fix(agents): land #31002 from @yfge

Co-authored-by: yfge <geyunfei@gmail.com>

src/agents/pi-embedded-helpers/thinking.test.ts

@@ -0,0 +1,60 @@
+import { describe, expect, it } from "vitest";
+import { pickFallbackThinkingLevel } from "./thinking.js";
+
+describe("pickFallbackThinkingLevel", () => {
+  it("returns undefined for empty message", () => {
+    expect(pickFallbackThinkingLevel({ message: "", attempted: new Set() })).toBeUndefined();
+  });
+
+  it("returns undefined for undefined message", () => {
+    expect(pickFallbackThinkingLevel({ message: undefined, attempted: new Set() })).toBeUndefined();
+  });
+
+  it("extracts supported values from error message", () => {
+    const result = pickFallbackThinkingLevel({
+      message: 'Supported values are: "high", "medium"',
+      attempted: new Set(),
+    });
+    expect(result).toBe("high");
+  });
+
+  it("skips already attempted values", () => {
+    const result = pickFallbackThinkingLevel({
+      message: 'Supported values are: "high", "medium"',
+      attempted: new Set(["high"]),
+    });
+    expect(result).toBe("medium");
+  });
+
+  it('falls back to "off" when error says "not supported" without listing values', () => {
+    const result = pickFallbackThinkingLevel({
+      message: '400 think value "low" is not supported for this model',
+      attempted: new Set(),
+    });
+    expect(result).toBe("off");
+  });
+
+  it('falls back to "off" for generic not-supported messages', () => {
+    const result = pickFallbackThinkingLevel({
+      message: "thinking level not supported by this provider",
+      attempted: new Set(),
+    });
+    expect(result).toBe("off");
+  });
+
+  it('returns undefined if "off" was already attempted', () => {
+    const result = pickFallbackThinkingLevel({
+      message: '400 think value "low" is not supported for this model',
+      attempted: new Set(["off"]),
+    });
+    expect(result).toBeUndefined();
+  });
+
+  it("returns undefined for unrelated error messages", () => {
+    const result = pickFallbackThinkingLevel({
+      message: "rate limit exceeded, please retry after 30 seconds",
+      attempted: new Set(),
+    });
+    expect(result).toBeUndefined();
+  });
+});

src/agents/pi-embedded-helpers/thinking.ts

@@ -29,6 +29,14 @@ export function pickFallbackThinkingLevel(params: {
   }
   const supported = extractSupportedValues(raw);
   if (supported.length === 0) {
+    // When the error clearly indicates the thinking level is unsupported but doesn't
+    // list supported values (e.g. OpenAI's "think value \"low\" is not supported for
+    // this model"), fall back to "off" to allow the request to succeed.
+    // This commonly happens during model fallback when switching from Anthropic
+    // (which supports thinking levels) to providers that don't.
+    if (/not supported/i.test(raw) && !params.attempted.has("off")) {
+      return "off";
+    }
     return undefined;
   }
   for (const entry of supported) {