fix(client): send MCP standard POST headers

.changeset/fresh-headers-flow.md

@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/client": patch
+---
+
+Send MCP standard headers on Streamable HTTP POST requests.

packages/client/src/client/streamableHttp.ts

@@ -163,6 +163,25 @@ export type StreamableHTTPClientTransportOptions = {
     protocolVersion?: string;
 };
 
+function setMcpStandardPostHeaders(headers: Headers, message: JSONRPCMessage | JSONRPCMessage[]): void {
+    if (Array.isArray(message) || !('method' in message) || typeof message.method !== 'string') {
+        return;
+    }
+
+    headers.set('Mcp-Method', message.method);
+
+    const params = 'params' in message ? message.params : undefined;
+    if (!params || typeof params !== 'object' || Array.isArray(params)) {
+        return;
+    }
+
+    const requestParams = params as { name?: unknown; uri?: unknown };
+    const mcpName = typeof requestParams.name === 'string' ? requestParams.name : requestParams.uri;
+    if (typeof mcpName === 'string') {
+        headers.set('Mcp-Name', mcpName);
+    }
+}
+
 /**
  * Client transport for Streamable HTTP: this implements the MCP Streamable HTTP transport specification.
  * It will connect to a server using HTTP `POST` for sending messages and HTTP `GET` with Server-Sent Events
@@ -545,6 +564,7 @@ export class StreamableHTTPClientTransport implements Transport {
             const userAccept = headers.get('accept');
             const types = [...(userAccept?.split(',').map(s => s.trim().toLowerCase()) ?? []), 'application/json', 'text/event-stream'];
             headers.set('accept', [...new Set(types)].join(', '));
+            setMcpStandardPostHeaders(headers, message);
 
             const init = {
                 ...this._requestInit,

packages/client/test/client/streamableHttp.test.ts

@@ -62,6 +62,112 @@ describe('StreamableHTTPClientTransport', () => {
         );
     });
 
+    it('should include MCP standard headers on POST requests', async () => {
+        const message: JSONRPCMessage = {
+            jsonrpc: '2.0',
+            method: 'tools/call',
+            params: {
+                name: 'read_file',
+                arguments: { path: 'README.md' }
+            },
+            id: 'test-id'
+        };
+
+        (globalThis.fetch as Mock).mockResolvedValueOnce({
+            ok: true,
+            status: 202,
+            headers: new Headers()
+        });
+
+        await transport.send(message);
+
+        const headers = (globalThis.fetch as Mock).mock.calls.at(-1)![1].headers as Headers;
+        expect(headers.get('mcp-method')).toBe('tools/call');
+        expect(headers.get('mcp-name')).toBe('read_file');
+    });
+
+    it('should use params.uri for the MCP name header', async () => {
+        const message: JSONRPCMessage = {
+            jsonrpc: '2.0',
+            method: 'resources/read',
+            params: {
+                uri: 'file:///README.md'
+            },
+            id: 'test-id'
+        };
+
+        (globalThis.fetch as Mock).mockResolvedValueOnce({
+            ok: true,
+            status: 202,
+            headers: new Headers()
+        });
+
+        await transport.send(message);
+
+        const headers = (globalThis.fetch as Mock).mock.calls.at(-1)![1].headers as Headers;
+        expect(headers.get('mcp-method')).toBe('resources/read');
+        expect(headers.get('mcp-name')).toBe('file:///README.md');
+    });
+
+    it('should include the MCP method header for notifications', async () => {
+        const message: JSONRPCMessage = {
+            jsonrpc: '2.0',
+            method: 'notifications/initialized'
+        };
+
+        (globalThis.fetch as Mock).mockResolvedValueOnce({
+            ok: true,
+            status: 202,
+            headers: new Headers()
+        });
+
+        await transport.send(message);
+
+        const headers = (globalThis.fetch as Mock).mock.calls.at(-1)![1].headers as Headers;
+        expect(headers.get('mcp-method')).toBe('notifications/initialized');
+        expect(headers.get('mcp-name')).toBeNull();
+    });
+
+    it('should not include MCP method headers for JSON-RPC responses', async () => {
+        const message: JSONRPCMessage = {
+            jsonrpc: '2.0',
+            id: 'test-id',
+            result: {}
+        };
+
+        (globalThis.fetch as Mock).mockResolvedValueOnce({
+            ok: true,
+            status: 202,
+            headers: new Headers()
+        });
+
+        await transport.send(message);
+
+        const headers = (globalThis.fetch as Mock).mock.calls.at(-1)![1].headers as Headers;
+        expect(headers.get('mcp-method')).toBeNull();
+        expect(headers.get('mcp-name')).toBeNull();
+    });
+
+    it('should omit the MCP name header when params are absent', async () => {
+        const message: JSONRPCMessage = {
+            jsonrpc: '2.0',
+            method: 'initialize',
+            id: 'test-id'
+        };
+
+        (globalThis.fetch as Mock).mockResolvedValueOnce({
+            ok: true,
+            status: 202,
+            headers: new Headers()
+        });
+
+        await transport.send(message);
+
+        const headers = (globalThis.fetch as Mock).mock.calls.at(-1)![1].headers as Headers;
+        expect(headers.get('mcp-method')).toBe('initialize');
+        expect(headers.get('mcp-name')).toBeNull();
+    });
+
     it('should send batch messages', async () => {
         const messages: JSONRPCMessage[] = [
             { jsonrpc: '2.0', method: 'test1', params: {}, id: 'id1' },
@@ -85,6 +191,10 @@ describe('StreamableHTTPClientTransport', () => {
                 body: JSON.stringify(messages)
             })
         );
+
+        const headers = (globalThis.fetch as Mock).mock.calls.at(-1)![1].headers as Headers;
+        expect(headers.get('mcp-method')).toBeNull();
+        expect(headers.get('mcp-name')).toBeNull();
     });
 
     it('should store session ID received during initialization', async () => {