fix(server): reject initialize with mismatched MCP-Protocol-Version header

.changeset/fix-init-protocol-version-mismatch.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/server': patch
+---
+
+fix(server): reject initialize when MCP-Protocol-Version header disagrees with body protocolVersion

packages/server/src/server/streamableHttp.ts

@@ -675,6 +675,21 @@ export class WebStandardStreamableHTTPServerTransport implements Transport {
                     this.onerror?.(new Error('Invalid Request: Only one initialization request is allowed'));
                     return this.createJsonErrorResponse(400, -32_600, 'Invalid Request: Only one initialization request is allowed');
                 }
+                // When the MCP-Protocol-Version header is present on initialize, it must agree
+                // with the body's params.protocolVersion. The spec does not require this check,
+                // but a silent header/body mismatch lets middleware misconfiguration through
+                // (the body wins) and routes responses to a version the client never asked for.
+                // See issue #2108.
+                const headerProtocolVersion = req.headers.get('mcp-protocol-version');
+                if (headerProtocolVersion !== null) {
+                    const initRequest = messages.find(element => isInitializeRequest(element));
+                    const bodyProtocolVersion = initRequest?.params.protocolVersion;
+                    if (bodyProtocolVersion !== undefined && headerProtocolVersion !== bodyProtocolVersion) {
+                        const error = `Bad Request: MCP-Protocol-Version header (${headerProtocolVersion}) does not match initialize body protocolVersion (${bodyProtocolVersion})`;
+                        this.onerror?.(new Error(error));
+                        return this.createJsonErrorResponse(400, -32_600, error);
+                    }
+                }
                 this.sessionId = this.sessionIdGenerator?.();
                 this._initialized = true;
 

packages/server/test/server/streamableHttp.test.ts

@@ -752,6 +752,68 @@ describe('Zod v4', () => {
             const errorData = await response.json();
             expectErrorResponse(errorData, -32_000, /Unsupported protocol version/);
         });
+
+        it('should reject initialize when MCP-Protocol-Version header disagrees with body (header older)', async () => {
+            const request = new Request('http://localhost/mcp', {
+                method: 'POST',
+                headers: {
+                    'Content-Type': 'application/json',
+                    Accept: 'application/json, text/event-stream',
+                    'mcp-protocol-version': '2025-03-26'
+                },
+                body: JSON.stringify(TEST_MESSAGES.initialize)
+            });
+
+            const response = await transport.handleRequest(request);
+
+            expect(response.status).toBe(400);
+            const errorData = await response.json();
+            expectErrorResponse(errorData, -32_600, /MCP-Protocol-Version header.*does not match initialize body protocolVersion/);
+        });
+
+        it('should reject initialize when MCP-Protocol-Version header disagrees with body (header newer)', async () => {
+            const request = new Request('http://localhost/mcp', {
+                method: 'POST',
+                headers: {
+                    'Content-Type': 'application/json',
+                    Accept: 'application/json, text/event-stream',
+                    'mcp-protocol-version': '2025-11-25'
+                },
+                body: JSON.stringify(TEST_MESSAGES.initializeOldVersion)
+            });
+
+            const response = await transport.handleRequest(request);
+
+            expect(response.status).toBe(400);
+            const errorData = await response.json();
+            expectErrorResponse(errorData, -32_600, /MCP-Protocol-Version header.*does not match initialize body protocolVersion/);
+        });
+
+        it('should accept initialize when MCP-Protocol-Version header matches body', async () => {
+            const request = new Request('http://localhost/mcp', {
+                method: 'POST',
+                headers: {
+                    'Content-Type': 'application/json',
+                    Accept: 'application/json, text/event-stream',
+                    'mcp-protocol-version': '2025-11-25'
+                },
+                body: JSON.stringify(TEST_MESSAGES.initialize)
+            });
+
+            const response = await transport.handleRequest(request);
+
+            expect(response.status).toBe(200);
+            expect(response.headers.get('mcp-session-id')).toBeDefined();
+        });
+
+        it('should accept initialize when MCP-Protocol-Version header is absent (existing behavior)', async () => {
+            const request = createRequest('POST', TEST_MESSAGES.initialize);
+
+            const response = await transport.handleRequest(request);
+
+            expect(response.status).toBe(200);
+            expect(response.headers.get('mcp-session-id')).toBeDefined();
+        });
     });
 
     describe('HTTPServerTransport - start() method', () => {