Pin fmt layer writer to stderr#97
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tracing-subscriber's default writer is stdout, so logs were leaking into the JSON-RPC stream when mcp serve runs over stdio. The doc comment already promised stderr, so the code was lying about its own behavior. Set with_writer(`std::io::stderr`) explicitly on both json and text branches. Signed-off-by: Avelino <31996+avelino@users.noreply.github.com>
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Pull request overview
This PR fixes mcp’s tracing output stream selection so that structured logs no longer leak into stdout (and therefore into the JSON-RPC/stdout protocol stream when running over stdio). It makes the implementation match the existing doc comment that promised stderr for JSON logging.
Changes:
- Pin
tracing_subscriber::fmtlayer output tostderrforMCP_LOG_FORMAT=json. - Pin
tracing_subscriber::fmtlayer output tostderrfor the default/text branch as well.
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tracing-subscriber's default writer is stdout, so logs were leaking into the JSON-RPC stream when mcp serve runs over stdio. The doc comment already promised stderr, so the code was lying about its own behavior.
Set with_writer(
std::io::stderr) explicitly on both json and text branches.fixed: #96