fix(core): route fallback Copilot Claude models to Anthropic Messages

packages/core/src/plugin/provider/github-copilot.ts

@@ -12,6 +12,7 @@ import { Agent } from "../../agent.js"
 import { define } from "@opencode/plugin/effect/plugin"
 import { Provider } from "../../provider.js"
 import { SessionAffinity } from "../../session/affinity.js"
+import { Variant } from "../../variant.js"
 import type { PluginInternal } from "../internal.js"
 
 const clientID = "Ov23li8tweQw6odWQebz"
@@ -228,6 +229,22 @@ export const GithubCopilotPlugin = define({
       }
       for (const id of item.models.keys()) {
         evt.models.update(item.provider.id, id, (model) => {
+          // Without the live model list there is no advertised /v1/messages endpoint, but Claude
+          // still needs it: over chat completions each thinking tool turn adds another
+          // reasoning_opaque, which the Copilot chat model rejects.
+          if ((model.modelID ?? model.id).startsWith("claude-")) {
+            const efforts = model.variants.flatMap((variant) =>
+              typeof variant.settings?.reasoningEffort === "string" ? [variant.settings.reasoningEffort] : [],
+            )
+            model.package = "@opencode/ai/providers/anthropic"
+            model.settings = Provider.mergeOverlay(model.settings, { baseURL: `${loaded.baseURL ?? baseURL()}/v1` })
+            // Catalog variants were spelled for chat completions; Messages ignores reasoningEffort.
+            model.variants = [
+              ...model.variants.filter((variant) => typeof variant.settings?.reasoningEffort !== "string"),
+              ...Variant.resolve(model, efforts.length ? [{ type: "effort", values: efforts }] : undefined),
+            ]
+            return
+          }
           model.package = Provider.aisdk("@ai-sdk/github-copilot")
           if (loaded.baseURL) model.settings = Provider.mergeOverlay(model.settings, { baseURL: loaded.baseURL })
         })

packages/core/test/plugin/provider-github-copilot.test.ts

@@ -391,6 +391,42 @@ describe("GithubCopilotPlugin", () => {
     }),
   )
 
+  it.effect("routes models.dev fallback Claude models to Anthropic Messages", () =>
+    Effect.gen(function* () {
+      const providers = yield* Provider.Service
+      const models = yield* Model.Service
+      yield* providers.transform((editor) => {
+        editor.update(Provider.ID.githubCopilot, (provider) => {
+          provider.activation = "enabled"
+        })
+        editor.models.update(Provider.ID.githubCopilot, Model.ID.make("claude-opus-5.5"), (model) => {
+          model.package = "@opencode/ai/providers/openai-compatible"
+          model.limit = { context: 200_000, output: 64_000 }
+          model.variants = [
+            { id: Model.VariantID.make("low"), settings: { reasoningEffort: "low" } },
+            { id: Model.VariantID.make("max"), settings: { reasoningEffort: "max" } },
+            { id: Model.VariantID.make("fast"), settings: { speed: "fast" } },
+          ]
+        })
+      })
+      yield* addPlugin()
+      const fallback = required(yield* models.get(Provider.ID.githubCopilot, Model.ID.make("claude-opus-5.5")))
+      expect(fallback.package).toBe("@opencode/ai/providers/anthropic")
+      expect(fallback.settings?.baseURL).toBe("https://api.githubcopilot.com/v1")
+      expect(fallback.variants).toEqual([
+        { id: Model.VariantID.make("fast"), settings: { speed: "fast" } },
+        {
+          id: Model.VariantID.make("low"),
+          settings: { thinking: { type: "adaptive", display: "summarized" }, effort: "low" },
+        },
+        {
+          id: Model.VariantID.make("max"),
+          settings: { thinking: { type: "adaptive", display: "summarized" }, effort: "max" },
+        },
+      ])
+    }),
+  )
+
   it.effect("selects languageModel when responses and chat are absent", () =>
     Effect.gen(function* () {
       const aisdk = yield* AISDK.Service