run })
+  yield* Deferred.succeed(
+    ready,
+    ACPService.make({ client, connection: promiseConnection, catalog, sessions, capabilities, turn }),
+  )
   return connection
 })
 
 const spanName = (method: string) => `cli.acp.${method.replaceAll("/", ".")}`
 
-const promise = <A>(evaluate: () => Promise<A>) =>
-  Effect.tryPromise({
-    try: evaluate,
-    // A catalog load failure is classified by the client error that caused it.
-    catch: (cause) => (cause instanceof ACPCatalog.LoadError ? cause.cause : cause),
-  }).pipe(
-    Effect.catch((cause) => {
-      if (cause instanceof RequestError || ACPError.is(cause)) return Effect.fail(cause)
-      if (cause instanceof ClientError && cause.reason === "Transport")
-        return Effect.fail(new ACPError.ServerUnavailableError())
-      return Effect.die(cause)
-    }),
-  )
-
 export * as ACP from "./agent"

packages/cli/src/acp/catalog.ts

@@ -1,14 +1,20 @@
 import type { CommandInfo, ModelInfo, ModelRef, OpenCodeClient, OpenCodeEvent } from "@opencode/client/promise"
 import { FSUtil } from "@opencode/util/fs-util"
-import { Context, Deferred, Effect, Exit, Schedule, Schema, Scope, Semaphore, Stream, SubscriptionRef } from "effect"
+import { Context, Deferred, Effect, Exit, Schedule, Schema, Semaphore, Stream, SubscriptionRef } from "effect"
 import type { ConfigOptionProvider } from "./config-option"
 
+// ACP runs these itself; they take precedence over server commands with the same name.
+export const builtinCommands = new Map([
+  ["compact", { description: "Compact the session", start: "compaction" as const }],
+])
+
 export type Catalog = {
   readonly providers: ConfigOptionProvider[]
   readonly models: ModelInfo[]
   readonly defaultModel: ModelRef
   readonly modes: Array<{ id: string; name: string; description?: string }>
   readonly defaultModeID: string
+  /** Server commands, without those shadowed by a built-in. */
   readonly commands: CommandInfo[]
 }
 
@@ -133,52 +139,6 @@ export const make = Effect.fnUntraced(function* (client: OpenCodeClient) {
   })
 })
 
-export type Live = {
-  readonly cwd: string
-  readonly current: Catalog
-}
-
-/** Temporary adapter for the promise-based `ACPService` until sessions consume the service directly. */
-export function promise(input: {
-  readonly catalog: Interface
-  readonly run: <A, E>(effect: Effect.Effect<A, E, Scope.Scope>) => Promise<A>
-  readonly changed: (live: Live, previous: Catalog) => Promise<unknown>
-}) {
-  const lives = new Map<string, Live>()
-  return {
-    get: (cwd: string) =>
-      input.run(
-        Effect.gen(function* () {
-          const current = yield* input.catalog.get(cwd)
-          const key = FSUtil.resolve(cwd)
-          const existing = lives.get(key)
-          if (existing) return existing
-          const live: Live = {
-            cwd,
-            // A loaded entry's read never suspends.
-            get current() {
-              return Effect.runSync(input.catalog.get(cwd))
-            },
-          }
-          lives.set(key, live)
-          yield* input.catalog.changes(cwd).pipe(
-            Stream.runFoldEffect(
-              () => current,
-              (previous, next) =>
-                next === previous
-                  ? Effect.succeed(previous)
-                  : Effect.promise(() => input.changed(live, previous).catch(() => {})).pipe(Effect.as(next)),
-            ),
-            Effect.ignore,
-            Effect.forkScoped({ startImmediately: true }),
-          )
-          return live
-        }),
-      ),
-    reload: (live: Live) => input.run(input.catalog.reload(live.cwd)),
-  }
-}
-
 const load = (client: OpenCodeClient, cwd: string) =>
   read(client, cwd).pipe(
     // Some providers discover models in the background after plugin startup begins.
@@ -222,7 +182,7 @@ const read = Effect.fnUntraced(function* (client: OpenCodeClient, cwd: string) {
     },
     modes: agents.map((agent) => ({ id: agent.id, name: agent.name, description: agent.description })),
     defaultModeID: defaultAgent.id,
-    commands: commandResult.data,
+    commands: commandResult.data.filter((command) => !builtinCommands.has(command.name)),
   } satisfies Catalog
 })
 

packages/cli/src/acp/config-option.ts

@@ -1,4 +1,6 @@
 import type { SessionConfigOption } from "@agentclientprotocol/sdk"
+import type { ModelRef } from "@opencode/client/promise"
+import { builtinCommands, type Catalog } from "./catalog"
 
 export const DEFAULT_VARIANT_VALUE = "default"
 
@@ -25,6 +27,34 @@ export type ModelSelection = {
   variant?: string
 }
 
+/** A session's model and mode. Unset fields follow the catalog defaults. */
+export type Selection = {
+  readonly model?: ModelRef
+  readonly modeID?: string
+}
+
+export function currentModel(catalog: Catalog, selection: Selection) {
+  return selection.model ?? catalog.defaultModel
+}
+
+export function configOptions(catalog: Catalog, selection: Selection) {
+  const model = currentModel(catalog, selection)
+  return buildConfigOptions({
+    providers: catalog.providers,
+    currentModel: { providerID: model.providerID, modelID: model.id },
+    currentVariant: model.variant,
+    modes: catalog.modes,
+    currentModeId: selection.modeID ?? catalog.defaultModeID,
+  })
+}
+
+export function availableCommands(catalog: Catalog) {
+  return [
+    ...catalog.commands.map((command) => ({ name: command.name, description: command.description ?? "" })),
+    ...Array.from(builtinCommands, ([name, command]) => ({ name, description: command.description })),
+  ]
+}
+
 export function buildConfigOptions(input: {
   providers: readonly ConfigOptionProvider[]
   currentModel: ModelSelection["model"]

packages/cli/src/acp/connection.ts

@@ -1,11 +1,28 @@
 import {
   methods,
   type AgentConnection,
+  type RequestError,
   type RequestPermissionRequest,
   type RequestPermissionResponse,
   type SendRequestOptions,
   type SessionNotification,
 } from "@agentclientprotocol/sdk"
+import { Context, type Effect } from "effect"
+import type { ACPError } from "./error"
+import { ACPPromise } from "./promise"
+
+export interface Interface {
+  readonly sessionUpdate: (params: SessionNotification) => Effect.Effect<void, ACPError.Error | RequestError>
+}
+
+export class Service extends Context.Service<Service, Interface>()("@opencode/cli/acp/Connection") {}
+
+export function make(connection: AgentConnection) {
+  return Service.of({
+    sessionUpdate: (params) =>
+      ACPPromise.promise(() => connection.client.notify(methods.client.session.update, params)),
+  })
+}
 
 export type Connection = {
   readonly signal?: AbortSignal
@@ -14,7 +31,8 @@ export type Connection = {
   extNotification?(method: string, params: Record<string, unknown>): Promise<void>
 }
 
-export function make(connection: AgentConnection): Connection {
+/** Promise view for the turn, permission, and replay code until they run as effects. */
+export function promise(connection: AgentConnection): Connection {
   return {
     signal: connection.signal,
     sessionUpdate: (params) => connection.client.notify(methods.client.session.update, params),

packages/cli/src/acp/promise.ts

@@ -0,0 +1,17 @@
+import { RequestError } from "@agentclientprotocol/sdk"
+import { ClientError } from "@opencode/client/promise"
+import { Effect } from "effect"
+import { ACPError } from "./error"
+
+/** Runs a promise, keeping ACP failures typed. Any other rejection is a defect. */
+export const promise = <A>(evaluate: (signal: AbortSignal) => Promise<A>) =>
+  Effect.tryPromise({ try: evaluate, catch: (cause) => cause }).pipe(Effect.catch(classify))
+
+export function classify(cause: unknown): Effect.Effect<never, ACPError.Error | RequestError> {
+  if (cause instanceof RequestError || ACPError.is(cause)) return Effect.fail(cause)
+  if (cause instanceof ClientError && cause.reason === "Transport")
+    return Effect.fail(new ACPError.ServerUnavailableError())
+  return Effect.die(cause)
+}
+
+export * as ACPPromise from "./promise"

packages/cli/src/acp/service.ts

@@ -1,14 +1,6 @@
-import { isDeepStrictEqual } from "node:util"
-import {
-  isSessionNotFoundError,
-  type CommandInfo,
-  type ModelRef,
-  type OpenCodeClient,
-  type SessionInfo,
-  type SessionMessageInfo,
-} from "@opencode/client/promise"
+import { isSessionNotFoundError, type ModelRef, type OpenCodeClient } from "@opencode/client/promise"
 import { FSUtil } from "@opencode/util/fs-util"
-import type { Effect, Scope } from "effect"
+import { Effect, Option, Ref, Stream } from "effect"
 import { withTimestampedFallback } from "@opencode/util/session-title-fallback"
 import type {
   AuthenticateRequest,
@@ -27,11 +19,11 @@ import type {
   ListSessionsResponse,
   LoadSessionRequest,
   LoadSessionResponse,
-  McpServer,
   NewSessionRequest,
   NewSessionResponse,
   PromptRequest,
   PromptResponse,
+  RequestError,
   ResumeSessionRequest,
   ResumeSessionResponse,
   SetSessionConfigOptionRequest,
@@ -40,177 +32,116 @@ import type {
   SetSessionModeResponse,
 } from "@agentclientprotocol/sdk"
 import { OPENCODE_VERSION } from "../version"
-import { SessionMessage } from "@opencode/schema/session-message"
-import { ACPCatalog, type Catalog } from "./catalog"
-import { buildConfigOptions, DEFAULT_VARIANT_VALUE, parseModelSelection } from "./config-option"
+import type { ACPCatalog, Catalog } from "./catalog"
+import { configOptions, currentModel, DEFAULT_VARIANT_VALUE, parseModelSelection } from "./config-option"
 import type { ACPConnection } from "./connection"
-import { promptContentToParts } from "./content"
-import {
-  ChildSessionUpdateMethod,
-  ChildSessionUpdatesCapability,
-  replayMessages,
-  streamTurn,
-  type ChildSessionUpdate,
-  type TurnControl,
-  type TurnStart,
-} from "./event"
+import { ChildSessionUpdatesCapability, replayMessages } from "./event"
 import { ACPError } from "./error"
+import { ACPPromise } from "./promise"
+import type { ACPSessions, Attached } from "./sessions"
+import type { ACPTurn } from "./turn"
 
 export const AuthMethodID = "opencode-login"
 
-// ACP runs these itself; they take precedence over server commands with the same name.
-const builtinCommands = new Map([["compact", { description: "Compact the session", start: "compaction" as const }]])
-
-// Model and mode are unset while the session follows the server defaults.
-type Attached = {
-  readonly id: string
-  readonly cwd: string
-  readonly abort: AbortController
-  readonly catalog: ACPCatalog.Live
-  model?: ModelRef
-  modeID?: string
-}
-
-type PreparedPrompt = {
-  readonly start: TurnStart
-  readonly text: string
-  readonly files: Array<{ readonly uri: string; readonly name?: string }>
-  readonly synthetic: ReadonlyArray<string>
-  readonly slash?: { readonly name: string; readonly args: string }
-  readonly command?: CommandInfo
-}
+export type Failure = ACPError.Error | RequestError | ACPCatalog.Error
 
 export interface Interface {
-  initialize(input: InitializeRequest): Promise<InitializeResponse>
-  authenticate(input: AuthenticateRequest): Promise<AuthenticateResponse>
-  newSession(input: NewSessionRequest): Promise<NewSessionResponse>
-  loadSession(input: LoadSessionRequest): Promise<LoadSessionResponse>
-  listSessions(input: ListSessionsRequest): Promise<ListSessionsResponse>
-  deleteSession(input: DeleteSessionRequest): Promise<DeleteSessionResponse>
-  resumeSession(input: ResumeSessionRequest): Promise<ResumeSessionResponse>
-  closeSession(input: CloseSessionRequest): Promise<CloseSessionResponse>
-  forkSession(input: ForkSessionRequest): Promise<ForkSessionResponse>
-  setSessionConfigOption(input: SetSessionConfigOptionRequest): Promise<SetSessionConfigOptionResponse>
-  setSessionMode(input: SetSessionModeRequest): Promise<SetSessionModeResponse>
+  readonly initialize: (input: InitializeRequest) => Effect.Effect<InitializeResponse>
+  readonly authenticate: (input: AuthenticateRequest) => Effect.Effect<AuthenticateResponse, Failure>
+  readonly newSession: (input: NewSessionRequest) => Effect.Effect<NewSessionResponse, Failure>
+  readonly loadSession: (input: LoadSessionRequest) => Effect.Effect<LoadSessionResponse, Failure>
+  readonly listSessions: (input: ListSessionsRequest) => Effect.Effect<ListSessionsResponse, Failure>
+  readonly deleteSession: (input: DeleteSessionRequest) => Effect.Effect<DeleteSessionResponse, Failure>
+  readonly resumeSession: (input: ResumeSessionRequest) => Effect.Effect<ResumeSessionResponse, Failure>
+  readonly closeSession: (input: CloseSessionRequest) => Effect.Effect<CloseSessionResponse, Failure>
+  readonly forkSession: (input: ForkSessionRequest) => Effect.Effect<ForkSessionResponse, Failure>
+  readonly setSessionConfigOption: (
+    input: SetSessionConfigOptionRequest,
+  ) => Effect.Effect<SetSessionConfigOptionResponse, Failure>
+  readonly setSessionMode: (input: SetSessionModeRequest) => Effect.Effect<SetSessionModeResponse, Failure>
   prompt(input: PromptRequest, signal?: AbortSignal): Promise<PromptResponse>
   cancel(input: CancelNotification): Promise<void>
 }
 
 export function make(input: {
   readonly client: OpenCodeClient
+  /** Replay still writes through the promise view. */
   readonly connection: ACPConnection.Connection
   readonly catalog: ACPCatalog.Interface
-  readonly run: <A, E>(effect: Effect.Effect<A, E, Scope.Scope>) => Promise<A>
+  readonly sessions: ACPSessions.Interface
+  readonly capabilities: Ref.Ref<{ readonly childSessionUpdates: boolean }>
+  readonly turn: ACPTurn.Interface
 }): Interface {
-  const sessions = new Map<string, Attached>()
-  const registeredMcp = new Map<string, Set<string>>()
-  const active = new Map<string, { readonly control: TurnControl; readonly turn: Promise<PromptResponse> }>()
-  const capabilities = { childSessionUpdates: false }
-
-  const catalogs = ACPCatalog.promise({
-    catalog: input.catalog,
-    run: input.run,
-    changed: (live, previous) =>
-      Promise.all(
-        Array.from(sessions.values())
-          .filter((state) => state.catalog === live)
-          .map(async (state) => {
-            const options = configOptions(state)
-            if (!isDeepStrictEqual(options, configOptions(state, previous))) {
-              await input.connection.sessionUpdate({
-                sessionId: state.id,
-                update: { sessionUpdate: "config_option_update", configOptions: options },
-              })
-            }
-            if (!isDeepStrictEqual(live.current.commands, previous.commands)) await sendCommands(state)
-          }),
-      ),
+  const currentOptions = Effect.fnUntraced(function* (attached: Attached) {
+    return configOptions(yield* input.catalog.get(attached.cwd), yield* Ref.get(attached.selection))
   })
 
-  const sendCommands = (state: Attached) =>
-    input.connection.sessionUpdate({
-      sessionId: state.id,
-      update: {
-        sessionUpdate: "available_commands_update",
-        availableCommands: [
-          ...state.catalog.current.commands
-            .filter((command) => !builtinCommands.has(command.name))
-            .map((command) => ({ name: command.name, description: command.description ?? "" })),
-          ...Array.from(builtinCommands, ([name, command]) => ({ name, description: command.description })),
-        ],
-      },
-    })
+  // A selection the catalog has not seen may be new on the server, so reload once before rejecting it.
+  const withReload = <A>(attached: Attached, select: Effect.Effect<A, Failure>) => {
+    const retry = () => input.catalog.reload(attached.cwd).pipe(Effect.andThen(select))
+    return select.pipe(
+      Effect.catchTags({ ACPInvalidModelError: retry, ACPInvalidModeError: retry, ACPInvalidEffortError: retry }),
+    )
+  }
 
-  const withReload = <A>(state: Attached, select: () => Promise<A>) =>
-    select().catch(async (error: unknown) => {
-      if (
-        !(
-          error instanceof ACPError.InvalidModelError ||
-          error instanceof ACPError.InvalidModeError ||
-          error instanceof ACPError.InvalidEffortError
-        )
-      )
+  const selectOption = Effect.fnUntraced(function* (attached: Attached, configId: string, value: string) {
+    const catalog = yield* input.catalog.get(attached.cwd)
+    const current = currentModel(catalog, yield* Ref.get(attached.selection))
+    switch (configId) {
+      case "model":
+        return yield* selectModel(attached, yield* requireModel(catalog, value, current))
+      case "effort":
+        return yield* selectModel(attached, yield* requireEffort(catalog, value, current))
+      case "mode":
+        return yield* selectMode(attached, value)
+      default:
+        return yield* new ACPError.InvalidConfigOptionError({ configId })
+    }
+  })
+
+  const selectModel = Effect.fnUntraced(function* (attached: Attached, model: ModelRef) {
+    yield* Ref.update(attached.selection, (selection) => ({ ...selection, model }))
+    yield* ACPPromise.promise(() => input.client.session.switchModel({ sessionID: attached.id, model }))
+  })
+
+  const selectMode = Effect.fnUntraced(function* (attached: Attached, modeID: string) {
+    const catalog = yield* input.catalog.get(attached.cwd)
+    if (!catalog.modes.some((mode) => mode.id === modeID)) return yield* new ACPError.InvalidModeError({ mode: modeID })
+    yield* Ref.update(attached.selection, (selection) => ({ ...selection, modeID }))
+    yield* ACPPromise.promise(() => input.client.session.switchAgent({ sessionID: attached.id, agent: modeID }))
+  })
+
+  const getSession = Effect.fnUntraced(function* (sessionID: string, cwd: string) {
+    const session = yield* ACPPromise.promise(() =>
+      input.client.session.get({ sessionID }).catch((error) => {
+        if (isSessionNotFoundError(error)) throw new ACPError.SessionNotFoundError({ sessionId: sessionID })
         throw error
-      await catalogs.reload(state.catalog)
-      return select()
-    })
+      }),
+    )
+    if (FSUtil.resolve(cwd) !== FSUtil.resolve(session.location.directory))
+      return yield* new ACPError.SessionDirectoryMismatchError({ sessionId: sessionID, cwd })
+    return session
+  })
 
-  const requireSession = async (sessionID: string) => {
-    const current = sessions.get(sessionID)
-    if (current) return current
-    throw new ACPError.SessionNotFoundError({ sessionId: sessionID })
-  }
-
-  const detach = (sessionID: string) => {
-    sessions.get(sessionID)?.abort.abort()
-    sessions.delete(sessionID)
-    registeredMcp.delete(sessionID)
-  }
-
-  const cancelTurn = (sessionID: string) => {
-    const turn = active.get(sessionID)
-    if (turn) {
-      turn.control.cancelled = true
-      turn.control.admission.abort()
-    }
-    return input.client.session.interrupt({ sessionID })
-  }
-
-  const attach = async (session: SessionInfo, cwd: string, mcpServers: readonly McpServer[]) => {
-    const catalog = await catalogs.get(cwd)
-    sessions.get(session.id)?.abort.abort()
-    const state: Attached = {
-      id: session.id,
-      cwd,
-      abort: new AbortController(),
-      catalog,
-      model: session.model,
-      modeID: session.agent,
-    }
-    sessions.set(session.id, state)
-    await registerMcpServers(input.client, registeredMcp, state, mcpServers)
-    await sendCommands(state)
-    return state
-  }
-
-  const replay = async (state: Attached) => {
-    await replayMessages(input.connection, state.id, state.cwd, await messages(input.client, state.id))
-  }
-
-  const configOptions = (state: Attached, catalog = state.catalog.current) => {
-    const model = currentModel(state, catalog)
-    return buildConfigOptions({
-      providers: catalog.providers,
-      currentModel: { providerID: model.providerID, modelID: model.id },
-      currentVariant: model.variant,
-      modes: catalog.modes,
-      currentModeId: state.modeID ?? catalog.defaultModeID,
-    })
-  }
+  const replay = (attached: Attached) =>
+    Stream.paginate(undefined, (cursor: string | undefined) =>
+      ACPPromise.promise(() =>
+        cursor
+          ? input.client.message.list({ sessionID: attached.id, limit: 200, cursor })
+          : input.client.message.list({ sessionID: attached.id, limit: 200, order: "asc" }),
+      ).pipe(Effect.map((page) => [page.data, Option.fromNullishOr(page.cursor.next)] as const)),
+    ).pipe(
+      Stream.runCollect,
+      Effect.flatMap((messages) =>
+        ACPPromise.promise(() => replayMessages(input.connection, attached.id, attached.cwd, messages)),
+      ),
+    )
 
   return {
-    initialize: async (params) => {
-      capabilities.childSessionUpdates = params.clientCapabilities?._meta?.[ChildSessionUpdatesCapability] === true
+    initialize: Effect.fnUntraced(function* (params) {
+      yield* Ref.set(input.capabilities, {
+        childSessionUpdates: params.clientCapabilities?._meta?.[ChildSessionUpdatesCapability] === true,
+      })
       const authMethod: AuthMethod = {
         description: "Run `opencode auth login` in the terminal",
         name: "Login with opencode",
@@ -233,32 +164,37 @@ export function make(input: {
         authMethods: [authMethod],
         agentInfo: { name: "OpenCode", version: OPENCODE_VERSION },
       }
-    },
-    authenticate: async (params) => {
-      if (params.methodId !== AuthMethodID) throw new ACPError.UnknownAuthMethodError({ methodId: params.methodId })
+    }),
+    authenticate: Effect.fnUntraced(function* (params) {
+      if (params.methodId !== AuthMethodID)
+        return yield* new ACPError.UnknownAuthMethodError({ methodId: params.methodId })
       return {}
-    },
-    newSession: async (params) => {
+    }),
+    newSession: Effect.fnUntraced(function* (params) {
       // Load before creating so a catalog failure leaves no session behind. Agent and model stay unset
       // so the server resolves its defaults after plugins activate.
-      await catalogs.get(params.cwd)
-      const created = await input.client.session.create({ location: { directory: params.cwd } })
-      const state = await attach(created, params.cwd, params.mcpServers)
-      return { sessionId: state.id, configOptions: configOptions(state) }
-    },
-    loadSession: async (params) => {
-      const session = await getSession(input.client, params.sessionId, params.cwd)
-      const state = await attach(session, session.location.directory, params.mcpServers)
-      await replay(state)
-      return { configOptions: configOptions(state) }
-    },
-    listSessions: async (params) => {
-      const page = await input.client.session.list({
-        ...(params.cwd ? { directory: params.cwd } : {}),
-        order: "desc",
-        limit: 100,
-        ...(params.cursor ? { cursor: params.cursor } : {}),
-      })
+      yield* input.catalog.get(params.cwd)
+      const created = yield* ACPPromise.promise(() =>
+        input.client.session.create({ location: { directory: params.cwd } }),
+      )
+      const attached = yield* input.sessions.attach(created, params.cwd, params.mcpServers)
+      return { sessionId: attached.id, configOptions: yield* currentOptions(attached) }
+    }),
+    loadSession: Effect.fnUntraced(function* (params) {
+      const session = yield* getSession(params.sessionId, params.cwd)
+      const attached = yield* input.sessions.attach(session, session.location.directory, params.mcpServers)
+      yield* replay(attached)
+      return { configOptions: yield* currentOptions(attached) }
+    }),
+    listSessions: Effect.fnUntraced(function* (params) {
+      const page = yield* ACPPromise.promise(() =>
+        input.client.session.list({
+          ...(params.cwd ? { directory: params.cwd } : {}),
+          order: "desc",
+          limit: 100,
+          ...(params.cursor ? { cursor: params.cursor } : {}),
+        }),
+      )
       return {
         sessions: page.data.map((session) => ({
           sessionId: session.id,
@@ -268,178 +204,57 @@ export function make(input: {
         })),
         ...(page.cursor.next ? { nextCursor: page.cursor.next } : {}),
       }
-    },
-    deleteSession: async (params) => {
-      await input.client.session.remove({ sessionID: params.sessionId }).catch((error) => {
-        if (!isSessionNotFoundError(error)) throw error
-      })
-      detach(params.sessionId)
+    }),
+    deleteSession: Effect.fnUntraced(function* (params) {
+      yield* ACPPromise.promise(() =>
+        input.client.session.remove({ sessionID: params.sessionId }).catch((error) => {
+          if (!isSessionNotFoundError(error)) throw error
+        }),
+      )
+      yield* input.sessions.detach(params.sessionId)
       return {}
-    },
-    resumeSession: async (params) => {
-      const session = await getSession(input.client, params.sessionId, params.cwd)
-      const state = await attach(session, session.location.directory, params.mcpServers ?? [])
-      return { configOptions: configOptions(state) }
-    },
-    closeSession: async (params) => {
-      const turn = active.get(params.sessionId)
-      await cancelTurn(params.sessionId).catch((error) => {
-        if (!isSessionNotFoundError(error)) throw error
-      })
-      await turn?.turn.catch(() => {})
-      detach(params.sessionId)
+    }),
+    resumeSession: Effect.fnUntraced(function* (params) {
+      const session = yield* getSession(params.sessionId, params.cwd)
+      const attached = yield* input.sessions.attach(session, session.location.directory, params.mcpServers ?? [])
+      return { configOptions: yield* currentOptions(attached) }
+    }),
+    closeSession: Effect.fnUntraced(function* (params) {
+      yield* input.turn.close(params.sessionId)
+      yield* input.sessions.detach(params.sessionId)
       return {}
-    },
-    forkSession: async (params) => {
-      const forked = await input.client.session.fork({
-        sessionID: params.sessionId,
-      })
-      const state = await attach(forked, forked.location.directory, params.mcpServers ?? [])
-      await replay(state)
-      return { sessionId: state.id, configOptions: configOptions(state) }
-    },
-    setSessionConfigOption: async (params) => {
-      const state = await requireSession(params.sessionId)
+    }),
+    forkSession: Effect.fnUntraced(function* (params) {
+      const forked = yield* ACPPromise.promise(() => input.client.session.fork({ sessionID: params.sessionId }))
+      const attached = yield* input.sessions.attach(forked, forked.location.directory, params.mcpServers ?? [])
+      yield* replay(attached)
+      return { sessionId: attached.id, configOptions: yield* currentOptions(attached) }
+    }),
+    setSessionConfigOption: Effect.fnUntraced(function* (params) {
+      const attached = yield* input.sessions.require(params.sessionId)
       const value = params.value
-      if (typeof value !== "string") throw new ACPError.InvalidConfigOptionError({ configId: params.configId })
-      await withReload(state, async () => {
-        switch (params.configId) {
-          case "model": {
-            const selected = requireModel(state.catalog.current, value, currentModel(state))
-            state.model = selected
-            await input.client.session.switchModel({ sessionID: state.id, model: selected })
-            return
-          }
-          case "effort": {
-            const current = currentModel(state)
-            const model = state.catalog.current.models.find(
-              (item) => item.providerID === current.providerID && item.id === current.id,
-            )
-            if (!model || (value !== DEFAULT_VARIANT_VALUE && !model.variants.some((variant) => variant.id === value)))
-              throw new ACPError.InvalidEffortError({ effort: value })
-            state.model = { ...current, variant: value }
-            await input.client.session.switchModel({ sessionID: state.id, model: state.model })
-            return
-          }
-          case "mode":
-            return selectMode(input.client, state, value)
-          default:
-            throw new ACPError.InvalidConfigOptionError({ configId: params.configId })
-        }
-      })
-      return { configOptions: configOptions(state) }
-    },
-    setSessionMode: async (params) => {
-      const state = await requireSession(params.sessionId)
-      await withReload(state, () => selectMode(input.client, state, params.modeId))
+      if (typeof value !== "string") return yield* new ACPError.InvalidConfigOptionError({ configId: params.configId })
+      yield* withReload(attached, selectOption(attached, params.configId, value))
+      return { configOptions: yield* currentOptions(attached) }
+    }),
+    setSessionMode: Effect.fnUntraced(function* (params) {
+      const attached = yield* input.sessions.require(params.sessionId)
+      yield* withReload(attached, selectMode(attached, params.modeId))
       return {}
-    },
-    prompt: async (params, signal) => {
-      const state = await requireSession(params.sessionId)
-      if (active.has(state.id)) {
-        throw new ACPError.ServiceFailureError({
-          safeMessage: `Session already has an active ACP prompt: ${state.id}`,
-          service: "session",
-        })
-      }
-      const messageID = SessionMessage.ID.create()
-      const prepared = preparePrompt(state.catalog.current, params.prompt, messageID)
-      const control: TurnControl = { cancelled: false, admission: new AbortController() }
-      const extNotification = input.connection.extNotification
-      const childSessionUpdate =
-        capabilities.childSessionUpdates && extNotification
-          ? (update: ChildSessionUpdate) => extNotification(ChildSessionUpdateMethod, update).then(() => {})
-          : undefined
-      // A `$/cancel_request` for this prompt behaves like `session/cancel` for its turn.
-      const cancel = () => void cancelTurn(state.id).catch(() => {})
-      const turn = streamTurn({
-        client: input.client,
-        connection: input.connection,
-        sessionID: state.id,
-        cwd: state.cwd,
-        start: prepared.start,
-        action: prepared.command !== undefined,
-        control,
-        connectionSignal: input.connection.signal,
-        sessionSignal: state.abort.signal,
-        submit: (signal) => submitPrompt(input.client, state, prepared, signal),
-        ...(childSessionUpdate ? { childSessionUpdate } : {}),
-      })
-        .then(async (result) => {
-          await sendUsageUpdate(input.client, input.connection, state, result.contextTokens).catch(() => {})
-          return result.response
-        })
-        .finally(() => {
-          signal?.removeEventListener("abort", cancel)
-          if (active.get(state.id)?.control === control) active.delete(state.id)
-        })
-      active.set(state.id, { control, turn })
-      signal?.addEventListener("abort", cancel, { once: true })
-      // The cancel may already be buffered behind the awaits above.
-      if (signal?.aborted) cancel()
-      return turn
-    },
-    cancel: async (params) => {
-      await cancelTurn(params.sessionId).catch(() => {})
-    },
+    }),
+    prompt: input.turn.prompt,
+    cancel: input.turn.cancel,
   }
 }
 
-function preparePrompt(catalog: Catalog, prompt: PromptRequest["prompt"], messageID: string): PreparedPrompt {
-  const parts = promptContentToParts(prompt)
-  const visible = parts.filter((part) => part.type !== "text" || (!part.synthetic && !part.ignored))
-  const synthetic = parts.flatMap((part) => (part.type === "text" && part.synthetic ? [part.text] : []))
-  const text = visible.flatMap((part) => (part.type === "text" ? [part.text] : [])).join("\n")
-  const files = visible.flatMap((part) => (part.type === "file" ? [{ uri: part.url, name: part.filename }] : []))
-  const slash = detectSlashCommand(text)
-  const command =
-    slash && !builtinCommands.has(slash.name) ? catalog.commands.find((item) => item.name === slash.name) : undefined
-  const start = turnStart(messageID, slash)
-  return { start, text, files, synthetic, slash, command }
-}
-
-async function submitPrompt(client: OpenCodeClient, session: Attached, prompt: PreparedPrompt, signal: AbortSignal) {
-  if (prompt.synthetic.length > 0) {
-    await client.session.synthetic({
-      sessionID: session.id,
-      text: prompt.synthetic.join("\n\n"),
-      description: "ACP embedded context",
-      delivery: "steer",
-      resume: false,
-    })
-  }
-  if (prompt.start.type === "compaction") return client.session.compact({ sessionID: session.id, id: prompt.start.id })
-  if (prompt.command) {
-    return client.session.command(
-      {
-        sessionID: session.id,
-        name: prompt.command.name,
-        text: prompt.slash?.args ?? "",
-        files: prompt.files,
-        delivery: "steer",
-      },
-      { signal },
-    )
-  }
-  return client.session.prompt(
-    { sessionID: session.id, id: prompt.start.id, text: prompt.text, files: prompt.files, delivery: "steer" },
-    { signal },
-  )
-}
-
-function turnStart(messageID: string, slash: PreparedPrompt["slash"]): TurnStart {
-  if (slash && builtinCommands.get(slash.name)?.start === "compaction") return { type: "compaction", id: messageID }
-  return { type: "input", id: messageID }
-}
-
-function requireModel(catalog: Catalog, modelID: string, current: ModelRef): ModelRef {
+const requireModel = Effect.fnUntraced(function* (catalog: Catalog, modelID: string, current: ModelRef) {
   const selected = parseModelSelection(modelID, catalog.providers)
   const model = catalog.models.find(
     (item) => item.providerID === selected.model.providerID && item.id === selected.model.modelID,
   )
-  if (!model) throw new ACPError.InvalidModelError({ providerId: selected.model.providerID, modelId: modelID })
+  if (!model) return yield* new ACPError.InvalidModelError({ providerId: selected.model.providerID, modelId: modelID })
   if (selected.variant && !model.variants.some((variant) => variant.id === selected.variant))
-    throw new ACPError.InvalidEffortError({ effort: selected.variant })
+    return yield* new ACPError.InvalidEffortError({ effort: selected.variant })
   const variant =
     selected.variant ??
     (current.providerID === model.providerID &&
@@ -447,124 +262,14 @@ function requireModel(catalog: Catalog, modelID: string, current: ModelRef): Mod
     (current.variant === DEFAULT_VARIANT_VALUE || model.variants.some((variant) => variant.id === current.variant))
       ? current.variant
       : undefined)
-  return { providerID: model.providerID, id: model.id, variant }
-}
+  return { providerID: model.providerID, id: model.id, variant } satisfies ModelRef
+})
 
-function currentModel(state: Attached, catalog = state.catalog.current) {
-  return state.model ?? catalog.defaultModel
-}
-
-async function selectMode(client: OpenCodeClient, state: Attached, modeID: string) {
-  if (!state.catalog.current.modes.some((mode) => mode.id === modeID))
-    throw new ACPError.InvalidModeError({ mode: modeID })
-  state.modeID = modeID
-  await client.session.switchAgent({ sessionID: state.id, agent: modeID })
-}
-
-async function getSession(client: OpenCodeClient, sessionID: string, cwd: string) {
-  const session = await client.session.get({ sessionID }).catch((error) => {
-    if (isSessionNotFoundError(error)) throw new ACPError.SessionNotFoundError({ sessionId: sessionID })
-    throw error
-  })
-  if (FSUtil.resolve(cwd) !== FSUtil.resolve(session.location.directory)) {
-    throw new ACPError.SessionDirectoryMismatchError({ sessionId: sessionID, cwd })
-  }
-  return session
-}
-
-async function messages(client: OpenCodeClient, sessionID: string) {
-  const result: SessionMessageInfo[] = []
-  let cursor: string | undefined
-  do {
-    const page = cursor
-      ? await client.message.list({ sessionID, limit: 200, cursor })
-      : await client.message.list({ sessionID, limit: 200, order: "asc" })
-    result.push(...page.data)
-    cursor = page.cursor.next ?? undefined
-  } while (cursor)
-  return result
-}
-
-async function registerMcpServers(
-  client: OpenCodeClient,
-  registered: Map<string, Set<string>>,
-  session: Attached,
-  servers: readonly McpServer[],
-) {
-  const current = registered.get(session.id) ?? new Set<string>()
-  registered.set(session.id, current)
-  await Promise.all(
-    servers.flatMap((server) => {
-      const config = mcpConfig(server)
-      const key = `${server.name}:${stableStringify(config)}`
-      if (current.has(key)) return []
-      current.add(key)
-      return [
-        client.mcp.add({ server: server.name, location: { directory: session.cwd }, config }).catch((error) => {
-          current.delete(key)
-          throw error
-        }),
-      ]
-    }),
-  )
-}
-
-function mcpConfig(server: McpServer) {
-  if ("type" in server) {
-    if (server.type === "acp") throw new Error("MCP-over-ACP is not supported")
-    return {
-      type: "remote" as const,
-      url: server.url,
-      headers: Object.fromEntries(server.headers.map((header) => [header.name, header.value])),
-      oauth: false as const,
-    }
-  }
-  return {
-    type: "local" as const,
-    command: [server.command, ...server.args],
-    environment: Object.fromEntries(server.env.map((entry) => [entry.name, entry.value])),
-  }
-}
-
-function stableStringify(value: unknown): string {
-  if (Array.isArray(value)) return `[${value.map(stableStringify).join(",")}]`
-  if (!value || typeof value !== "object") return JSON.stringify(value)
-  return `{${Object.entries(value)
-    .toSorted(([a], [b]) => a.localeCompare(b))
-    .map(([key, item]) => `${JSON.stringify(key)}:${stableStringify(item)}`)
-    .join(",")}}`
-}
-
-async function sendUsageUpdate(
-  client: OpenCodeClient,
-  connection: ACPConnection.Connection,
-  session: Attached,
-  used?: number,
-) {
-  if (!used) return
-  const current = currentModel(session)
-  const model = session.catalog.current.models.find(
-    (item) => item.providerID === current.providerID && item.id === current.id,
-  )
-  if (!model?.limit.context) return
-  const info = await client.session.get({ sessionID: session.id })
-  await connection.sessionUpdate({
-    sessionId: session.id,
-    update: {
-      sessionUpdate: "usage_update",
-      used,
-      size: model.limit.context,
-      cost: { amount: info.cost, currency: "USD" },
-    },
-  })
-}
-
-function detectSlashCommand(text: string): { readonly name: string; readonly args: string } | undefined {
-  const value = text.trim()
-  if (!value.startsWith("/")) return undefined
-  const [name, ...rest] = value.slice(1).split(/\s+/)
-  if (!name) return undefined
-  return { name, args: rest.join(" ").trim() }
-}
+const requireEffort = Effect.fnUntraced(function* (catalog: Catalog, effort: string, current: ModelRef) {
+  const model = catalog.models.find((item) => item.providerID === current.providerID && item.id === current.id)
+  if (!model || (effort !== DEFAULT_VARIANT_VALUE && !model.variants.some((variant) => variant.id === effort)))
+    return yield* new ACPError.InvalidEffortError({ effort })
+  return { ...current, variant: effort } satisfies ModelRef
+})
 
 export * as ACPService from "./service"

packages/cli/src/acp/sessions.ts

@@ -0,0 +1,176 @@
+import { isDeepStrictEqual } from "node:util"
+import type { McpServer, RequestError } from "@agentclientprotocol/sdk"
+import type { OpenCodeClient, SessionInfo } from "@opencode/client/promise"
+import { Context, Effect, Exit, Ref, Scope, Stream } from "effect"
+import type { ACPCatalog, Catalog } from "./catalog"
+import { availableCommands, configOptions, type Selection } from "./config-option"
+import type { ACPConnection } from "./connection"
+import { ACPError } from "./error"
+import { ACPPromise } from "./promise"
+
+export type Attached = {
+  readonly id: string
+  readonly cwd: string
+  readonly selection: Ref.Ref<Selection>
+  /** Aborted when the session detaches, for the promise-based turn. */
+  readonly signal: AbortSignal
+}
+
+export interface Interface {
+  /**
+   * Attaches a session in its own scope, closing any previous attachment of the same ID. The scope follows the
+   * cwd's catalog and pushes config option and command updates while it is open. A failed attach leaves the
+   * session detached.
+   */
+  readonly attach: (
+    session: SessionInfo,
+    cwd: string,
+    mcpServers: readonly McpServer[],
+  ) => Effect.Effect<Attached, ACPError.Error | RequestError | ACPCatalog.Error>
+  /** Closes the session scope. No-op when the session is not attached. */
+  readonly detach: (sessionID: string) => Effect.Effect<void>
+  readonly require: (sessionID: string) => Effect.Effect<Attached, ACPError.SessionNotFoundError>
+}
+
+export class Service extends Context.Service<Service, Interface>()("@opencode/cli/acp/Sessions") {}
+
+type Entry = { readonly attached: Attached; readonly scope: Scope.Closeable }
+
+export const make = Effect.fnUntraced(function* (input: {
+  readonly client: OpenCodeClient
+  readonly connection: ACPConnection.Interface
+  readonly catalog: ACPCatalog.Interface
+}) {
+  const scope = yield* Effect.scope
+  const sessions = new Map<string, Entry>()
+  // Kept across re-attachment so resuming with the same servers does not add them again.
+  const registeredMcp = new Map<string, Set<string>>()
+
+  const sendCommands = (sessionID: string, catalog: Catalog) =>
+    input.connection.sessionUpdate({
+      sessionId: sessionID,
+      update: { sessionUpdate: "available_commands_update", availableCommands: availableCommands(catalog) },
+    })
+
+  const changed = Effect.fnUntraced(function* (attached: Attached, previous: Catalog, next: Catalog) {
+    const selection = yield* Ref.get(attached.selection)
+    const options = configOptions(next, selection)
+    if (!isDeepStrictEqual(options, configOptions(previous, selection))) {
+      yield* input.connection.sessionUpdate({
+        sessionId: attached.id,
+        update: { sessionUpdate: "config_option_update", configOptions: options },
+      })
+    }
+    if (!isDeepStrictEqual(next.commands, previous.commands)) yield* sendCommands(attached.id, next)
+  })
+
+  const registerMcp = (attached: Attached, servers: readonly McpServer[]) =>
+    Effect.suspend(() => {
+      const registered = registeredMcp.get(attached.id) ?? new Set<string>()
+      registeredMcp.set(attached.id, registered)
+      return Effect.forEach(
+        servers,
+        (server) =>
+          Effect.suspend(() => {
+            const config = mcpConfig(server)
+            const key = `${server.name}:${stableStringify(config)}`
+            if (registered.has(key)) return Effect.void
+            registered.add(key)
+            return ACPPromise.promise(() =>
+              input.client.mcp.add({ server: server.name, location: { directory: attached.cwd }, config }),
+            ).pipe(
+              Effect.onError(() => Effect.sync(() => registered.delete(key))),
+              Effect.uninterruptible,
+            )
+          }),
+        { concurrency: "unbounded", discard: true },
+      )
+    })
+
+  const remove = (sessionID: string, entry: Entry) =>
+    Effect.suspend(() => {
+      if (sessions.get(sessionID) === entry) {
+        sessions.delete(sessionID)
+        registeredMcp.delete(sessionID)
+      }
+      return Scope.close(entry.scope, Exit.void)
+    })
+
+  return Service.of({
+    attach: Effect.fn("cli.acp.sessions.attach")(function* (session, cwd, mcpServers) {
+      const current = yield* input.catalog.get(cwd)
+      const abort = new AbortController()
+      const entry: Entry = {
+        attached: {
+          id: session.id,
+          cwd,
+          selection: yield* Ref.make<Selection>({ model: session.model, modeID: session.agent }),
+          signal: abort.signal,
+        },
+        scope: Scope.forkUnsafe(scope),
+      }
+      // Swap synchronously so concurrent attaches of one ID cannot both keep a scope.
+      const replaced = sessions.get(session.id)
+      sessions.set(session.id, entry)
+      if (replaced) yield* Scope.close(replaced.scope, Exit.void)
+      yield* Scope.addFinalizer(
+        entry.scope,
+        Effect.sync(() => abort.abort()),
+      )
+      yield* registerMcp(entry.attached, mcpServers).pipe(
+        Effect.andThen(sendCommands(session.id, current)),
+        Effect.onError(() => remove(session.id, entry)),
+      )
+      // `changes` emits the latest catalog first, so a reload since `current` is still pushed.
+      yield* input.catalog.changes(cwd).pipe(
+        Stream.runFoldEffect(
+          () => current,
+          (previous, next) =>
+            next === previous
+              ? Effect.succeed(previous)
+              : changed(entry.attached, previous, next).pipe(Effect.ignore, Effect.as(next)),
+        ),
+        Effect.ignore,
+        Effect.forkIn(entry.scope),
+      )
+      return entry.attached
+    }),
+    detach: Effect.fn("cli.acp.sessions.detach")(function* (sessionID) {
+      const entry = sessions.get(sessionID)
+      if (entry) yield* remove(sessionID, entry)
+    }),
+    require: Effect.fn("cli.acp.sessions.require")(function* (sessionID) {
+      const entry = sessions.get(sessionID)
+      if (!entry) return yield* new ACPError.SessionNotFoundError({ sessionId: sessionID })
+      return entry.attached
+    }),
+  })
+})
+
+function mcpConfig(server: McpServer) {
+  if ("type" in server) {
+    if (server.type === "acp") throw new Error("MCP-over-ACP is not supported")
+    return {
+      type: "remote" as const,
+      url: server.url,
+      headers: Object.fromEntries(server.headers.map((header) => [header.name, header.value])),
+      oauth: false as const,
+    }
+  }
+  return {
+    type: "local" as const,
+    command: [server.command, ...server.args],
+    environment: Object.fromEntries(server.env.map((entry) => [entry.name, entry.value])),
+  }
+}
+
+function stableStringify(value: unknown): string {
+  if (Array.isArray(value)) return `[${value.map(stableStringify).join(",")}]`
+  if (!value || typeof value !== "object") return JSON.stringify(value)
+  return `{${Object.entries(value)
+    .toSorted(([a], [b]) => a.localeCompare(b))
+    .map(([key, item]) => `${JSON.stringify(key)}:${stableStringify(item)}`)
+    .join(",")}}`
+}
+
+export * as ACPSessions from "./sessions"

packages/cli/src/acp/turn.ts

@@ -0,0 +1,203 @@
+import type { CancelNotification, PromptRequest, PromptResponse, RequestError } from "@agentclientprotocol/sdk"
+import { isSessionNotFoundError, type CommandInfo, type OpenCodeClient } from "@opencode/client/promise"
+import { SessionMessage } from "@opencode/schema/session-message"
+import { Effect, Ref, type Scope } from "effect"
+import { builtinCommands, type ACPCatalog, type Catalog } from "./catalog"
+import { currentModel } from "./config-option"
+import type { ACPConnection } from "./connection"
+import { promptContentToParts } from "./content"
+import { ACPError } from "./error"
+import {
+  ChildSessionUpdateMethod,
+  streamTurn,
+  type ChildSessionUpdate,
+  type TurnControl,
+  type TurnStart,
+} from "./event"
+import { ACPPromise } from "./promise"
+import type { ACPSessions, Attached } from "./sessions"
+
+type PreparedPrompt = {
+  readonly start: TurnStart
+  readonly text: string
+  readonly files: Array<{ readonly uri: string; readonly name?: string }>
+  readonly synthetic: ReadonlyArray<string>
+  readonly slash?: { readonly name: string; readonly args: string }
+  readonly command?: CommandInfo
+}
+
+export interface Interface {
+  prompt(input: PromptRequest, signal?: AbortSignal): Promise<PromptResponse>
+  cancel(input: CancelNotification): Promise<void>
+  /** Cancels the session's active turn and waits for it to settle. */
+  readonly close: (sessionID: string) => Effect.Effect<void, ACPError.Error | RequestError>
+}
+
+export function make(input: {
+  readonly client: OpenCodeClient
+  readonly connection: ACPConnection.Connection
+  readonly sessions: ACPSessions.Interface
+  readonly catalog: ACPCatalog.Interface
+  readonly capabilities: Ref.Ref<{ readonly childSessionUpdates: boolean }>
+  readonly run: <A, E>(effect: Effect.Effect<A, E, Scope.Scope>) => Promise<A>
+}): Interface {
+  const active = new Map<string, { readonly control: TurnControl; readonly turn: Promise<PromptResponse> }>()
+
+  const cancelTurn = (sessionID: string) => {
+    const turn = active.get(sessionID)
+    if (turn) {
+      turn.control.cancelled = true
+      turn.control.admission.abort()
+    }
+    return input.client.session.interrupt({ sessionID })
+  }
+
+  const sendUsageUpdate = async (state: Attached, used?: number) => {
+    if (!used) return
+    const model = await input.run(
+      Effect.gen(function* () {
+        const catalog = yield* input.catalog.get(state.cwd)
+        const current = currentModel(catalog, yield* Ref.get(state.selection))
+        return catalog.models.find((item) => item.providerID === current.providerID && item.id === current.id)
+      }),
+    )
+    if (!model?.limit.context) return
+    const info = await input.client.session.get({ sessionID: state.id })
+    await input.connection.sessionUpdate({
+      sessionId: state.id,
+      update: {
+        sessionUpdate: "usage_update",
+        used,
+        size: model.limit.context,
+        cost: { amount: info.cost, currency: "USD" },
+      },
+    })
+  }
+
+  return {
+    prompt: async (params, signal) => {
+      // Read everything first so the active check and registration below stay synchronous.
+      const resolved = await input.run(
+        Effect.gen(function* () {
+          const attached = yield* input.sessions.require(params.sessionId)
+          return {
+            attached,
+            catalog: yield* input.catalog.get(attached.cwd),
+            childSessionUpdates: (yield* Ref.get(input.capabilities)).childSessionUpdates,
+          }
+        }),
+      )
+      const state = resolved.attached
+      if (active.has(state.id)) {
+        throw new ACPError.ServiceFailureError({
+          safeMessage: `Session already has an active ACP prompt: ${state.id}`,
+          service: "session",
+        })
+      }
+      const messageID = SessionMessage.ID.create()
+      const prepared = preparePrompt(resolved.catalog, params.prompt, messageID)
+      const control: TurnControl = { cancelled: false, admission: new AbortController() }
+      const extNotification = input.connection.extNotification
+      const childSessionUpdate =
+        resolved.childSessionUpdates && extNotification
+          ? (update: ChildSessionUpdate) => extNotification(ChildSessionUpdateMethod, update).then(() => {})
+          : undefined
+      // A `$/cancel_request` for this prompt behaves like `session/cancel` for its turn.
+      const cancel = () => void cancelTurn(state.id).catch(() => {})
+      const turn = streamTurn({
+        client: input.client,
+        connection: input.connection,
+        sessionID: state.id,
+        cwd: state.cwd,
+        start: prepared.start,
+        action: prepared.command !== undefined,
+        control,
+        connectionSignal: input.connection.signal,
+        sessionSignal: state.signal,
+        submit: (signal) => submitPrompt(input.client, state, prepared, signal),
+        ...(childSessionUpdate ? { childSessionUpdate } : {}),
+      })
+        .then(async (result) => {
+          await sendUsageUpdate(state, result.contextTokens).catch(() => {})
+          return result.response
+        })
+        .finally(() => {
+          signal?.removeEventListener("abort", cancel)
+          if (active.get(state.id)?.control === control) active.delete(state.id)
+        })
+      active.set(state.id, { control, turn })
+      signal?.addEventListener("abort", cancel, { once: true })
+      // The cancel may already be buffered behind the awaits above.
+      if (signal?.aborted) cancel()
+      return turn
+    },
+    cancel: async (params) => {
+      await cancelTurn(params.sessionId).catch(() => {})
+    },
+    close: Effect.fnUntraced(function* (sessionID) {
+      const turn = active.get(sessionID)
+      yield* ACPPromise.promise(() =>
+        cancelTurn(sessionID).catch((error) => {
+          if (!isSessionNotFoundError(error)) throw error
+        }),
+      )
+      if (turn) yield* Effect.promise(() => turn.turn.catch(() => {}))
+    }),
+  }
+}
+
+function preparePrompt(catalog: Catalog, prompt: PromptRequest["prompt"], messageID: string): PreparedPrompt {
+  const parts = promptContentToParts(prompt)
+  const visible = parts.filter((part) => part.type !== "text" || (!part.synthetic && !part.ignored))
+  const synthetic = parts.flatMap((part) => (part.type === "text" && part.synthetic ? [part.text] : []))
+  const text = visible.flatMap((part) => (part.type === "text" ? [part.text] : [])).join("\n")
+  const files = visible.flatMap((part) => (part.type === "file" ? [{ uri: part.url, name: part.filename }] : []))
+  const slash = detectSlashCommand(text)
+  const command = slash ? catalog.commands.find((item) => item.name === slash.name) : undefined
+  const start = turnStart(messageID, slash)
+  return { start, text, files, synthetic, slash, command }
+}
+
+async function submitPrompt(client: OpenCodeClient, session: Attached, prompt: PreparedPrompt, signal: AbortSignal) {
+  if (prompt.synthetic.length > 0) {
+    await client.session.synthetic({
+      sessionID: session.id,
+      text: prompt.synthetic.join("\n\n"),
+      description: "ACP embedded context",
+      delivery: "steer",
+      resume: false,
+    })
+  }
+  if (prompt.start.type === "compaction") return client.session.compact({ sessionID: session.id, id: prompt.start.id })
+  if (prompt.command) {
+    return client.session.command(
+      {
+        sessionID: session.id,
+        name: prompt.command.name,
+        text: prompt.slash?.args ?? "",
+        files: prompt.files,
+        delivery: "steer",
+      },
+      { signal },
+    )
+  }
+  return client.session.prompt(
+    { sessionID: session.id, id: prompt.start.id, text: prompt.text, files: prompt.files, delivery: "steer" },
+    { signal },
+  )
+}
+
+function turnStart(messageID: string, slash: PreparedPrompt["slash"]): TurnStart {
+  if (slash && builtinCommands.get(slash.name)?.start === "compaction") return { type: "compaction", id: messageID }
+  return { type: "input", id: messageID }
+}
+
+function detectSlashCommand(text: string): { readonly name: string; readonly args: string } | undefined {
+  const value = text.trim()
+  if (!value.startsWith("/")) return undefined
+  const [name, ...rest] = value.slice(1).split(/\s+/)
+  if (!name) return undefined
+  return { name, args: rest.join(" ").trim() }
+}
+
+export * as ACPTurn from "./turn"

packages/cli/test/acp/catalog.test.ts

@@ -118,24 +118,71 @@ describe("acp catalog and config options over the wire", () => {
     const session = await acp.newSession()
     acp.server.catalog.models = [testModel, secondModel]
     acp.server.catalog.agents = [buildAgent, planAgent, configured]
+    const set = (configId: string, value: string) =>
+      acp.request("session/set_config_option", { sessionId: session.sessionId, configId, value })
+    const initialModelReads = modelReads(acp)
 
-    const model = await acp.request("session/set_config_option", {
-      sessionId: session.sessionId,
-      configId: "model",
-      value: "test/second-model",
-    })
+    const model = await set("model", "test/second-model")
+    const reloadedModelReads = modelReads(acp)
+    const missingModel = await rpcError(set("model", "test/missing-model"))
+    const missingModelReads = modelReads(acp)
     await acp.request("session/set_mode", { sessionId: session.sessionId, modeId: "copilot-build" })
     const reads = agentReads(acp)
-    const missing = await rpcError(
-      acp.request("session/set_config_option", { sessionId: session.sessionId, configId: "mode", value: "missing" }),
-    )
+    const missing = await rpcError(set("mode", "missing"))
 
     expect(currentValue(model, "model")).toBe("test/second-model")
+    expect([reloadedModelReads, missingModelReads]).toEqual([initialModelReads + 1, initialModelReads + 2])
+    expect(missingModel).toMatchObject({ code: -32602, data: { modelId: "test/missing-model" } })
     expect(acp.server.selections).toContainEqual({ sessionID: session.sessionId, agent: "copilot-build" })
     expect(missing).toMatchObject({ code: -32602, data: { mode: "missing" } })
     expect(agentReads(acp)).toBeGreaterThan(reads)
   })
 
+  test.each([
+    [
+      "a sibling session closes",
+      async (acp: Wire) => {
+        const closed = await acp.newSession()
+        const open = await acp.newSession()
+        await acp.request("session/close", { sessionId: closed.sessionId })
+        return open.sessionId
+      },
+    ],
+    ...(["session/load", "session/resume"] as const).map(
+      (method) =>
+        [
+          `${method} re-attaches the session`,
+          async (acp: Wire) => {
+            const session = await acp.newSession()
+            const params = { cwd: "/workspace", sessionId: session.sessionId, mcpServers: [] }
+            await acp.request(method, params)
+            await acp.request(method, params)
+            return session.sessionId
+          },
+        ] as const,
+    ),
+  ])("pushes exactly one update per catalog change after %s", async (_, setup) => {
+    await using acp = await startWire()
+    acp.server.catalog.models = [testModel]
+    await acp.initialize()
+    const sessionId = await setup(acp)
+    const since = acp.updates.length
+
+    await change(acp, sessionId, "config_option_update", () => {
+      acp.server.catalog.models = [testModel, secondModel]
+      acp.server.send(ephemeralEvent("model.updated", {}))
+    })
+    await change(acp, sessionId, "available_commands_update", () => {
+      acp.server.catalog.commands = [reviewCommand, { name: "ship", description: "Ship it" }]
+      acp.server.send(ephemeralEvent("command.updated", {}, { directory: "/workspace" }))
+    })
+
+    expect(updateKinds(acp, since)).toEqual([
+      [sessionId, "config_option_update"],
+      [sessionId, "available_commands_update"],
+    ])
+  })
+
   test.each(["empty", "missing the default"])(
     "retries when the model list is %s but the default is ready",
     async (initial) => {
@@ -212,25 +259,52 @@ describe("acp catalog and config options over the wire", () => {
     await using acp = await startSession()
     const advertised = await acp.waitForUpdate((item) => commandNames(item) !== undefined)
 
-    acp.server.catalog.commands = [reviewCommand, { name: "compact", description: "Server compact" }]
+    acp.server.catalog.commands = [
+      reviewCommand,
+      { name: "compact", description: "Server compact" },
+      { name: "ship", description: "Ship it" },
+    ]
     acp.server.send(ephemeralEvent("command.updated", {}, { directory: "/workspace" }))
     const replaced = await acp.waitForUpdate((item) => item !== advertised && commandNames(item) !== undefined)
     const compacted = await acp.prompt(acp.sessionId, "/compact")
 
-    expect([advertised, replaced].map((item) => item.update)).toEqual(
-      Array.from({ length: 2 }, () => ({
+    expect([advertised, replaced].map((item) => item.update)).toEqual([
+      {
         sessionUpdate: "available_commands_update",
         availableCommands: [
           { name: "review", description: "Review changes" },
           { name: "compact", description: "Compact the session" },
         ],
-      })),
-    )
+      },
+      {
+        sessionUpdate: "available_commands_update",
+        availableCommands: [
+          { name: "review", description: "Review changes" },
+          { name: "ship", description: "Ship it" },
+          { name: "compact", description: "Compact the session" },
+        ],
+      },
+    ])
     expect(compacted.stopReason).toBe("end_turn")
     expect(acp.server.submissions.map((item) => item.kind)).toEqual(["compact"])
   })
 })
 
+// Each change waits on a catalog reload over HTTP, so a stray update for an earlier change lands before the next one.
+async function change(acp: Wire, sessionId: string, kind: string, trigger: () => void) {
+  const seen = acp.updates.filter((item) => item.sessionId === sessionId && item.update.sessionUpdate === kind).length
+  trigger()
+  await acp.until(
+    () =>
+      acp.updates.filter((item) => item.sessionId === sessionId && item.update.sessionUpdate === kind).length > seen,
+    kind,
+  )
+}
+
+function updateKinds(acp: Wire, since: number) {
+  return acp.updates.slice(since).map((item) => [item.sessionId, item.update.sessionUpdate])
+}
+
 function commandNames(item: SessionNotification) {
   if (item.update.sessionUpdate !== "available_commands_update") return undefined
   return item.update.availableCommands.map((command) => command.name)

packages/cli/test/acp/session.test.ts

@@ -1,7 +1,7 @@
 import { describe, expect, test } from "bun:test"
 import type { McpServer } from "@agentclientprotocol/sdk"
 import { currentValue } from "./select-options"
-import { makeSession, rpcError, secondModel, startSession, startWire } from "./wire-fixture"
+import { ephemeralEvent, makeSession, rpcError, secondModel, startSession, startWire, testModel } from "./wire-fixture"
 
 describe("acp session lifecycle over the wire", () => {
   test("initialize advertises capabilities and terminal auth only when the client asks", async () => {
@@ -226,4 +226,33 @@ describe("acp session lifecycle over the wire", () => {
       config: { type: "remote", url: "https://example.com/mcp", headers: { Authorization: "Bearer x" }, oauth: false },
     })
   })
+  test("leaves a session detached when re-attaching it fails", async () => {
+    const broken: McpServer = { name: "broken", command: "bun", args: [], env: [] }
+    await using acp = await startWire({
+      fetch: (request) =>
+        request.method === "PUT" && request.path === "/api/experimental/mcp/broken"
+          ? new Response(null, { status: 500 })
+          : undefined,
+    })
+    await acp.initialize()
+    const failed = await acp.newSession()
+    const other = await acp.newSession()
+
+    expect(
+      await rpcError(
+        acp.request("session/resume", { cwd: "/workspace", sessionId: failed.sessionId, mcpServers: [broken] }),
+      ),
+    ).toMatchObject({ code: -32603 })
+    const since = acp.updates.length
+    acp.server.catalog.models = [testModel]
+    acp.server.send(ephemeralEvent("model.updated", {}))
+    await acp.until(() => acp.updates.length > since, "config options for the attached session")
+
+    expect(acp.updates.slice(since).map((item) => item.sessionId)).toEqual([other.sessionId])
+    expect(
+      await rpcError(
+        acp.request("session/set_config_option", { sessionId: failed.sessionId, configId: "mode", value: "plan" }),
+      ),
+    ).toMatchObject({ code: -32602, data: { sessionId: failed.sessionId } })
+  })
 })