mcp: keep the stdio session alive on a malformed JSON frame - #1210
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mcp: keep the stdio session alive on a malformed JSON frame#1210blackwell-systems wants to merge 1 commit into
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The stdio transport feeds os.Stdin into a single streaming json.Decoder. A syntactically malformed frame makes Decode return a *json.SyntaxError, and the read goroutine returned on that error, so one bad frame terminated the whole session. A streaming decoder also cannot resynchronize on its own: a syntax error poisons its buffered stream state. Per JSON-RPC 2.0, a parse error should be answered with a -32700 response and the session should continue. That is what mark3labs/mcp-go does (it reads newline-delimited frames and unmarshals each independently), and it is the same recoverable-decode direction taken for empty-method requests in modelcontextprotocol#1000. The stdio read loop now: - replies with a -32700 parse-error response (id: null) for a malformed frame, - resynchronizes to the next newline-delimited frame and keeps reading, - terminates only on a genuine EOF or I/O error, as before. An EOF immediately after a malformed frame (with or without a trailing newline) still ends the session cleanly. Only *json.SyntaxError is recovered; the existing "invalid trailing data" handling is unchanged. Tests cover a malformed frame followed by a valid request (the request is still delivered and a -32700 is written), consecutive malformed frames, and malformed frames at end-of-stream. Verified with -race. Surfaced by an MCP conformance study run against a downstream server (blackwell-systems/agent-lsp#14). The behavior is a property of this transport, shared by every server built on the SDK's stdio transport. Fixes modelcontextprotocol#1209
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The stdio transport feeds os.Stdin into a single streaming json.Decoder. A
syntactically malformed frame makes Decode return a *json.SyntaxError, and the
read goroutine returned on that error, so one bad frame terminated the whole
session. A streaming decoder also cannot resynchronize on its own: a syntax
error poisons its buffered stream state.
Per JSON-RPC 2.0, a parse error should be answered with a -32700 response and
the session should continue. That is what mark3labs/mcp-go does (it reads
newline-delimited frames and unmarshals each independently), and it is the same
recoverable-decode direction taken for empty-method requests in #1000.
The stdio read loop now:
An EOF immediately after a malformed frame (with or without a trailing newline)
still ends the session cleanly. Only *json.SyntaxError is recovered; the
existing "invalid trailing data" handling is unchanged.
Tests cover a malformed frame followed by a valid request (the request is still
delivered and a -32700 is written), consecutive malformed frames, and malformed
frames at end-of-stream. Verified with -race.
Surfaced by an MCP conformance study run against a downstream server
(blackwell-systems/agent-lsp#14). The behavior is a property of this transport,
shared by every server built on the SDK's stdio transport.
Fixes #1209