diff --git a/docs/tools/thinking.md b/docs/tools/thinking.md
index d23f446b71a..950a3614144 100644
--- a/docs/tools/thinking.md
+++ b/docs/tools/thinking.md
@@ -13,7 +13,7 @@ title: "Thinking levels"
   - low → “think hard”
   - medium → “think harder”
   - high → “ultrathink” (max budget)
-  - xhigh → “ultrathink+” (GPT-5.2 + Codex models and Anthropic Claude Opus 4.7 effort)
+  - xhigh → “ultrathink+” (GPT-5.2+ and Codex models, plus Anthropic Claude Opus 4.7 effort)
   - adaptive → provider-managed adaptive thinking (supported for Claude 4.6 on Anthropic/Bedrock and Anthropic Claude Opus 4.7)
   - max → provider max reasoning (currently Anthropic Claude Opus 4.7)
   - `x-high`, `x_high`, `extra-high`, `extra high`, and `extra_high` map to `xhigh`.
diff --git a/src/agents/openai-reasoning-effort.test.ts b/src/agents/openai-reasoning-effort.test.ts
new file mode 100644
index 00000000000..09e9af74e5a
--- /dev/null
+++ b/src/agents/openai-reasoning-effort.test.ts
@@ -0,0 +1,27 @@
+import { describe, expect, it } from "vitest";
+import {
+  resolveOpenAIReasoningEffortForModel,
+  resolveOpenAISupportedReasoningEfforts,
+} from "./openai-reasoning-effort.js";
+
+describe("OpenAI reasoning effort support", () => {
+  it.each([
+    { provider: "openai", id: "gpt-5.5" },
+    { provider: "openai-codex", id: "gpt-5.5" },
+  ])("preserves xhigh for $provider/$id", (model) => {
+    expect(resolveOpenAISupportedReasoningEfforts(model)).toContain("xhigh");
+    expect(resolveOpenAIReasoningEffortForModel({ model, effort: "xhigh" })).toBe("xhigh");
+  });
+
+  it("does not downgrade xhigh when Pi compat metadata declares it explicitly", () => {
+    const model = {
+      provider: "openai-codex",
+      id: "gpt-5.5",
+      compat: {
+        supportedReasoningEfforts: ["low", "medium", "high", "xhigh"],
+      },
+    };
+
+    expect(resolveOpenAIReasoningEffortForModel({ model, effort: "xhigh" })).toBe("xhigh");
+  });
+});
diff --git a/src/agents/pi-embedded-runner/openai-stream-wrappers.test.ts b/src/agents/pi-embedded-runner/openai-stream-wrappers.test.ts
index a29001c5d11..663727a8922 100644
--- a/src/agents/pi-embedded-runner/openai-stream-wrappers.test.ts
+++ b/src/agents/pi-embedded-runner/openai-stream-wrappers.test.ts
@@ -158,4 +158,25 @@ describe("createOpenAIThinkingLevelWrapper", () => {
       expect(payloads[0]?.reasoning).toEqual({ effort: level });
     }
   });
+
+  it.each([
+    {
+      api: "openai-responses",
+      provider: "openai",
+      id: "gpt-5.5",
+    },
+    {
+      api: "openai-codex-responses",
+      provider: "openai-codex",
+      id: "gpt-5.5",
+    },
+  ] as const)("preserves xhigh for $provider/$id", (model) => {
+    const { baseStreamFn, payloads } = createPayloadCapture({
+      initialReasoning: { effort: "high" },
+    });
+    const wrapped = createOpenAIThinkingLevelWrapper(baseStreamFn, "xhigh");
+    void wrapped(model as Model<typeof model.api>, { messages: [] }, {});
+
+    expect(payloads[0]?.reasoning).toEqual({ effort: "xhigh" });
+  });
 });