fix(errors): dedupe identical messages when traversing error .cause chain (#84556)

Merged via squash.

Prepared head SHA: 46aa27fa12
Co-authored-by: RomneyDa <6581799+RomneyDa@users.noreply.github.com>
Co-authored-by: altaywtf <9790196+altaywtf@users.noreply.github.com>
Reviewed-by: @altaywtf

src/infra/errors.test.ts

@@ -88,6 +88,13 @@ describe("error helpers", () => {
     expect(formatted).toBe("error A | error B");
   });
 
+  it("dedupes repeated cause messages while preserving deeper distinct causes", () => {
+    const rootCause = new Error("provider auth lookup failed");
+    const inner = new Error('No API key found for provider "openai-codex".', { cause: rootCause });
+    const wrapper = new Error(inner.message, { cause: inner });
+    expect(formatErrorMessage(wrapper)).toBe(`${inner.message} | ${rootCause.message}`);
+  });
+
   it("redacts sensitive tokens from formatted error messages", () => {
     const token = "sk-abcdefghijklmnopqrstuv";
     const formatted = formatErrorMessage(new Error(`Authorization: Bearer ${token}`));

src/infra/errors.ts

@@ -72,15 +72,22 @@ export function formatErrorMessage(err: unknown): string {
     // Traverse .cause chain to include nested error messages (e.g. grammY HttpError wraps network errors in .cause)
     let cause: unknown = err.cause;
     const seen = new Set<unknown>([err]);
+    // Skip causes that repeat a message already emitted (e.g. coerceToFailoverError).
+    const seenMessages = new Set<string>([formatted]);
+    const appendCauseMessage = (message: string): void => {
+      if (!message || seenMessages.has(message)) {
+        return;
+      }
+      formatted += ` | ${message}`;
+      seenMessages.add(message);
+    };
     while (cause && !seen.has(cause)) {
       seen.add(cause);
       if (cause instanceof Error) {
-        if (cause.message) {
-          formatted += ` | ${cause.message}`;
-        }
+        appendCauseMessage(cause.message);
         cause = cause.cause;
       } else if (typeof cause === "string") {
-        formatted += ` | ${cause}`;
+        appendCauseMessage(cause);
         break;
       } else {
         break;