fix(copilot): disable eager tool streaming for Claude 4.5 (#75393)

Disable Anthropic eager tool streaming only for GitHub Copilot Claude 4.5 proxy routes, preserving newer Claude behavior.

Fixes #75348

Co-authored-by: Kailigithub <Kailigithub@users.noreply.github.com>

extensions/github-copilot/model-metadata.ts

@@ -56,6 +56,10 @@ function isCopilotGeminiModelId(modelId: string): boolean {
   return /(?:^|[-_.])gemini(?:$|[-_.])/.test(modelId);
 }
 
+function isCopilotClaude45ModelId(modelId: string): boolean {
+  return /^claude-(?:haiku|opus|sonnet)-4[.-]5(?:$|[-.])/.test(modelId);
+}
+
 export function resolveCopilotTransportApi(modelId: string): CopilotRuntimeApi {
   const normalized = normalizeOptionalLowercaseString(modelId) ?? "";
   if (normalized.includes("claude")) {
@@ -71,7 +75,15 @@ export function resolveCopilotModelCompat(
   modelId: string,
 ): ModelDefinitionConfig["compat"] | undefined {
   const normalized = normalizeOptionalLowercaseString(modelId) ?? "";
-  return isCopilotGeminiModelId(normalized) ? { ...COPILOT_CHAT_COMPLETIONS_COMPAT } : undefined;
+  if (isCopilotGeminiModelId(normalized)) {
+    return { ...COPILOT_CHAT_COMPLETIONS_COMPAT };
+  }
+  // Copilot's Claude 4.5 endpoints reject Anthropic's eager tool extension,
+  // while current Claude 4.6+ endpoints accept it.
+  if (isCopilotClaude45ModelId(normalized)) {
+    return { supportsEagerToolInputStreaming: false };
+  }
+  return undefined;
 }
 
 function compatSupportsEffort(

extensions/github-copilot/models.test.ts

@@ -90,8 +90,18 @@ describe("github-copilot model defaults", () => {
       const def = buildCopilotModelDefinition("claude-sonnet-4.6");
       expect(def.id).toBe("claude-sonnet-4.6");
       expect(def.api).toBe("anthropic-messages");
+      expect(def.compat).toBeUndefined();
     });
 
+    it.each(["claude-haiku-4.5", "claude-sonnet-4-5"])(
+      "disables eager tool streaming for Copilot Claude 4.5 model %s",
+      (modelId) => {
+        expect(buildCopilotModelDefinition(modelId).compat).toEqual({
+          supportsEagerToolInputStreaming: false,
+        });
+      },
+    );
+
     it("uses static metadata overrides for gpt-5.5 fallback rows", () => {
       const def = buildCopilotModelDefinition("gpt-5.5");
       expect(def).toEqual({
@@ -243,6 +253,12 @@ describe("resolveCopilotForwardCompatModel", () => {
     expect((result as unknown as Record<string, unknown>).input).toEqual(["text", "image"]);
   });
 
+  it("disables eager tool streaming for synthetic Copilot Claude 4.5 models", () => {
+    const result = requireResolvedModel(createMockCtx("claude-haiku-4.5"));
+    expect(result.api).toBe("anthropic-messages");
+    expect(result.compat).toEqual({ supportsEagerToolInputStreaming: false });
+  });
+
   it("creates synthetic Gemini models with Chat Completions compatibility", () => {
     const result = requireResolvedModel(createMockCtx("gemini-3.1-pro-preview"));
     expect((result as unknown as Record<string, unknown>).api).toBe("openai-completions");
@@ -620,6 +636,7 @@ describe("fetchCopilotModelCatalog", () => {
     const opus45 = out.find((m) => m.id === "claude-opus-4-5");
     expect(opus45?.thinkingLevelMap).toEqual({ xhigh: null, max: null });
     expect(opus45?.compat).toEqual({
+      supportsEagerToolInputStreaming: false,
       supportedReasoningEfforts: ["low", "medium", "high", "max"],
     });
   });