**Confirm the transport.** Check the server config is an http or streamable http URL, not a stdio command. Expected: this fix only applies to the HTTP transport.

**Inspect the header the client sends.** If the client allows custom headers, look at its configured Accept value. Success check: it lists only one media type, or no Accept value is set at all.

**Set the combined Accept header.** Configure the header value as application/json, text/event-stream with both types present and comma separated. Expected: the saved client config shows both types in one header.

**Reconnect and initialize.** Restart the client connection so the new header is used from the first initialize request. Success check: the server returns a session response instead of the not acceptable error.

**Verify a tools call.** List tools or call a harmless read only tool. Expected: a normal result returns over the same connection, proving the header fix holds beyond initialization.

**Check for a proxy stripping headers.** If the error persists with the header set, test the same server URL without any local proxy or gateway in between. Success check: a direct connection initializes, pointing at the intermediary as the place the header is lost.