Expose optional stdin/stdout on MCPServer stdio entrypoints#2343
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mangelajo wants to merge 3 commits intomodelcontextprotocol:mainfrom
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Expose optional stdin/stdout on MCPServer stdio entrypoints#2343mangelajo wants to merge 3 commits intomodelcontextprotocol:mainfrom
mangelajo wants to merge 3 commits intomodelcontextprotocol:mainfrom
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MCPServer.run_stdio_async and run(..., transport="stdio") now accept optional async text streams, matching mcp.server.stdio.stdio_server. This supports hosts that redirect process stdout for logging while keeping a dedicated MCP JSON-RPC stream. Made-with: Cursor
Made-with: Cursor
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mangelajo
commented
Mar 25, 2026
The SSE and streamable-http overloads declare transport-specific params but the implementation catches them all via **kwargs. Follow the same pattern for stdin/stdout so the implementation signature stays uniform. Made-with: Cursor
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Summary
mcp.server.stdio.stdio_serveralready supports optionalstdinandstdout. This PR threads those parameters throughMCPServer.run_stdio_asyncand thestdiobranch ofMCPServer.run, so applications can use the public API instead of callingstdio_servermanually and depending on internal wiring.Motivation
MCP over stdio is JSON-RPC on a dedicated pair of streams. In real processes, fd 1 is also used for logging,
print(), subprocesses, and native code. Any extra bytes on the protocol stream corrupt JSON-RPC and break clients.A common pattern is to duplicate the real stdout fd for MCP-only I/O and redirect fd 1 (and often
sys.stdout) to stderr so other output stays off the wire. That requires passing customstdin/stdoutinto the same code path the server uses for stdio.stdio_serveralready accepts them; this exposes the same capability onMCPServer.Testing
Adds
tests/server/mcpserver/test_run_stdio_custom_streams.py, which asserts custom streams are forwarded tostdio_server(withstdio_serverand low-levelrunstubbed).Example consumer
Downstream servers jumpstarter-dev/jumpstarter#383 need this to keep logs and tooling output off the MCP stream while staying on supported APIs.
We have a lot of logging originating from gRPC and other libraries that we want to keep out of the channel, and we are using async io.