mcp: derive Mcp-Protocol-Version header from the outgoing message

mcp/streamable.go

@@ -2249,7 +2249,7 @@ func (c *streamableClientConn) Write(ctx context.Context, msg jsonrpc.Message) e
 		req.Header.Set("Content-Type", "application/json")
 		req.Header.Set("Accept", "application/json, text/event-stream")
 
-		if err := c.setMCPHeaders(req); err != nil {
+		if err := c.setMCPHeaders(req, msg); err != nil {
 			// Failure to set headers means that the request was not sent.
 			// Wrap with ErrRejected so the jsonrpc2 connection doesn't set writeErr
 			// and permanently break the connection.
@@ -2373,7 +2373,7 @@ func (c *streamableClientConn) Write(ctx context.Context, msg jsonrpc.Message) e
 	return nil
 }
 
-func (c *streamableClientConn) setMCPHeaders(req *http.Request) error {
+func (c *streamableClientConn) setMCPHeaders(req *http.Request, msg jsonrpc.Message) error {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
@@ -2403,10 +2403,13 @@ func (c *streamableClientConn) setMCPHeaders(req *http.Request) error {
 			}
 		}
 	}
-	if c.initializedResult != nil {
+	switch {
+	case protocolVersionFromMessage(msg) != "":
+		req.Header.Set(protocolVersionHeader, protocolVersionFromMessage(msg))
+	case c.initializedResult != nil:
 		req.Header.Set(protocolVersionHeader, c.initializedResult.ProtocolVersion)
-	} else if v := protocolVersionFromContext(req.Context()); v != "" {
-		req.Header.Set(protocolVersionHeader, v)
+	case protocolVersionFromContext(req.Context()) != "":
+		req.Header.Set(protocolVersionHeader, protocolVersionFromContext(req.Context()))
 	}
 	if c.sessionID != "" {
 		req.Header.Set(sessionIDHeader, c.sessionID)
@@ -2415,6 +2418,23 @@ func (c *streamableClientConn) setMCPHeaders(req *http.Request) error {
 	return nil
 }
 
+// protocolVersionFromMessage recovers the SEP-2575 `_meta.protocolVersion`
+// value from an outgoing JSON-RPC request, if present. It returns "" for
+// notifications, responses, requests without a `_meta.protocolVersion`, or a
+// nil msg.
+func protocolVersionFromMessage(msg jsonrpc.Message) string {
+	req, ok := msg.(*jsonrpc.Request)
+	if !ok {
+		return ""
+	}
+	meta := extractRequestMeta(req.Params)
+	if meta == nil {
+		return ""
+	}
+	v, _ := meta[MetaKeyProtocolVersion].(string)
+	return v
+}
+
 func (c *streamableClientConn) handleJSON(requestSummary string, resp *http.Response) {
 	body, err := io.ReadAll(resp.Body)
 	resp.Body.Close()
@@ -2676,7 +2696,7 @@ func (c *streamableClientConn) connectSSE(ctx context.Context, lastEventID strin
 			if err != nil {
 				return nil, err
 			}
-			if err := c.setMCPHeaders(req); err != nil {
+			if err := c.setMCPHeaders(req, nil); err != nil {
 				return nil, err
 			}
 			if lastEventID != "" {
@@ -2712,7 +2732,7 @@ func (c *streamableClientConn) Close() error {
 			if err != nil {
 				c.closeErr = err
 			} else {
-				if err := c.setMCPHeaders(req); err != nil {
+				if err := c.setMCPHeaders(req, nil); err != nil {
 					c.closeErr = err
 				} else if resp, err := c.client.Do(req); err != nil {
 					c.closeErr = err

mcp/streamable_client_test.go

@@ -1364,6 +1364,76 @@ func TestStreamableClientConnect_DiscoverSuccess(t *testing.T) {
 	}
 }
 
+// TestStreamableClientConnSetMCPHeaders_ProtocolVersion covers
+// streamableClientConn.setMCPHeaders' selection of the Mcp-Protocol-Version
+// header value.
+//
+// Ordinarily initializedResult is populated by sessionUpdated, called
+// through a type assertion to the unexported clientConnection interface
+// (see Client.Connect). That assertion silently fails, leaving
+// initializedResult nil for the life of the session, whenever the
+// Connection returned by a Transport is wrapped by another type exposing
+// only the exported Connection interface (a real pattern for transports
+// that intercept traffic, e.g. to filter notifications): Go does not
+// promote unexported interface methods across an embedded interface
+// boundary. Every SEP-2575 (>= 2026-07-28) request already carries its own
+// `_meta.protocolVersion` field, so setMCPHeaders falls back to reading it
+// from the outgoing message when initializedResult is unset.
+func TestStreamableClientConnSetMCPHeaders_ProtocolVersion(t *testing.T) {
+	tests := []struct {
+		name              string
+		initializedResult *InitializeResult
+		msg               jsonrpc.Message
+		want              string
+	}{
+		{
+			name:              "message meta wins when initializedResult unset",
+			initializedResult: nil,
+			msg:               req(1, methodListTools, &ListToolsParams{Meta: Meta{MetaKeyProtocolVersion: protocolVersion20260728}}),
+			want:              protocolVersion20260728,
+		},
+		{
+			name:              "initializedResult used when message has no meta",
+			initializedResult: &InitializeResult{ProtocolVersion: protocolVersion20251125},
+			msg:               req(1, methodListTools, &ListToolsParams{}),
+			want:              protocolVersion20251125,
+		},
+		{
+			name:              "initializedResult used for nil message (GET/DELETE)",
+			initializedResult: &InitializeResult{ProtocolVersion: protocolVersion20251125},
+			msg:               nil,
+			want:              protocolVersion20251125,
+		},
+		{
+			name:              "message meta preferred over stale initializedResult",
+			initializedResult: &InitializeResult{ProtocolVersion: protocolVersion20251125},
+			msg:               req(1, methodListTools, &ListToolsParams{Meta: Meta{MetaKeyProtocolVersion: protocolVersion20260728}}),
+			want:              protocolVersion20260728,
+		},
+		{
+			name:              "no header when neither source is set",
+			initializedResult: nil,
+			msg:               req(1, methodListTools, &ListToolsParams{}),
+			want:              "",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			conn := &streamableClientConn{initializedResult: tt.initializedResult}
+			httpReq, err := http.NewRequest(http.MethodPost, "http://test.invalid", nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if err := conn.setMCPHeaders(httpReq, tt.msg); err != nil {
+				t.Fatalf("setMCPHeaders: %v", err)
+			}
+			if got := httpReq.Header.Get(protocolVersionHeader); got != tt.want {
+				t.Errorf("Mcp-Protocol-Version header = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
 // TestStreamableClientConnect_DiscoverMethodNotFound verifies that Client.Connect
 // falls back to the legacy initialize handshake when the server responds to
 // server/discover with a JSON-RPC "Method not found" error.