Revert terminal task cancellation validation to restore idempotent behavior

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

docs/concepts/tasks/tasks.md

@@ -424,7 +424,7 @@ Task operations may throw <xref:ModelContextProtocol.McpException> with these er
 
 | Error Code | Scenario |
 |------------|----------|
-| `InvalidParams` | Invalid or nonexistent task ID, invalid cursor, or attempting to cancel a task already in a terminal status |
+| `InvalidParams` | Invalid or nonexistent task ID or invalid cursor |
 | `InvalidParams` | Tool with `taskSupport: forbidden` called with task metadata, or tool with `taskSupport: required` called without task metadata |
 | `InternalError` | Task execution failure or result unavailable |
 

src/ModelContextProtocol.Core/Client/McpClientImpl.cs

@@ -461,21 +461,6 @@ private void RegisterTaskHandlers(RequestHandlers requestHandlers, IMcpTaskStore
                     throw new McpProtocolException("Missing required parameter 'taskId'", McpErrorCode.InvalidParams);
                 }
 
-                // Get current task status
-                var currentTask = await taskStore.GetTaskAsync(taskId, SessionId, cancellationToken).ConfigureAwait(false);
-                if (currentTask is null)
-                {
-                    throw new McpProtocolException($"Task not found: '{taskId}'", McpErrorCode.InvalidParams);
-                }
-
-                // Validate not already in terminal status
-                if (currentTask.Status is McpTaskStatus.Completed or McpTaskStatus.Failed or McpTaskStatus.Cancelled)
-                {
-                    throw new McpProtocolException(
-                        $"Cannot cancel task '{taskId}' in terminal status '{currentTask.Status}'.",
-                        McpErrorCode.InvalidParams);
-                }
-
                 // Signal cancellation if task is still running
                 _taskCancellationTokenProvider!.Cancel(taskId);
 

src/ModelContextProtocol.Core/McpErrorCode.cs

@@ -67,7 +67,7 @@ public enum McpErrorCode
     /// <item><description><b>Prompts</b>: Unknown prompt name or missing required arguments.</description></item>
     /// <item><description><b>Pagination</b>: Invalid or expired cursor values.</description></item>
     /// <item><description><b>Logging</b>: Invalid log level.</description></item>
-    /// <item><description><b>Tasks</b>: Invalid or nonexistent task ID, invalid cursor, or attempting to cancel a task already in a terminal status.</description></item>
+    /// <item><description><b>Tasks</b>: Invalid or nonexistent task ID or invalid cursor.</description></item>
     /// <item><description><b>Elicitation</b>: Server requests an elicitation mode not declared in client capabilities.</description></item>
     /// <item><description><b>Sampling</b>: Missing tool result or tool results mixed with other content.</description></item>
     /// </list>

src/ModelContextProtocol.Core/Server/McpServerImpl.cs

@@ -771,21 +771,6 @@ async Task<JsonElement> GetTaskResultAsync(RequestContext<GetTaskPayloadRequestP
                 throw new McpProtocolException("Missing required parameter 'taskId'", McpErrorCode.InvalidParams);
             }
 
-            // Get current task status
-            var currentTask = await taskStore.GetTaskAsync(taskId, SessionId, cancellationToken).ConfigureAwait(false);
-            if (currentTask is null)
-            {
-                throw new McpProtocolException($"Task not found: '{taskId}'", McpErrorCode.InvalidParams);
-            }
-
-            // Validate not already in terminal status
-            if (currentTask.Status is McpTaskStatus.Completed or McpTaskStatus.Failed or McpTaskStatus.Cancelled)
-            {
-                throw new McpProtocolException(
-                    $"Cannot cancel task '{taskId}' in terminal status '{currentTask.Status}'.",
-                    McpErrorCode.InvalidParams);
-            }
-
             // Signal cancellation if task is still running
             _taskCancellationTokenProvider!.Cancel(taskId);
 

tests/ModelContextProtocol.Tests/Server/TaskCancellationIntegrationTests.cs

@@ -460,14 +460,13 @@ public async Task CompletedTask_CannotTransitionToOtherStatus()
 
         Assert.Equal(McpTaskStatus.Completed, taskStatus.Status);
 
-        // Act & Assert - Try to cancel a completed task should throw InvalidParams
-        var exception = await Assert.ThrowsAsync<McpProtocolException>(async () =>
-            await client.CancelTaskAsync(taskId, cancellationToken: TestContext.Current.CancellationToken));
+        // Act - Try to cancel a completed task (should be idempotent)
+        var cancelResult = await client.CancelTaskAsync(taskId, cancellationToken: TestContext.Current.CancellationToken);
 
-        Assert.Equal(McpErrorCode.InvalidParams, exception.ErrorCode);
-        Assert.Contains("terminal status", exception.Message, StringComparison.OrdinalIgnoreCase);
+        // Assert - Status should still be completed (not cancelled)
+        Assert.Equal(McpTaskStatus.Completed, cancelResult.Status);
 
-        // Verify task status unchanged
+        // Verify via get
         var verifyStatus = await client.GetTaskAsync(taskId, cancellationToken: TestContext.Current.CancellationToken);
         Assert.Equal(McpTaskStatus.Completed, verifyStatus.Status);
     }
@@ -501,11 +500,10 @@ public async Task FailedTask_CannotTransitionToOtherStatus()
 
         Assert.Equal(McpTaskStatus.Failed, taskStatus.Status);
 
-        // Act & Assert - Try to cancel a failed task should throw InvalidParams
-        var exception = await Assert.ThrowsAsync<McpProtocolException>(async () =>
-            await client.CancelTaskAsync(taskId, cancellationToken: TestContext.Current.CancellationToken));
+        // Act - Try to cancel a failed task (should be idempotent)
+        var cancelResult = await client.CancelTaskAsync(taskId, cancellationToken: TestContext.Current.CancellationToken);
 
-        Assert.Equal(McpErrorCode.InvalidParams, exception.ErrorCode);
-        Assert.Contains("terminal status", exception.Message, StringComparison.OrdinalIgnoreCase);
+        // Assert - Status should still be failed
+        Assert.Equal(McpTaskStatus.Failed, cancelResult.Status);
     }
 }