Servlet transports block Tomcat thread

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
48/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Quiet
Tech stack
java
Domain
api, backend

Research direction

Start with HttpServletStreamableServerTransportProvider, HttpServletSseServerTransportProvider, and HttpServletStatelessServerTransport, tracing where incoming requests call .block() and how AsyncContext is used. Compare the SyncToolSpecification and AsyncToolSpecification paths, then verify that request threads are released while the reactive operation continues and completes successfully.

Written by the indexing model from the issue text.

Description

needs confirmation

Bug Description

All Servlet-based transports in the SDK (e.g., HttpServletStreamableServerTransportProvider, HttpServletSseServerTransportProvider, HttpServletStatelessServerTransport) call .block() on the Tomcat request thread when processing incoming HTTP requests.

Although AsyncContext is used to hold connections for SSE/streaming, the Tomcat thread remains parked until tool execution completes. As a result, the reactive/async architecture does not provide any real thread efficiency: the request thread stays blocked for the entire duration of the tool call.

With SyncToolSpecification and immediateExecution=false, the tool handler is executed on a separate thread (typically from the boundedElastic pool). This prevents blocking logic from running on the Tomcat thread itself, but the request thread still remains blocked waiting for completion.

With AsyncToolSpecification(via McpAsyncServer), handlers are expected to return fully asynchronous operations (e.g., non-blocking I/O). However, because the transport calls .block(), the HTTP request handling remains synchronous. The handler is not automatically scheduled on another thread unless the developer explicitly does so.

As a result, both SyncToolSpecification and AsyncToolSpecification ultimately block the Tomcat request thread until completion.

I’m not sure if this behavior is intentional. Based on the design of McpAsyncServer, I would expect the transport layer to release the Tomcat thread immediately and allow the reactive pipeline to complete asynchronously. (AsyncContext + subscribe).

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@pratik2294

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