fix(client): preserve custom Accept headers in StreamableHTTPClientTransport

packages/client/src/client/streamableHttp.ts

@@ -214,7 +214,9 @@ export class StreamableHTTPClientTransport implements Transport {
             // Try to open an initial SSE stream with GET to listen for server messages
             // This is optional according to the spec - server may not support it
             const headers = await this._commonHeaders();
-            headers.set('Accept', 'text/event-stream');
+            if (!headers.has('accept')) {
+                headers.set('accept', 'text/event-stream');
+            }
 
             // Include Last-Event-ID header for resumable streams if provided
             if (resumptionToken) {
@@ -473,7 +475,9 @@ export class StreamableHTTPClientTransport implements Transport {
 
             const headers = await this._commonHeaders();
             headers.set('content-type', 'application/json');
-            headers.set('accept', 'application/json, text/event-stream');
+            if (!headers.has('accept')) {
+                headers.set('accept', 'application/json, text/event-stream');
+            }
 
             const init = {
                 ...this._requestInit,

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

@@ -601,6 +601,72 @@ describe('StreamableHTTPClientTransport', () => {
         expect((actualReqInit.headers as Headers).get('x-custom-header')).toBe('CustomValue');
     });
 
+    it('should preserve custom Accept header on POST requests', async () => {
+        transport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
+            requestInit: {
+                headers: {
+                    Accept: 'application/vnd.example.v1+json'
+                }
+            }
+        });
+
+        let actualReqInit: RequestInit = {};
+
+        (globalThis.fetch as Mock).mockImplementation(async (_url, reqInit) => {
+            actualReqInit = reqInit;
+            return new Response(JSON.stringify({ jsonrpc: '2.0', result: {} }), {
+                status: 200,
+                headers: { 'content-type': 'application/json' }
+            });
+        });
+
+        await transport.start();
+
+        await transport.send({ jsonrpc: '2.0', method: 'test', params: {} } as JSONRPCMessage);
+        expect((actualReqInit.headers as Headers).get('accept')).toBe('application/vnd.example.v1+json');
+    });
+
+    it('should preserve custom Accept header on GET SSE requests', async () => {
+        transport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
+            requestInit: {
+                headers: {
+                    Accept: 'text/event-stream, application/json'
+                }
+            }
+        });
+
+        let actualReqInit: RequestInit = {};
+
+        (globalThis.fetch as Mock).mockImplementation(async (_url, reqInit) => {
+            actualReqInit = reqInit;
+            return new Response(null, { status: 200, headers: { 'content-type': 'text/event-stream' } });
+        });
+
+        await transport.start();
+
+        await transport['_startOrAuthSse']({});
+        expect((actualReqInit.headers as Headers).get('accept')).toBe('text/event-stream, application/json');
+    });
+
+    it('should set default Accept header when none provided', async () => {
+        transport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'));
+
+        let actualReqInit: RequestInit = {};
+
+        (globalThis.fetch as Mock).mockImplementation(async (_url, reqInit) => {
+            actualReqInit = reqInit;
+            return new Response(JSON.stringify({ jsonrpc: '2.0', result: {} }), {
+                status: 200,
+                headers: { 'content-type': 'application/json' }
+            });
+        });
+
+        await transport.start();
+
+        await transport.send({ jsonrpc: '2.0', method: 'test', params: {} } as JSONRPCMessage);
+        expect((actualReqInit.headers as Headers).get('accept')).toBe('application/json, text/event-stream');
+    });
+
     it('should have exponential backoff with configurable maxRetries', () => {
         // This test verifies the maxRetries and backoff calculation directly