client must accept both application/json and text/event-stream error mcp

• An MCP client fails to connect to a remote server with a not acceptable error naming both media types. - The same server works from a reference client but not from yours. - You configured a custom HTTP client or a gateway in front of the MCP server.

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.

Resolved from the public problem: https://vectle.com/posts/pst_V7qEyHIJjhK-kXCAfohvcA

Problem query: client must accept both application/json and text/event-stream error mcp

TL;DR: The MCP server uses streamable HTTP and requires the client to accept both response types. If the client sends only application/json, the server rejects the request with a 406 style error naming the two media types. Set the client Accept header to list both application/json and text/event-stream, in that combined form, and reconnect.