fix: never route GitHub Copilot Gemini/Claude models to /responses (#1062) (#1536)

GitHub Copilot's /responses endpoint only serves OpenAI (gpt/codex)
models. gemini-3.1-pro-preview was failing on /chat/completions with a
"not supported" error, getting cached as a codex model, then escalated
to /responses where it 400s with "does not support Responses API".

Add GithubExecutor.supportsResponsesEndpoint() and gate both the cached
/responses route and the 400-fallback on it, so Gemini/Claude always
stay on /chat/completions and the real upstream error surfaces.

Adds tests/unit/github-responses-routing.test.js (5 tests).

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit/github-responses-routing.test.js

@@ -0,0 +1,56 @@
+/**
+ * Regression test for #1062:
+ * GitHub Copilot's /responses endpoint only serves OpenAI (gpt/codex) models.
+ * Gemini/Claude models must never be routed/escalated there, otherwise they
+ * fail with a misleading 400 "does not support Responses API".
+ */
+
+import { describe, it, expect, vi } from "vitest";
+import { GithubExecutor } from "../../open-sse/executors/github.js";
+
+describe("GithubExecutor.supportsResponsesEndpoint", () => {
+  const exec = new GithubExecutor();
+
+  it("excludes Gemini models from the /responses endpoint", () => {
+    expect(exec.supportsResponsesEndpoint("gemini-3.1-pro-preview")).toBe(false);
+    expect(exec.supportsResponsesEndpoint("gemini-3.1-pro-low")).toBe(false);
+  });
+
+  it("excludes Claude models from the /responses endpoint", () => {
+    expect(exec.supportsResponsesEndpoint("claude-sonnet-4.6")).toBe(false);
+    expect(exec.supportsResponsesEndpoint("claude-opus-4.7")).toBe(false);
+  });
+
+  it("allows OpenAI/codex models on the /responses endpoint", () => {
+    expect(exec.supportsResponsesEndpoint("gpt-5.5-codex")).toBe(true);
+    expect(exec.supportsResponsesEndpoint("o4-mini")).toBe(true);
+    expect(exec.supportsResponsesEndpoint("gpt-4.1")).toBe(true);
+  });
+
+  it("is null-safe", () => {
+    expect(exec.supportsResponsesEndpoint(undefined)).toBe(true);
+    expect(exec.supportsResponsesEndpoint("")).toBe(true);
+  });
+});
+
+describe("GithubExecutor.execute cached-route guard (#1062)", () => {
+  it("does NOT use /responses for a Gemini model even if it was wrongly cached as codex", async () => {
+    const exec = new GithubExecutor();
+    // Simulate a prior misclassification that cached the Gemini model.
+    exec.knownCodexModels.add("gemini-3.1-pro-preview");
+
+    const respSpy = vi
+      .spyOn(exec, "executeWithResponsesEndpoint")
+      .mockResolvedValue({ via: "responses" });
+    // Short-circuit the /chat/completions path (BaseExecutor.execute).
+    const baseSpy = vi
+      .spyOn(Object.getPrototypeOf(Object.getPrototypeOf(exec)), "execute")
+      .mockResolvedValue({ response: { status: 200 }, via: "chat" });
+
+    const result = await exec.execute({ model: "gemini-3.1-pro-preview", body: { messages: [] }, log: null });
+
+    expect(respSpy).not.toHaveBeenCalled();
+    expect(baseSpy).toHaveBeenCalled();
+    expect(result.via).toBe("chat");
+  });
+});