fix(translator): clamp thinking effort max->xhigh for OpenAI format (#2466)

Claude Code sends reasoning_effort "max" (its top level); OpenAI enum caps
at "xhigh" and rejects "max" with HTTP 400 "max effort not support".
applyFormat case "openai" now clamps "max"->"xhigh" before assigning
body.reasoning_effort; other levels pass through unchanged.

Add regression test covering client output_config.effort, direct
reasoning_effort, passthrough of xhigh/high, and budget_tokens capping.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/unit/thinking-effort-openai-max-clamp.test.js

@@ -0,0 +1,40 @@
+import { describe, expect, it } from "vitest";
+import { applyThinking } from "../../open-sse/translator/concerns/thinkingUnified.js";
+import { FORMATS } from "../../open-sse/translator/formats.js";
+
+// Regression: Claude Code sends thinking effort "max" (its top level). When
+// 9router routes to an OpenAI-format provider, applyThinking() case "openai"
+// must clamp "max"→"xhigh" because OpenAI's reasoning_effort enum has no "max"
+// (L.openai caps at "xhigh"). Without the clamp, upstream returns HTTP 400
+// "max effort not support". See open-sse/providers/thinkingLevels.js:10.
+describe("applyThinking (openai): clamp max effort to xhigh", () => {
+  it("client output_config.effort:\"max\" → reasoning_effort:\"xhigh\" (not \"max\")", () => {
+    const body = { output_config: { effort: "max" } };
+    const out = applyThinking(FORMATS.OPENAI, "gpt-5", body, "openai");
+    expect(out.reasoning_effort).toBe("xhigh");
+  });
+
+  it("direct reasoning_effort:\"max\" clamped to \"xhigh\"", () => {
+    const body = { reasoning_effort: "max" };
+    const out = applyThinking(FORMATS.OPENAI, "gpt-5", body, "openai");
+    expect(out.reasoning_effort).toBe("xhigh");
+  });
+
+  it("\"xhigh\" passes through unchanged (highest valid OpenAI level)", () => {
+    const body = { reasoning_effort: "xhigh" };
+    const out = applyThinking(FORMATS.OPENAI, "gpt-5", body, "openai");
+    expect(out.reasoning_effort).toBe("xhigh");
+  });
+
+  it("\"high\" passes through unchanged", () => {
+    const body = { reasoning_effort: "high" };
+    const out = applyThinking(FORMATS.OPENAI, "gpt-5", body, "openai");
+    expect(out.reasoning_effort).toBe("high");
+  });
+
+  it("max budget (thinking.budget_tokens:128000) → reasoning_effort:\"xhigh\" (budgetToLevel caps at xhigh)", () => {
+    const body = { thinking: { type: "enabled", budget_tokens: 128000 } };
+    const out = applyThinking(FORMATS.OPENAI, "gpt-5", body, "openai");
+    expect(out.reasoning_effort).toBe("xhigh");
+  });
+});