[copilot-finds] Bug: rewindInstance and restartOrchestration discard original gRPC error cause

[github-actions]

Problem

rewindInstance and restartOrchestration in TaskHubGrpcClient (packages/durabletask-js/src/client/client.ts, lines 570–588 and 628–642) catch gRPC errors and rethrow new Error objects with user-friendly messages, but discard the original error. The { cause: e } option (available since ES2022, which this repo targets) is not used.

This means when users catch these errors, they cannot access the original gRPC ServiceError details — including gRPC metadata, status details, and the original stack trace — making it significantly harder to debug orchestration failures.

Example

try {
  await client.rewindInstance(instanceId, "retry after fix");
} catch (e) {
  // e.message is "An orchestration with the instanceId ... was not found."
  // e.cause is undefined — the original gRPC ServiceError with metadata, 
  // status details, and stack trace is lost
}

Additionally, rewindInstance uses inconsistent gRPC error detection (typeof e === "object" && "code" in e) compared to restartOrchestration which uses the more type-safe e instanceof Error && "code" in e pattern.

Root Cause

The catch blocks in both methods create new Error objects without passing { cause: e } as the second argument. The codebase already uses { cause: e } correctly in other locations:

• packages/durabletask-js/src/worker/entity-executor.ts:102
• packages/durabletask-js/src/utils/pb-helper.util.ts:434

These two client methods were not updated to follow the same pattern.

Proposed Fix

1. Add { cause: e } to all rethrown errors in both rewindInstance and restartOrchestration
2. Standardize rewindInstance gRPC error detection to use instanceof Error (consistent with restartOrchestration)

Impact

Severity: Medium — does not cause incorrect behavior, but makes debugging gRPC failures significantly harder for SDK users.

Affected scenarios: Any error handling code that catches errors from rewindInstance or restartOrchestration and inspects error.cause for diagnostic purposes.