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fix: reject initialize protocol version mismatch #964

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fix: reject initialize protocol version mismatch #964

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21 changes: 20 additions & 1 deletion mcp/streamable.go
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Original file line number Diff line number Diff line change
Expand Up @@ -1258,7 +1258,8 @@ func (c *streamableServerConn) servePOST(w http.ResponseWriter, req *http.Reques
return
}

protocolVersion := protocolVersionFromContext(req.Context())
headerProtocolVersion := protocolVersionFromContext(req.Context())
protocolVersion := headerProtocolVersion
if protocolVersion == "" {
protocolVersion = protocolVersion20250326
}
Expand All @@ -1278,6 +1279,7 @@ func (c *streamableServerConn) servePOST(w http.ResponseWriter, req *http.Reques
calls := make(map[jsonrpc.ID]struct{})
tokenInfo := auth.TokenInfoFromContext(req.Context())
isInitialize := false
var initializeID jsonrpc.ID
var initializeProtocolVersion string
for _, msg := range incoming {
if jreq, ok := msg.(*jsonrpc.Request); ok {
Expand All @@ -1290,6 +1292,7 @@ func (c *streamableServerConn) servePOST(w http.ResponseWriter, req *http.Reques
}
if jreq.Method == methodInitialize {
isInitialize = true
initializeID = jreq.ID
// Extract the protocol version from InitializeParams.
var params InitializeParams
if err := internaljson.Unmarshal(jreq.Params, &params); err == nil {
Expand Down Expand Up @@ -1322,6 +1325,22 @@ func (c *streamableServerConn) servePOST(w http.ResponseWriter, req *http.Reques
}
}

if headerProtocolVersion != "" && initializeProtocolVersion != "" && headerProtocolVersion != initializeProtocolVersion {
resp := &jsonrpc.Response{
ID: initializeID,
Error: jsonrpc2.NewError(
CodeHeaderMismatch,
fmt.Sprintf("header mismatch: %s header value %q does not match body protocolVersion %q", protocolVersionHeader, headerProtocolVersion, initializeProtocolVersion),
),
}
w.Header().Set("Content-Type", "application/json")
w.WriteHeader(http.StatusBadRequest)
if data, err := jsonrpc2.EncodeMessage(resp); err == nil {
w.Write(data)
}
return
}

// Validate MCP standard headers (Mcp-Method, Mcp-Name, Mcp-Param-*)
if !isBatch && len(incoming) == 1 {
if err := validateMcpHeaders(req.Header, incoming[0], c.toolLookup); err != nil {
Expand Down
13 changes: 13 additions & 0 deletions mcp/streamable_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -966,6 +966,19 @@ func TestStreamableServerTransport(t *testing.T) {
},
wantSessions: 1,
},
{
name: "initialize protocol version header mismatch",
requests: []streamableRequest{
{
method: "POST",
headers: http.Header{protocolVersionHeader: {protocolVersion20251125}},
messages: []jsonrpc.Message{req(1, methodInitialize, &InitializeParams{ProtocolVersion: protocolVersion20250618})},
wantStatusCode: http.StatusBadRequest,
wantBodyContaining: "header mismatch",
},
},
wantSessions: 0,
},
{
name: "batch rejected on 2025-06-18",
requests: []streamableRequest{
Expand Down