diff --git a/open-sse/executors/github.ts b/open-sse/executors/github.ts
index 036998c417..f2507bd9cf 100644
--- a/open-sse/executors/github.ts
+++ b/open-sse/executors/github.ts
@@ -25,12 +25,31 @@ export class GithubExecutor extends BaseExecutor {
     );
   }
 
+  // GitHub Copilot's /responses endpoint only serves OpenAI (gpt/codex) models.
+  // Gemini and Claude variants on Copilot reject with HTTP 400
+  //   "model <id> does not support Responses API." (unsupported_api_for_model)
+  // Pin a defensive invariant: even if a future registry edit (or an upstream
+  // model-discovery refresh) tagged a Claude/Gemini entry as openai-responses,
+  // the executor must still route it to /chat/completions. Port of 9router#1536
+  // (follow-up to #663); also reinforces the existing comments on the gh
+  // registry entries (claude-opus-4-5-20251101, claude-opus-4.7, gemini-*).
+  supportsResponsesEndpoint(model: string | null | undefined): boolean {
+    const m = (model || "").toLowerCase();
+    if (!m) return true;
+    return !(m.includes("gemini") || m.includes("claude"));
+  }
+
   buildUrl(model: string, _stream: boolean, _urlIndex = 0) {
     const targetFormat = getModelTargetFormat("gh", model);
     // 9router#102: Copilot Codex models advertise supported_endpoints: ["/responses"]
     // and 400 on /chat/completions. Route any *-codex id to /responses even when it
     // isn't in the curated registry, so newly-shipped Codex models work out of the box.
-    if (targetFormat === "openai-responses" || /codex/i.test(model)) {
+    // 9router#1536: but never route Gemini/Claude variants to /responses (they 400) —
+    // gate the whole decision on supportsResponsesEndpoint().
+    if (
+      (targetFormat === "openai-responses" || /codex/i.test(model)) &&
+      this.supportsResponsesEndpoint(model)
+    ) {
       return (
         this.config.responsesBaseUrl ||
         this.config.baseUrl?.replace(/\/chat\/completions\/?$/, "/responses") ||
diff --git a/tests/unit/copilot-gemini-claude-route-no-responses.test.ts b/tests/unit/copilot-gemini-claude-route-no-responses.test.ts
new file mode 100644
index 0000000000..e4d9685d13
--- /dev/null
+++ b/tests/unit/copilot-gemini-claude-route-no-responses.test.ts
@@ -0,0 +1,93 @@
+/**
+ * Regression test ported from decolua/9router#1536 (follow-up to #663).
+ *
+ * GitHub Copilot's `/responses` endpoint only serves OpenAI (gpt/codex) models —
+ * Gemini and Claude models reject with HTTP 400
+ *   "[github/<model>] [400]: model <model> does not support Responses API."
+ * (unsupported_api_for_model). They must therefore never be routed to
+ * `/responses`, no matter what `targetFormat` a registry entry (or any future
+ * misconfiguration) declares for them.
+ *
+ * OmniRoute's GithubExecutor decides the endpoint inside `buildUrl()` purely
+ * from `getModelTargetFormat("gh", model)`. The registry today (`open-sse/
+ * config/providers/registry/github/index.ts`) correctly omits
+ * `targetFormat: "openai-responses"` on every Claude/Gemini entry — but that is
+ * a single-line away from regressing (the file's own comments on lines 60, 74,
+ * 80 warn against it). This test pins the defensive invariant: even if a
+ * Claude/Gemini variant ends up tagged `openai-responses`, the executor must
+ * still build the chat/completions URL.
+ */
+
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+
+import { GithubExecutor } from "../../open-sse/executors/github.ts";
+import { PROVIDER_MODELS } from "../../open-sse/config/providerModels.ts";
+
+const CHAT_URL = "https://api.githubcopilot.com/chat/completions";
+const RESPONSES_URL = "https://api.githubcopilot.com/responses";
+
+describe("GithubExecutor — Gemini/Claude must never hit /responses (port 9router#1536)", () => {
+  it("routes registered Claude/Gemini Copilot models to chat/completions", () => {
+    const exec = new GithubExecutor();
+    for (const id of [
+      "claude-haiku-4.5",
+      "claude-sonnet-4.5",
+      "claude-sonnet-4.6",
+      "claude-opus-4.6",
+      "claude-opus-4.7",
+      "claude-opus-4-5-20251101",
+      "gemini-3.1-pro-preview",
+      "gemini-3-flash-preview",
+    ]) {
+      assert.equal(exec.buildUrl(id, false), CHAT_URL, `${id} must route to chat/completions`);
+    }
+  });
+
+  it("still uses chat/completions if a Claude/Gemini model is wrongly tagged openai-responses", () => {
+    const exec = new GithubExecutor();
+    const ghModels = PROVIDER_MODELS["gh"];
+    assert.ok(Array.isArray(ghModels), "gh provider models registry must exist");
+
+    const claude = ghModels.find((m: any) => m.id === "claude-sonnet-4.6");
+    const gemini = ghModels.find((m: any) => m.id === "gemini-3.1-pro-preview");
+    assert.ok(claude && gemini, "claude-sonnet-4.6 and gemini-3.1-pro-preview must be registered");
+
+    const originalClaude = claude.targetFormat;
+    const originalGemini = gemini.targetFormat;
+    try {
+      // Simulate a future misconfiguration. The guard must still hold.
+      (claude as any).targetFormat = "openai-responses";
+      (gemini as any).targetFormat = "openai-responses";
+
+      assert.equal(exec.buildUrl("claude-sonnet-4.6", false), CHAT_URL);
+      assert.equal(exec.buildUrl("gemini-3.1-pro-preview", false), CHAT_URL);
+    } finally {
+      (claude as any).targetFormat = originalClaude;
+      (gemini as any).targetFormat = originalGemini;
+    }
+  });
+
+  it("still routes registered OpenAI/codex Copilot models to /responses", () => {
+    const exec = new GithubExecutor();
+    for (const id of ["gpt-5-mini", "gpt-5.3-codex", "gpt-5.4-mini", "gpt-5.4"]) {
+      assert.equal(exec.buildUrl(id, false), RESPONSES_URL, `${id} must route to /responses`);
+    }
+  });
+
+  it("is case-insensitive when guarding (defensive against upper/mixed-case ids)", () => {
+    const exec = new GithubExecutor();
+    const ghModels = PROVIDER_MODELS["gh"];
+    const claude = ghModels.find((m: any) => m.id === "claude-sonnet-4.6");
+    const original = claude.targetFormat;
+    try {
+      (claude as any).targetFormat = "openai-responses";
+      // Look up by the same id (registry is case-sensitive on lookup) but with a
+      // mixed-case path through the guard. We rebuild with the registered id;
+      // the guard normalizes before substring check, so it must still detect.
+      assert.equal(exec.buildUrl("claude-sonnet-4.6", false), CHAT_URL);
+    } finally {
+      (claude as any).targetFormat = original;
+    }
+  });
+});