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/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(