[dotnet] Fix SendAndWaitAsync to throw OperationCanceledException on external cancellation

[Copilot]

• Understand the issue: SendAndWaitAsync always throws TimeoutException when the linked CTS fires, even if the user's cancellation token was cancelled externally
• Fix Session.cs: check if original cancellationToken is cancelled; if so call TrySetCanceled, else throw TimeoutException
• Update XML doc comment to mention OperationCanceledException
• Add E2E test SendAndWait_Throws_OperationCanceledException_When_Token_Cancelled that properly waits for ToolExecutionStartEvent before cancelling
• Add snapshot YAML auto-generated by running the test (not manually written)
• All SendAndWait tests pass, CodeQL clean

Original prompt

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This section details on the original issue you should resolve

<issue_title>[dotnet] SendAndWaitAsync throws timeout for cancelled tokens</issue_title>
<issue_description>Ref:

copilot-sdk/dotnet/src/Session.cs

Lines 204 to 205 in c263dfc

	tcs.TrySetException(new TimeoutException($"SendAndWaitAsync timed out after {effectiveTimeout}")));
	return await tcs.Task;

If the cancellation token passed to SendAndWaitAsync is cancelled externally it always throws a TimeoutException - instead of a OperationCanceledException.

This make it hard to tell to the user what's the real issue - if it's just a user cancellation or if the action indeed timed out.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [dotnet] SendAndWaitAsync throws timeout for cancelled tokens #518

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[github-actions]

Cross-SDK Consistency Review

This PR fixes a .NET-specific bug in SendAndWaitAsync where external cancellation was incorrectly reported as a timeout. The fix properly distinguishes between OperationCanceledException (user cancellation) and TimeoutException (actual timeout).

Cross-SDK Comparison

After reviewing the equivalent sendAndWait/send_and_wait/SendAndWait implementations across all SDKs:

✅ Go SDK - Already handles this correctly

• Uses context.Context which combines timeout and cancellation
• Returns ctx.Err() on line 211, which is either context.Canceled or context.DeadlineExceeded
• Reference: go/session.go:160-213

⚠️ Node.js SDK - Limited cancellation support

• Only accepts a timeout parameter (no AbortSignal)
• Cannot distinguish between user cancellation and timeout
• Reference: nodejs/src/session.ts:153-206

⚠️ Python SDK - Limited cancellation support

• Only accepts a timeout parameter (no cancellation token)
• Uses asyncio.wait_for() which raises asyncio.TimeoutError for all timeout scenarios
• Reference: python/copilot/session.py:139-198

.NET SDK - Now fixed with this PR ✅

• Accepts both TimeSpan? timeout and CancellationToken
• Properly distinguishes external cancellation from timeout after this fix

Recommendation

No action required for this PR - the fix is correct and maintains the existing .NET API surface. The .NET SDK's ability to accept an explicit CancellationToken is actually a .NET-specific pattern that aligns with framework conventions (e.g., HttpClient.SendAsync, Task.Delay).

The other SDKs use different patterns appropriate to their ecosystems:

• Go's context.Context is idiomatic and already correct
• Node.js and Python could potentially add cancellation support in future feature work, but this is not a blocking inconsistency

This PR maintains cross-SDK consistency by ensuring .NET behaves correctly within its own idiomatic patterns. ✅

AI generated by SDK Consistency Review Agent

[Copilot]

Pull request overview

Fixes .NET SDK CopilotSession.SendAndWaitAsync cancellation semantics so that externally-canceled CancellationTokens surface as cancellation (instead of being reported as a timeout), aligning the API behavior with .NET expectations and improving diagnosability for callers.

Changes:

• Update SendAndWaitAsync to complete the wait task as canceled when the caller’s CancellationToken is canceled; otherwise keep throwing TimeoutException on timeout.
• Update XML docs to include OperationCanceledException.
• Add a new E2E test + snapshot covering external cancellation during an in-flight SendAndWaitAsync.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
dotnet/src/Session.cs	Adjusts SendAndWaitAsync completion to distinguish external cancellation vs timeout and documents cancellation behavior.
dotnet/test/SessionTests.cs	Adds an E2E regression test validating cancellation behavior.
test/snapshots/session/sendandwait_throws_operationcanceledexception_when_token_cancelled.yaml	Adds the replay snapshot for the new E2E test scenario.

[Copilot]

This test creates a session without an OnPermissionRequest handler but then waits for ToolExecutionStartEvent from a shell command. In this repo, tool operations are expected to be denied-by-default when no permission handler is provided (see PermissionTests.Should_Deny_Tool_Operations_By_Default_When_No_Handler_Is_Provided), which can prevent tool.execution_start from ever arriving or can cause the turn to complete quickly with “Permission denied”, making this test flaky/slow (60s timeout in GetNextEventOfTypeAsync). Consider creating the session with new SessionConfig { OnPermissionRequest = PermissionHandler.ApproveAll } (or a handler that approves shell), and optionally pass a shorter timeout to GetNextEventOfTypeAsync to fail fast.

[Copilot]

Method name uses Token_Cancelled (double-l). In .NET APIs and the rest of the framework, the standard spelling is Canceled (single-l), e.g. OperationCanceledException / CancellationToken. Consider renaming the test (and corresponding snapshot filename) to ..._When_Token_Canceled for consistency/searchability.

Suggested change

	public async Task SendAndWait_Throws_OperationCanceledException_When_Token_Cancelled()
	public async Task SendAndWait_Throws_OperationCanceledException_When_Token_Canceled()