fix(client): preserve custom Accept headers in StreamableHTTPClientTransport (#1655)

Co-authored-by: Felix Weinberger <3823880+felixweinberger@users.noreply.github.com>
Co-authored-by: Felix Weinberger <fweinberger@anthropic.com>

Author: nielskaspers

.changeset/odd-forks-enjoy.md

@@ -0,0 +1,7 @@
+---
+"@modelcontextprotocol/client": patch
+---
+
+fix(client): append custom Accept headers to spec-required defaults in StreamableHTTPClientTransport
+
+Custom Accept headers provided via `requestInit.headers` are now appended to the spec-mandated Accept types instead of being overwritten. This ensures the required media types (`application/json, text/event-stream` for POST; `text/event-stream` for GET SSE) are always present while allowing users to include additional types for proxy/gateway routing.

packages/client/src/client/streamableHttp.ts

@@ -237,7 +237,9 @@ 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');
+            const userAccept = headers.get('accept');
+            const types = [...(userAccept?.split(',').map(s => s.trim().toLowerCase()) ?? []), 'text/event-stream'];
+            headers.set('accept', [...new Set(types)].join(', '));
 
             // Include Last-Event-ID header for resumable streams if provided
             if (resumptionToken) {
@@ -538,7 +540,9 @@ export class StreamableHTTPClientTransport implements Transport {
 
             const headers = await this._commonHeaders();
             headers.set('content-type', 'application/json');
-            headers.set('accept', 'application/json, text/event-stream');
+            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(', '));
 
             const init = {
                 ...this._requestInit,

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

@@ -628,6 +628,99 @@ describe('StreamableHTTPClientTransport', () => {
         expect((actualReqInit.headers as Headers).get('x-custom-header')).toBe('CustomValue');
     });
 
+    it('should append custom Accept header to required types 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, application/json, text/event-stream'
+        );
+    });
+
+    it('should append custom Accept header to required types on GET SSE requests', async () => {
+        transport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
+            requestInit: {
+                headers: {
+                    Accept: '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('application/json, text/event-stream');
+    });
+
+    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 not duplicate Accept media types when user-provided value overlaps required types', async () => {
+        transport = new StreamableHTTPClientTransport(new URL('http://localhost:1234/mcp'), {
+            requestInit: {
+                headers: {
+                    Accept: 'application/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/json, text/event-stream');
+    });
+
     it('should have exponential backoff with configurable maxRetries', () => {
         // This test verifies the maxRetries and backoff calculation directly