Cancellation support via CancellationToken's fails

[mitchcapper]

Describe the bug
On a tool call the client does take a CancellationToken however it seems to just abort the http request and not send a cancellation notification. The server side tool calls are shown taking CancellationToken's but I don't see what ever triggers them in that normal tool flow.

The client should send a cancel notification when the cancel token is used, and the server should trigger the tool cancel token both if it gets the cancel notification or if the connection drops.

working on a PR to do so.

[stephentoub]

however it seems to just abort the http request and not send a cancellation notification

It does send a cancellation notification:

csharp-sdk/src/ModelContextProtocol.Core/McpSessionHandler.cs

Line 533 in 9b2aaed

// Now that the request has been sent, register for cancellation. If we registered before,

csharp-sdk/src/ModelContextProtocol.Core/McpSessionHandler.cs

Line 444 in 9b2aaed

return cancellationToken.Register(static objState =>

[stephentoub]

and the server should trigger the tool cancel token both if it gets the cancel notification or if the connection drops.

It does:

csharp-sdk/src/ModelContextProtocol.Core/McpSessionHandler.cs

Line 80 in ce2a5e4

/// <summary>

csharp-sdk/src/ModelContextProtocol.Core/McpSessionHandler.cs

Line 383 in ce2a5e4

// Special-case cancellation to cancel a pending operation. (We'll still subsequently invoke a user-specified handler if one exists.)

[stephentoub]

Please share a repro of what's not working.

[mitchcapper]

Sorry should have included that upfront was trying to figure out a good way to do a socket close in the test cases but here is a console app that at least shows it off well.

Here is a simple repro:
https://github.com/mitchcapper/__mcp_cancel_fail

by default it just uses a cancelation token, which fails.
if you pass --disposeMCS it will call dispose on the MCP client itself, this does work, server properly gets cancelled.
if you pass --disposeSocket it closes the underlying socket not giving the MCP client a chance to gracefully abort (say an app crash).

With preview2 only disposeMCS works, with the PR it ties more cancel tokens together and all 3 cases abort properly.

You can see the output log of with pr: https://github.com/mitchcapper/__mcp_cancel_fail/blob/master/preview_ours.log

and without: https://github.com/mitchcapper/__mcp_cancel_fail/blob/master/preview2.log