diff --git a/src/infra/errors.test.ts b/src/infra/errors.test.ts
index 5f8d274a628..e2b6e856c9a 100644
--- a/src/infra/errors.test.ts
+++ b/src/infra/errors.test.ts
@@ -88,10 +88,11 @@ describe("error helpers", () => {
     expect(formatted).toBe("error A | error B");
   });
 
-  it("dedupes causes whose message repeats the parent (e.g. FailoverError wrapping)", () => {
-    const inner = new Error('No API key found for provider "openai-codex".');
+  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);
+    expect(formatErrorMessage(wrapper)).toBe(`${inner.message} | ${rootCause.message}`);
   });
 
   it("redacts sensitive tokens from formatted error messages", () => {
diff --git a/src/infra/errors.ts b/src/infra/errors.ts
index 07a3ffcc68f..b9f62dd3236 100644
--- a/src/infra/errors.ts
+++ b/src/infra/errors.ts
@@ -74,18 +74,20 @@ export function formatErrorMessage(err: unknown): string {
     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 && !seenMessages.has(cause.message)) {
-          formatted += ` | ${cause.message}`;
-          seenMessages.add(cause.message);
-        }
+        appendCauseMessage(cause.message);
         cause = cause.cause;
       } else if (typeof cause === "string") {
-        if (!seenMessages.has(cause)) {
-          formatted += ` | ${cause}`;
-        }
+        appendCauseMessage(cause);
         break;
       } else {
         break;