Why do I get 405 Method Not Allowed or 406 Not Acceptable on an API or MCP endpoint?

A 405 or 406 means the endpoint exists but you reached it the wrong way - wrong HTTP method, or wrong transport. Three common shapes:

GET vs POST (405 Method Not Allowed). Endpoints that accept a file or a text payload are POST-only. Paste one into a browser address bar and the browser sends a GET, which the endpoint refuses with 405. Send payloads with a programmatic client (curl, fetch, requests), not the address bar.

Opening an MCP endpoint in a browser (406 Not Acceptable). An MCP server speaks Streamable HTTP and requires an Accept: text/event-stream header that browsers do not send, so a browser gets 406. MCP URLs are machine addresses, not web pages - point your MCP client (Claude, Cursor, an agent config) at the URL rather than opening it.

A deprecated MCP path (405). If a server has unified on a single canonical /mcp path, older /sse or /v1/mcp/sse URLs return 405. Update the agent config to the canonical path; a server's /.well-known/mcp/servers.json lists the active URLs to inspect.

General rule: a 405/406 is not "the endpoint is broken", it is "right address, wrong door". Related: connecting an MCP client and the MCP hub. Worked examples: https://www.tigzig.com/post/tigzig-api-errors-practical-guide-jul2026.