fix: preserve custom Accept headers instead of unconditionally overwriting

The SDK unconditionally overwrites Accept headers with headers.set(),
discarding any custom Accept values provided via requestInit.headers.
This breaks APIs that require specific Accept header values.

Now checks headers.has('Accept') before setting the default value,
preserving user-provided Accept headers in both Streamable HTTP and
SSE transports.

Fixes #1646

Author: Charlie Tonneslan

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,