mcp: emit UnsupportedProtocolVersionError for stateful servers rejecting SEP-2575 requests - #1111
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mcp: emit UnsupportedProtocolVersionError for stateful servers rejecting SEP-2575 requests#1111jhrozek wants to merge 1 commit into
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…ing SEP-2575 requests
A stateful (Stateless == false) Streamable HTTP server rejected requests
carrying `_meta."io.modelcontextprotocol/protocolVersion"` >= 2026-07-28
with a plain-text http.Error body, instead of the JSON-RPC
UnsupportedProtocolVersionError the draft spec requires for a known
version the server has chosen not to support. The body also leaked a
Go-API hint ("set StreamableHTTPOptions.Stateless = true") meant for the
server author, not the client.
Reuse the existing CodeUnsupportedProtocolVersion (-32022) and
UnsupportedProtocolVersionData shape already used by the discover and
initialize paths, listing only the server's legacy-era supported
versions. This lets Client.Connect's existing renegotiation logic
recognize and handle the rejection instead of falling back on an
unrecognized text/plain body. The developer hint moves to a server-side
log line.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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Summary
A stateful (
Stateless == false) Streamable HTTP server rejected requests carrying_meta."io.modelcontextprotocol/protocolVersion"for MCP2026-07-28with a plain-texthttp.Errorbody, instead of the JSON-RPC error the draft spec requires. Per the draft's Protocol Version Header section, a server that won't serve a known-but-unsupported version MUST respond400 Bad Requestwith anUnsupportedProtocolVersionErrorlisting its supported versions — this is exactly that case (a stateful server that hasn't opted intoStateless).Two things made this clearly a bug rather than a design choice:
The very next branch in the same function already calls
writeJSONRPCErrorwithCodeHeaderMismatchfor an adjacent condition — this branch usedhttp.Errorinstead, inconsistently.The body text (
"set StreamableHTTPOptions.Stateless = true") is a Go-API hint for whoever wrote the server, not something a remote client can act on.Reuse the existing
CodeUnsupportedProtocolVersion(-32022) andUnsupportedProtocolVersionData{Supported, Requested}shape already used by thediscover/initializepaths, soClient.Connect's existing renegotiation logic can recognize this response and fall back cleanly, instead of hitting an unrecognizedtext/plainbody.Supportedlists only the server's legacy-era versions (excludes2026-07-28), since that's the whole point of the rejection.The developer-facing hint moves to a server-side log line instead of the wire body.
server/discoverstays exempt, as before — it's how a client is meant to learn a stateful server's restricted version set in the first place.Test plan
go build ./...go vet ./...go test ./mcp/...(full package)TestStreamableStateful_RejectsNewProtocolto assert the JSON-RPC error shape (code-32022,Requested,Supportedexcluding2026-07-28) instead of the old text-contains-"stateless" checkTestStreamableStateless_AcceptsNewProtocol,TestStreamableClientUnsupportedVersionFallback,TestStreamableStateful_AcceptsDiscoverall still pass unmodified🤖 Generated with Claude Code