From b0408f2821d7a686b6cccff52c81539a4315b229 Mon Sep 17 00:00:00 2001
From: gwyszynski-sa <gwyszynski@secureauth.com>
Date: Tue, 18 Aug 2026 09:42:58 +0200
Subject: [PATCH] mcp: prefer the negotiated protocol version over the request
 context (#1163)

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>
---
 mcp/streamable.go             |  4 ++--
 mcp/streamable_client_test.go | 28 ++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/mcp/streamable.go b/mcp/streamable.go
index b2190fa..b761f9c 100644
--- a/mcp/streamable.go
+++ b/mcp/streamable.go
@@ -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)
diff --git a/mcp/streamable_client_test.go b/mcp/streamable_client_test.go
index a5957bf..389f87e 100644
--- a/mcp/streamable_client_test.go
+++ b/mcp/streamable_client_test.go
@@ -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)
 			}