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Potential encoding issue in STDIO protocol #295
Description
Bug description
It appears the implementation for StdioServerTransportProvider could potentially mismatch encodings between input and output.
The startOutboundProcessing method will always force the encoding to be UTF-8:
synchronized (outputStream) {
outputStream.write(jsonMessage.getBytes(StandardCharsets.UTF_8));
outputStream.write("\n".getBytes(StandardCharsets.UTF_8));
outputStream.flush();
}However, the startInboundProcessing method doesn't appear to specify an encoding when creating the InputStreamReader object, and so it could be subject to the default charset selected by the JRE:
reader = new BufferedReader(new InputStreamReader(inputStream));Environment
Testing with 0.10.0
Steps to reproduce
Running a STDIO-based MCP server with the JRE default charset set to something other than UTF-8.
Expected behavior
I'd expect charset/encoding handling should be consistent between input and output.
Activity
chemicL commentedon Feb 18, 2026
Thank you for pointing it out. I think #260 is related. I'm marking this as a good first issue and would welcome analysis and a contribution if someone is able to provide a unit test that fails and a fix.
Fix potential encoding issue in STDIO transport
Fix potential encoding issue in STDIO transport
Specify UTF-8 charset explicitly in STDIO transport InputStreamReader
Specify UTF-8 charset explicitly in STDIO transport InputStreamReader
Remove client-side changes outside the scope of issue modelcontextpro…
Use explicit UTF-8 charset in StdioServerTransportProvider (#826)
Use explicit UTF-8 charset in StdioServerTransportProvider (#826)