Is the Streamable HTTP servlet actually async?

[JKobyP]

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Question Category

• Protocol Specification
• SDK Usage
• Server Implementation
• General Implementation
• Documentation
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Your Question

When instantiating an async MCP server and requesting a tool, I expected the server to execute the tool call in an asynchronous thread. It appears that the call is being handled in a servlet thread instead.

I took a look at the call stack when the call handler was being invoked and I spotted this line -

java-sdk/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletStreamableServerTransportProvider.java

Line 487 in 95323b3

.block();

Is the async MCP server truly async? What does it mean for the server to be async if the tool calls are blocking on the servlet thread? Am I misunderstanding the intention of this design?

[JKobyP]

It seems like this test class would help to establish whether the async tool call is using an asynchronous thread, but it seems like the test class that exercises this may have been refactored out (or at least renamed) in recent versions - https://github.com/modelcontextprotocol/java-sdk/blob/main/mcp-core/src/test/java/io/modelcontextprotocol/server/transport/McpTestServletFilter.java