fix(async-retryer): preserve non-Error message and cause in normalized errors

.changeset/big-bikes-wash.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/pacer': patch
+---
+
+Fix AsyncRetryer error normalization to preserve plain-object message values and attach the original thrown value as cause.

packages/pacer/src/async-retryer.ts

@@ -538,7 +538,17 @@ export class AsyncRetryer<TFn extends AnyAsyncFunction> {
         ) {
           return undefined
         }
-        lastError = error instanceof Error ? error : new Error(String(error))
+        lastError =
+          error instanceof Error
+            ? error
+            : new Error(
+                typeof error === 'object' &&
+                  error !== null &&
+                  typeof (error as any).message === 'string'
+                  ? (error as any).message
+                  : String(error),
+                { cause: error },
+              )
         this.#setState({ lastError })
 
         // Call onError for every error (including during retries)

packages/pacer/tests/async-retryer.test.ts

@@ -387,6 +387,24 @@ describe('AsyncRetryer', () => {
       expect(onLastError).toHaveBeenCalledTimes(1)
       expect(onLastError).toHaveBeenCalledWith(error, retryer)
     })
+
+    it('should preserve plain object error message and cause', async () => {
+      const originalError = { message: 'readable error', code: 'E001' }
+      const mockFn = vi.fn().mockRejectedValue(originalError)
+      const retryer = new AsyncRetryer(mockFn, {
+        maxAttempts: 1,
+        throwOnError: true,
+      })
+
+      try {
+        await retryer.execute()
+        expect.unreachable('Expected execute to throw')
+      } catch (error) {
+        expect(error).toBeInstanceOf(Error)
+        expect((error as Error).message).toBe('readable error')
+        expect((error as Error & { cause?: unknown }).cause).toBe(originalError)
+      }
+    })
   })
 
   describe('State Management', () => {