Fix: Honor CancellationToken in DownloadModelWithProgressAsync

sdk/cs/src/FoundryLocalManager.cs

@@ -429,6 +429,9 @@ public async IAsyncEnumerable<ModelDownloadProgress> DownloadModelWithProgressAs
 
         while (!completed && (line = await reader.ReadLineAsync(ct)) is not null)
         {
+            // Check for cancellation at the start of each iteration
+            ct.ThrowIfCancellationRequested();
+
             // Check if this line contains download percentage
             if (line.StartsWith("Total", StringComparison.CurrentCultureIgnoreCase) && line.Contains("Downloading") && line.Contains('%'))
             {
@@ -437,6 +440,8 @@ public async IAsyncEnumerable<ModelDownloadProgress> DownloadModelWithProgressAs
                 if (double.TryParse(percentStr, out var percentage))
                 {
                     yield return ModelDownloadProgress.Progress(percentage);
+                    // Check cancellation after yielding progress to ensure timely response
+                    ct.ThrowIfCancellationRequested();
                 }
             }
             else if (line.Contains("[DONE]") || line.Contains("All Completed"))

sdk/cs/test/FoundryLocal.Tests/FoundryLocalManagerTest.cs

@@ -499,6 +499,85 @@ public async Task DownloadModelWithProgressAsync_Error()
         Assert.Equal("Download error occurred.", last.ErrorMessage);
     }
 
+    [Fact]
+    public async Task DownloadModelWithProgressAsync_CancellationToken_ThrowsOperationCanceledException()
+    {
+        // GIVEN - Simulate an in-progress download that will be cancelled mid-stream
+        MockCatalog(includeCuda: true);
+        MockLocalModels(); // empty to force a download
+
+        var stream = new MemoryStream();
+        // Simulate partial download progress (NOT complete)
+        // This mimics a real download that's in progress when user clicks cancel
+        for (int i = 0; i <= 50; i += 10)
+        {
+            var progressLine = Encoding.UTF8.GetBytes($"Total {i}.00% Downloading model.onnx.data\n");
+            stream.Write(progressLine, 0, progressLine.Length);
+        }
+
+        // DO NOT add [DONE] or completion JSON - this simulates incomplete download
+        // The stream will end without completion, which is realistic for cancelled operations
+        stream.Position = 0;
+
+        _mockHttp.When("/openai/download").Respond("application/json", stream);
+
+        // WHEN - User cancels after receiving some progress updates
+        using var cts = new System.Threading.CancellationTokenSource();
+        var progressList = new List<ModelDownloadProgress>();
+
+        // THEN
+        await Assert.ThrowsAsync<OperationCanceledException>(async () =>
+        {
+            await foreach (var p in _manager.DownloadModelWithProgressAsync("model-3", ct: cts.Token))
+            {
+                progressList.Add(p);
+
+                // Cancel after receiving 2 progress updates (simulating user clicking cancel button)
+                // Using 2 instead of 3 to ensure we cancel before all mock data is consumed
+                if (progressList.Count >= 2)
+                {
+                    cts.Cancel();
+                }
+            }
+        });
+
+        // Verify cancellation was responsive and happened during download
+        Assert.InRange(progressList.Count, 2, 4); // Allow for 1-2 items in async pipeline
+        Assert.All(progressList, p => Assert.False(p.IsCompleted, "Should not reach completion status"));
+        Assert.All(progressList, p => Assert.True(p.Percentage < 100, $"Progress {p.Percentage}% should be less than 100%"));
+    }
+
+    [Fact]
+    public async Task DownloadModelWithProgressAsync_AlreadyCancelledToken_ThrowsImmediately()
+    {
+        // GIVEN - Token is already cancelled before download starts
+        MockCatalog(includeCuda: true);
+        MockLocalModels(); // empty to force a download
+
+        var stream = new MemoryStream();
+        var progressLine = Encoding.UTF8.GetBytes("Total 10.00% Downloading model.onnx.data\n");
+        stream.Write(progressLine, 0, progressLine.Length);
+        stream.Position = 0;
+
+        _mockHttp.When("/openai/download").Respond("application/json", stream);
+
+        using var cts = new System.Threading.CancellationTokenSource();
+        cts.Cancel(); // Cancel BEFORE starting download
+        var progressList = new List<ModelDownloadProgress>();
+
+        // WHEN/THEN - Should throw immediately without processing any progress
+        await Assert.ThrowsAsync<TaskCanceledException>(async () =>
+        {
+            await foreach (var p in _manager.DownloadModelWithProgressAsync("model-3", ct: cts.Token))
+            {
+                progressList.Add(p);
+            }
+        });
+
+        // Should not receive any progress updates since token was already cancelled
+        Assert.Empty(progressList);
+    }
+
     [Fact]
     public async Task LoadModelAsync_Succeeds_AndPassesEpOverrideWhenCudaPresent()
     {