fix: preserve custom Accept headers instead of unconditionally overwriting

.changeset/preserve-accept-header.md

@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/client": patch
+---
+
+Preserve custom Accept headers instead of unconditionally overwriting them

packages/client/src/client/sse.ts

@@ -143,7 +143,9 @@ export class SSEClientTransport implements Transport {
                 ...this._eventSourceInit,
                 fetch: async (url, init) => {
                     const headers = await this._commonHeaders();
-                    headers.set('Accept', 'text/event-stream');
+                    if (!headers.has('Accept')) {
+                        headers.set('Accept', 'text/event-stream');
+                    }
                     const response = await fetchImpl(url, {
                         ...init,
                         headers

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,