fix(agents): avoid classifying reasoning-required errors as context overflow (#24593)

* Agents: exclude reasoning-required errors from overflow detection

* Tests: cover reasoning-required overflow classification guard

* Tests: format reasoning-required endpoint errors

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

@@ -35,6 +35,15 @@ describe("formatAssistantErrorText", () => {
     );
     expect(formatAssistantErrorText(msg)).toContain("Context overflow");
   });
+  it("returns a reasoning-required message for mandatory reasoning endpoint errors", () => {
+    const msg = makeAssistantError(
+      "400 Reasoning is mandatory for this endpoint and cannot be disabled.",
+    );
+    const result = formatAssistantErrorText(msg);
+    expect(result).toContain("Reasoning is required");
+    expect(result).toContain("/think minimal");
+    expect(result).not.toContain("Context overflow");
+  });
   it("returns a friendly message for Anthropic role ordering", () => {
     const msg = makeAssistantError('messages: roles must alternate between "user" and "assistant"');
     expect(formatAssistantErrorText(msg)).toContain("Message ordering conflict");

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

@@ -208,6 +208,17 @@ describe("isContextOverflowError", () => {
     expect(isContextOverflowError("We're debugging context overflow issues")).toBe(false);
     expect(isContextOverflowError("Something is causing context overflow messages")).toBe(false);
   });
+
+  it("excludes reasoning-required invalid-request errors", () => {
+    const samples = [
+      "400 Reasoning is mandatory for this endpoint and cannot be disabled.",
+      '{"type":"error","error":{"type":"invalid_request_error","message":"Reasoning is mandatory for this endpoint and cannot be disabled."}}',
+      "This model requires reasoning to be enabled",
+    ];
+    for (const sample of samples) {
+      expect(isContextOverflowError(sample)).toBe(false);
+    }
+  });
 });
 
 describe("error classifiers", () => {
@@ -286,6 +297,17 @@ describe("isLikelyContextOverflowError", () => {
       expect(isLikelyContextOverflowError(sample)).toBe(false);
     }
   });
+
+  it("excludes reasoning-required invalid-request errors", () => {
+    const samples = [
+      "400 Reasoning is mandatory for this endpoint and cannot be disabled.",
+      '{"type":"error","error":{"type":"invalid_request_error","message":"Reasoning is mandatory for this endpoint and cannot be disabled."}}',
+      "This endpoint requires reasoning",
+    ];
+    for (const sample of samples) {
+      expect(isLikelyContextOverflowError(sample)).toBe(false);
+    }
+  });
 });
 
 describe("isTransientHttpError", () => {

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

@@ -34,6 +34,19 @@ function formatRateLimitOrOverloadedErrorCopy(raw: string): string | undefined {
   return undefined;
 }
 
+function isReasoningConstraintErrorMessage(raw: string): boolean {
+  if (!raw) {
+    return false;
+  }
+  const lower = raw.toLowerCase();
+  return (
+    lower.includes("reasoning is mandatory") ||
+    lower.includes("reasoning is required") ||
+    lower.includes("requires reasoning") ||
+    (lower.includes("reasoning") && lower.includes("cannot be disabled"))
+  );
+}
+
 export function isContextOverflowError(errorMessage?: string): boolean {
   if (!errorMessage) {
     return false;
@@ -45,6 +58,10 @@ export function isContextOverflowError(errorMessage?: string): boolean {
     return false;
   }
 
+  if (isReasoningConstraintErrorMessage(errorMessage)) {
+    return false;
+  }
+
   const hasRequestSizeExceeds = lower.includes("request size exceeds");
   const hasContextWindow =
     lower.includes("context window") ||
@@ -85,6 +102,10 @@ export function isLikelyContextOverflowError(errorMessage?: string): boolean {
     return false;
   }
 
+  if (isReasoningConstraintErrorMessage(errorMessage)) {
+    return false;
+  }
+
   if (CONTEXT_WINDOW_TOO_SMALL_RE.test(errorMessage)) {
     return false;
   }
@@ -464,6 +485,13 @@ export function formatAssistantErrorText(
     );
   }
 
+  if (isReasoningConstraintErrorMessage(raw)) {
+    return (
+      "Reasoning is required for this model endpoint. " +
+      "Use /think minimal (or any non-off level) and try again."
+    );
+  }
+
   // Catch role ordering errors - including JSON-wrapped and "400" prefix variants
   if (
     /incorrect role information|roles must alternate|400.*role|"message".*role.*information/i.test(