diff --git a/src/agents/pi-embedded-runner/extra-params.xai-tool-payload.test.ts b/src/agents/pi-embedded-runner/extra-params.xai-tool-payload.test.ts
index f832e83fd2e..f402256c3e2 100644
--- a/src/agents/pi-embedded-runner/extra-params.xai-tool-payload.test.ts
+++ b/src/agents/pi-embedded-runner/extra-params.xai-tool-payload.test.ts
@@ -43,6 +43,29 @@ describe("extra-params: xAI tool payload compatibility", () => {
     expect(payload.tools?.[0]?.function).not.toHaveProperty("strict");
   });
 
+  it("strips xai Responses reasoning payload fields", () => {
+    const payload = runExtraParamsCase({
+      applyProvider: "xai",
+      applyModelId: "grok-4.20-beta-latest-reasoning",
+      model: {
+        api: "openai-responses",
+        provider: "xai",
+        id: "grok-4.20-beta-latest-reasoning",
+      } as Model<"openai-responses">,
+      payload: {
+        model: "grok-4.20-beta-latest-reasoning",
+        input: [],
+        reasoning: { effort: "high", summary: "auto" },
+        reasoningEffort: "high",
+        reasoning_effort: "high",
+      },
+    }).payload as Record<string, unknown>;
+
+    expect(payload).not.toHaveProperty("reasoning");
+    expect(payload).not.toHaveProperty("reasoningEffort");
+    expect(payload).not.toHaveProperty("reasoning_effort");
+  });
+
   it("keeps function.strict for non-xai providers", () => {
     const payload = runToolPayloadCase("openai", "gpt-5.4");