refactor: remove redundant meta field from validatedMeta and simplify protocol version validation logic

Author: guglielmo-san

mcp/shared.go

@@ -482,7 +482,6 @@ func extractRequestMeta(rawParams json.RawMessage) Meta {
 
 type validatedMeta struct {
 	usesNewProtocol bool
-	meta            Meta
 }
 
 // validateRequestMeta inspects a JSON-RPC request to detect whether it follows
@@ -501,7 +500,7 @@ func validateRequestMeta(req *jsonrpc.Request) (*validatedMeta, error) {
 	}
 	// Notifications do not carry full client identity
 	if !req.IsCall() {
-		return &validatedMeta{usesNewProtocol: true, meta: meta}, nil
+		return &validatedMeta{usesNewProtocol: true}, nil
 	}
 	if _, ok := meta[MetaKeyClientInfo]; !ok {
 		return nil, &jsonrpc.Error{
@@ -515,7 +514,7 @@ func validateRequestMeta(req *jsonrpc.Request) (*validatedMeta, error) {
 			Message: fmt.Sprintf("missing required _meta field %q", MetaKeyClientCapabilities),
 		}
 	}
-	return &validatedMeta{usesNewProtocol: true, meta: meta}, nil
+	return &validatedMeta{usesNewProtocol: true}, nil
 }
 
 // A Request is a method request with parameters and additional information, such as the session.

mcp/shared_test.go

@@ -136,10 +136,6 @@ func TestValidateRequestMeta(t *testing.T) {
 
 			vmeta, err := validateRequestMeta(req)
 			usesNew := vmeta != nil && vmeta.usesNewProtocol
-			if err != nil {
-				meta := extractRequestMeta(req.Params)
-				usesNew = meta != nil && meta[MetaKeyProtocolVersion] != nil
-			}
 			if usesNew != tc.wantUsesNew {
 				t.Errorf("usesNewProtocol = %v, want %v", usesNew, tc.wantUsesNew)
 			}

mcp/streamable.go

@@ -390,17 +390,16 @@ func (h *StreamableHTTPHandler) serveStatelessLegacyDELETE(w http.ResponseWriter
 }
 
 // ephemeralConnectOpts peeks at the request body to determine whether it
-// contains an initialize or initialized message, or whether any of its
-// messages carry the per-request `_meta.protocolVersion` field that signals
-// the >= 2026-06-30 sessionless protocol (SEP-2575).
+// contains an initialize or initialized message or whether the protocol version
+// header indicates a protocol version >= 2026-06-30 (SEP-2575).
 //
 // For old-protocol requests, default session state is synthesized so that
 // the session's init gate doesn't reject the request.
 //
 // It is used for both stateless servers and stateful servers with no session ID.
 //
-// The returned usesNewProtocol bool reports whether any request in the body
-// carried `_meta.protocolVersion`.
+// The returned usesNewProtocol bool reports whether the protocol version
+// header indicates a protocol version >= 2026-06-30 (SEP-2575).
 func (h *StreamableHTTPHandler) ephemeralConnectOpts(req *http.Request) (opts *ServerSessionOptions, usesNewProtocol bool, err error) {
 	protocolVersion := protocolVersionFromContext(req.Context())
 	if protocolVersion == "" {
@@ -1325,25 +1324,25 @@ func (c *streamableServerConn) servePOST(w http.ResponseWriter, req *http.Reques
 					http.Error(w, fmt.Sprintf(
 						"Bad Request: protocol version %q is only supported on stateless HTTP servers (set StreamableHTTPOptions.Stateless = true)",
 						protocolVersion),
-							http.StatusBadRequest)
-						return
-					}
-					if headerVersion == "" {
-						http.Error(w, fmt.Sprintf(
-							"Bad Request: %s header is required for requests carrying %q",
-							protocolVersionHeader, MetaKeyProtocolVersion),
-							http.StatusBadRequest)
-						return
-					}
-					if headerVersion != metaVersion {
-						http.Error(w, fmt.Sprintf(
-							"Bad Request: %s header %q does not match request %s %q",
-							protocolVersionHeader, headerVersion,
-							MetaKeyProtocolVersion, metaVersion),
-							http.StatusBadRequest)
-						return
-					}
+						http.StatusBadRequest)
+					return
+				}
+				if headerVersion == "" {
+					http.Error(w, fmt.Sprintf(
+						"Bad Request: %s header is required for requests carrying %q",
+						protocolVersionHeader, MetaKeyProtocolVersion),
+						http.StatusBadRequest)
+					return
+				}
+				if headerVersion != metaVersion {
+					http.Error(w, fmt.Sprintf(
+						"Bad Request: %s header %q does not match request %s %q",
+						protocolVersionHeader, headerVersion,
+						MetaKeyProtocolVersion, metaVersion),
+						http.StatusBadRequest)
+					return
 				}
+			}
 			// Include metadata for all requests (including notifications).
 			jreq.Extra = &RequestExtra{
 				TokenInfo: tokenInfo,

mcp/streamable_test.go

@@ -1926,39 +1926,12 @@ func TestStreamableMcpHeaderValidation(t *testing.T) {
 			return &CallToolResult{}, nil
 		})
 
-	handler := NewStreamableHTTPHandler(func(req *http.Request) *Server { return server }, nil)
+	handler := NewStreamableHTTPHandler(func(req *http.Request) *Server { return server }, &StreamableHTTPOptions{
+		Stateless: true,
+	})
 	defer handler.closeAll()
 
-	initReq := req(1, methodInitialize, &InitializeParams{ProtocolVersion: minVersionForStandardHeaders})
-	initResp := resp(1, &InitializeResult{
-		Capabilities: &ServerCapabilities{
-			Logging: &LoggingCapabilities{},
-			Tools:   &ToolCapabilities{ListChanged: true},
-		},
-		ProtocolVersion: minVersionForStandardHeaders,
-		ServerInfo:      &Implementation{Name: "testServer", Version: "v1.0.0"},
-	}, nil)
-
-	initialize := streamableRequest{
-		method:         "POST",
-		messages:       []jsonrpc.Message{initReq},
-		wantStatusCode: http.StatusOK,
-		wantMessages:   []jsonrpc.Message{initResp},
-		wantSessionID:  true,
-	}
-	initialized := streamableRequest{
-		method: "POST",
-		headers: http.Header{
-			protocolVersionHeader: {minVersionForStandardHeaders},
-			methodHeader:          {notificationInitialized},
-		},
-		messages:       []jsonrpc.Message{req(0, notificationInitialized, &InitializedParams{})},
-		wantStatusCode: http.StatusAccepted,
-	}
-
 	testStreamableHandler(t, handler, []streamableRequest{
-		initialize,
-		initialized,
 		{
 			method: "POST",
 			headers: http.Header{