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mcp: prefer the negotiated protocol version over the request context (#1163)

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setMCPHeaders resolved Mcp-Protocol-Version from the request context
before
c.initializedResult. #1107 established the opposite order and said so in
its
description; the code it replaced also consulted the context last. The
two
branches were transposed by the switch-to-if rewrite in #1115, whose
subject
was an unrelated nil-pointer check.

The context branch serves the pre-negotiation server/discover probe,
which
sets its own value in Client.Connect. Once a version has been negotiated
it is
the authoritative one for that connection, so initializedResult should
win.
_meta.protocolVersion from the outgoing message stays on top, so #1109
stays
fixed.

The existing table built its request with http.NewRequest, so
protocolVersionFromContext returned "" in every case and the two
orderings
were indistinguishable, which is why the transposition went unnoticed.
Adds a
ctxVersion dimension and three cases pinning the precedence.

Fixes #1162

Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>
This commit is contained in:
gwyszynski-sa 2026-08-18 09:42:58 +02:00 • committed by GitHub
commit b0408f2821
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2 changed files with 30 additions and 2 deletions
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4
mcp/streamable.go
View file

@ -2446,10 +2446,10 @@ func (c *streamableClientConn) setMCPHeaders(req *http.Request, msg jsonrpc.Mess
}
if pv := protocolVersionFromMessage(msg); pv != "" {
req.Header.Set(protocolVersionHeader, pv)
} else if pv := protocolVersionFromContext(req.Context()); pv != "" {
req.Header.Set(protocolVersionHeader, pv)
} else if c.initializedResult != nil {
req.Header.Set(protocolVersionHeader, c.initializedResult.ProtocolVersion)
} else if pv := protocolVersionFromContext(req.Context()); pv != "" {
req.Header.Set(protocolVersionHeader, pv)
}
if c.sessionID != "" {
req.Header.Set(sessionIDHeader, c.sessionID)

28
mcp/streamable_client_test.go
View file

@ -1384,6 +1384,7 @@ func TestStreamableClientConnSetMCPHeaders_ProtocolVersion(t *testing.T) {
name string
initializedResult *InitializeResult
msg jsonrpc.Message
ctxVersion string
want string
}{
{
@ -1422,6 +1423,30 @@ func TestStreamableClientConnSetMCPHeaders_ProtocolVersion(t *testing.T) {
msg: req(1, methodListTools, &ListToolsParams{}),
want: "",
},
{
// A process that is both a server and a client (a proxy) carries the
// inbound request's version on the context of the outgoing call. The
// version this connection negotiated must win over it.
name: "initializedResult preferred over request context",
initializedResult: &InitializeResult{ProtocolVersion: protocolVersion20251125},
msg: req(1, methodListTools, &ListToolsParams{}),
ctxVersion: protocolVersion20260728,
want: protocolVersion20251125,
},
{
name: "request context used when initializedResult unset",
initializedResult: nil,
msg: req(1, methodListTools, &ListToolsParams{}),
ctxVersion: protocolVersion20260728,
want: protocolVersion20260728,
},
{
name: "message meta preferred over request context",
initializedResult: nil,
msg: req(1, methodListTools, &ListToolsParams{Meta: Meta{MetaKeyProtocolVersion: protocolVersion20251125}}),
ctxVersion: protocolVersion20260728,
want: protocolVersion20251125,
},
}
for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
@ -1430,6 +1455,9 @@ func TestStreamableClientConnSetMCPHeaders_ProtocolVersion(t *testing.T) {
if err != nil {
t.Fatal(err)
}
if tt.ctxVersion != "" {
httpReq = httpReq.WithContext(context.WithValue(httpReq.Context(), protocolVersionContextKey{}, tt.ctxVersion))
}
if err := conn.setMCPHeaders(httpReq, tt.msg); err != nil {
t.Fatalf("setMCPHeaders: %v", err)
}