Prevent cancellation of initialize request per MCP spec#1350
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Prevent cancellation of initialize request per MCP spec#1350
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Per MCP spec: "The initialize request MUST NOT be cancelled by clients" Client side: Skip RegisterCancellation in SendRequestAsync for initialize requests so that no notifications/cancelled is sent even if the cancellation token fires (e.g. from timeout). Server side: Don't store initialize request in _handlingRequests so incoming cancellation notifications can't cancel it. Added tests for both behaviors. Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
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@copilot fix the pr title and description |
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Updated the PR description. I'm unable to update the PR title through my available tools — could you update it to "Prevent cancellation of initialize request per MCP spec"? |
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[WIP] Fix initialize cancellation handling based on MCP spec
Prevent cancellation of initialize request per MCP spec
Feb 21, 2026
jeffhandley
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halter73 and
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Pull request overview
This pull request implements MCP specification compliance for preventing cancellation of the initialize request. Per the MCP spec (cancellation.mdx, behavior requirement #2), "The initialize request MUST NOT be cancelled by clients." The PR correctly implements this requirement on both the client and server sides.
Changes:
- Client-side: Modified
McpSessionHandler.SendRequestAsyncto skip registering cancellation callbacks for theinitializerequest method, preventing cancellation notifications from being sent - Server-side: Modified
McpSessionHandler.ProcessMessagesCoreAsyncto excludeinitializerequests from the_handlingRequeststracking dictionary, preventing incoming cancellation notifications from canceling the initialize handler - Tests: Added two comprehensive tests to verify both client-side and server-side behavior
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
| src/ModelContextProtocol.Core/McpSessionHandler.cs | Implements spec-compliant initialize request handling by skipping cancellation registration on client side (line 539) and excluding from tracking on server side (lines 199-202) |
| tests/ModelContextProtocol.Tests/Protocol/CancellationTests.cs | Adds InitializeTimeout_DoesNotSendCancellationNotification test to verify client doesn't send cancellation notification when initialize times out |
| tests/ModelContextProtocol.Tests/Server/McpServerTests.cs | Adds Server_IgnoresCancellationNotificationForInitializeRequest test to verify server ignores cancellation notifications for initialize requests |
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Per the MCP spec (cancellation.mdx, behavior requirement #2): "The
initializerequest MUST NOT be cancelled by clients."Changes
Client side (
McpSessionHandler.SendRequestAsync): SkipRegisterCancellationwhen the request method isinitialize, so nonotifications/cancellednotification is sent even if the cancellation token fires (e.g., from timeout).Server side (
McpSessionHandler.ProcessMessagesAsync): Don't store the initialize request in_handlingRequests, so incoming cancellation notifications can't cancel the initialize handler.Tests
InitializeTimeout_DoesNotSendCancellationNotification: Verifies the client doesn't send a cancellation notification when initialize times out.Server_IgnoresCancellationNotificationForInitializeRequest: Verifies the server ignores a cancellation notification targeting the initialize request and still responds successfully.🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.