[dotnet] Fix SendAndWaitAsync to throw OperationCanceledException on external cancellation (#543)

Author: Copilot

dotnet/src/Session.cs

@@ -147,6 +147,7 @@ public async Task<string> SendAsync(MessageOptions options, CancellationToken ca
     /// <param name="cancellationToken">A <see cref="CancellationToken"/> that can be used to cancel the operation.</param>
     /// <returns>A task that resolves with the final assistant message event, or null if none was received.</returns>
     /// <exception cref="TimeoutException">Thrown if the timeout is reached before the session becomes idle.</exception>
+    /// <exception cref="OperationCanceledException">Thrown if the <paramref name="cancellationToken"/> is cancelled.</exception>
     /// <exception cref="InvalidOperationException">Thrown if the session has been disposed.</exception>
     /// <remarks>
     /// <para>
@@ -201,7 +202,12 @@ void Handler(SessionEvent evt)
         cts.CancelAfter(effectiveTimeout);
 
         using var registration = cts.Token.Register(() =>
-            tcs.TrySetException(new TimeoutException($"SendAndWaitAsync timed out after {effectiveTimeout}")));
+        {
+            if (cancellationToken.IsCancellationRequested)
+                tcs.TrySetCanceled(cancellationToken);
+            else
+                tcs.TrySetException(new TimeoutException($"SendAndWaitAsync timed out after {effectiveTimeout}"));
+        });
         return await tcs.Task;
     }
 

dotnet/test/SessionTests.cs

@@ -403,6 +403,30 @@ public async Task SendAndWait_Throws_On_Timeout()
         Assert.Contains("timed out", ex.Message);
     }
 
+    [Fact]
+    public async Task SendAndWait_Throws_OperationCanceledException_When_Token_Cancelled()
+    {
+        var session = await Client.CreateSessionAsync();
+
+        // Set up wait for tool execution to start BEFORE sending
+        var toolStartTask = TestHelper.GetNextEventOfTypeAsync<ToolExecutionStartEvent>(session);
+
+        using var cts = new CancellationTokenSource();
+
+        // Start SendAndWaitAsync - don't await it yet
+        var sendTask = session.SendAndWaitAsync(
+            new MessageOptions { Prompt = "run the shell command 'sleep 10' (note this works on both bash and PowerShell)" },
+            cancellationToken: cts.Token);
+
+        // Wait for the tool to begin executing before cancelling
+        await toolStartTask;
+
+        // Cancel the token
+        cts.Cancel();
+
+        await Assert.ThrowsAnyAsync<OperationCanceledException>(() => sendTask);
+    }
+
     [Fact]
     public async Task Should_Create_Session_With_Custom_Config_Dir()
     {

test/snapshots/session/sendandwait_throws_operationcanceledexception_when_token_cancelled.yaml

@@ -0,0 +1,24 @@
+models:
+  - claude-sonnet-4.5
+conversations:
+  - messages:
+      - role: system
+        content: ${system}
+      - role: user
+        content: run the shell command 'sleep 10' (note this works on both bash and PowerShell)
+      - role: assistant
+        content: I'll run the sleep command for you.
+      - role: assistant
+        tool_calls:
+          - id: toolcall_0
+            type: function
+            function:
+              name: report_intent
+              arguments: '{"intent":"Running sleep command"}'
+      - role: assistant
+        tool_calls:
+          - id: toolcall_1
+            type: function
+            function:
+              name: ${shell}
+              arguments: '{"command":"sleep 10","description":"Execute sleep 10 command","initial_wait":15,"mode":"sync"}'