fix(agents): prevent false billing error replacing valid response text (#40616)

Merged via squash.

Prepared head SHA: 05179362b4
Co-authored-by: ingyukoh <6015960+ingyukoh@users.noreply.github.com>
Co-authored-by: altaywtf <9790196+altaywtf@users.noreply.github.com>
Reviewed-by: @altaywtf

src/agents/tools/sessions-helpers.ts

@@ -166,9 +166,9 @@ export function extractAssistantText(message: unknown): string | undefined {
       normalizeText: (text) => text.trim(),
     }) ?? "";
   const stopReason = (message as { stopReason?: unknown }).stopReason;
-  const errorMessage = (message as { errorMessage?: unknown }).errorMessage;
-  const errorContext =
-    stopReason === "error" || (typeof errorMessage === "string" && Boolean(errorMessage.trim()));
+  // Gate on stopReason only — a non-error response with a stale/background errorMessage
+  // should not have its content rewritten with error templates (#13935).
+  const errorContext = stopReason === "error";
 
   return joined ? sanitizeUserFacingText(joined, { errorContext }) : undefined;
 }

src/agents/tools/sessions.test.ts

@@ -199,6 +199,16 @@ describe("extractAssistantText", () => {
       "Firebase downgraded us to the free Spark plan. Check whether billing should be re-enabled.",
     );
   });
+
+  it("preserves successful turns with stale background errorMessage", () => {
+    const message = {
+      role: "assistant",
+      stopReason: "end_turn",
+      errorMessage: "insufficient credits for embedding model",
+      content: [{ type: "text", text: "Handle payment required errors in your API." }],
+    };
+    expect(extractAssistantText(message)).toBe("Handle payment required errors in your API.");
+  });
 });
 
 describe("resolveAnnounceTarget", () => {