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How to fix an MCP client 405 error with Streamable HTTP and OAuth

When an automation tool connects to a remote MCP server and reports 405 Method Not Allowed, the 405 is rarely the real problem. The MCP spec lets a server answer GET with 405, and clients that fall back to the older SSE transport show that error after their first POST has already failed.

At a glance

  1. Find the first failed requestLook at the server log for the first POST from the client (the `initialize` call).
  2. Check the OAuth setupOn a 401 the server should send `WWW-Authenticate` pointing to its protected-resource metadata at `/.well-known/oauth-protected-resource`.
  3. Test outside the automation toolRun the MCP Inspector (`npx @modelcontextprotocol/inspector`), choose Streamable HTTP and the same URL.

Find the first failed request

  • Look at the server log for the first POST from the client (the `initialize` call).
  • Note its status and whether an `Authorization: Bearer` header was present.

Check the OAuth setup

  • On a 401 the server should send `WWW-Authenticate` pointing to its protected-resource metadata at `/.well-known/oauth-protected-resource`.
  • That metadata should name the authorization server.
  • The token's audience or resource must match the MCP URL exactly, path included.

Test outside the automation tool

  • Run the MCP Inspector (`npx @modelcontextprotocol/inspector`), choose Streamable HTTP and the same URL.
  • If it connects and lists tools, the server is fine. Send the tool's support the request IDs from the failing attempt.
  • If it fails too, the server log from step one shows where.

Updated 2026-11-01. General guidance, not advice for your situation; we scope each project in writing before any work starts.

Questions people ask first

Is a 405 on GET a bug?

No. A server that doesn't offer a standalone event stream may answer GET with 405, and clients should accept it.

Do I need a proxy?

Usually not. Fix the authentication on the first POST and the fallback never runs.

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