[copilot-finds] Bug: TestOrchestrationClient serializes null values differently from TaskHubGrpcClient

[github-actions]

Problem

TestOrchestrationClient.raiseOrchestrationEvent and terminateOrchestration handle null data differently from the real TaskHubGrpcClient, causing behavioral divergence between test and production environments.

File: packages/durabletask-js/src/testing/test-client.ts (lines 95, 103)

Real client behavior (TaskHubGrpcClient)

Both raiseOrchestrationEvent (line 415 of client.ts) and terminateOrchestration (line 485) unconditionally call JSON.stringify(data):

const i = new StringValue();
i.setValue(JSON.stringify(data));  // null → "null"
req.setInput(i);

This means even when data is null (the default parameter), the string "null" is stored. When deserialized by the executor, the orchestrator receives null.

Test client behavior (TestOrchestrationClient)

Both methods used a guard that skipped serialization for null:

const encodedData = data !== null ? JSON.stringify(data) : undefined;

When data is null, encodedData becomes undefined, so no input is stored. When deserialized by the executor, the orchestrator receives undefined instead of null.

Root Cause

The conditional data !== null ? JSON.stringify(data) : undefined was intended to avoid serializing "no data," but it conflicts with the real client's behavior of always serializing — including null. The real client treats null as a valid serializable value, not as "absent."

Proposed Fix

Change both methods to unconditionally call JSON.stringify(data), matching the real client:

const encodedData = JSON.stringify(data);

Impact

Severity: Medium — Causes silent behavioral divergence between test and production.

Affected scenarios:

• Orchestrations that call waitForExternalEvent and check the received value with strict equality (=== null) would get true in production but false in tests.
• Orchestrations terminated with default output would see null in production but undefined in tests when reading serializedOutput.
• Any test relying on the in-memory backend to faithfully reproduce production semantics for null event data is unreliable.

[github-actions]

Ready for PR

The fix is implemented on branch copilot-finds/bug/fix-test-client-null-serialization.

The GitHub Actions token does not have permission to create PRs in this repository. A maintainer can create the PR from the branch above.

Changes

• packages/durabletask-js/src/testing/test-client.ts — Fixed raiseOrchestrationEvent and terminateOrchestration to unconditionally serialize data (matching TaskHubGrpcClient behavior)
• packages/durabletask-js/test/test-client-serialization.spec.ts — Added 7 unit tests verifying correct null serialization

Verification

• All 937 existing unit tests pass (818 core + 64 Azure managed + 55 export history)
• ESLint passes with zero errors
• All 7 new tests pass