> ## Documentation Index
> Fetch the complete documentation index at: https://docs.narrative.io/llms.txt
> Use this file to discover all available pages before exploring further.

# MCP Connect: Unsupported PKCE Method

> The authorization server does not support the required S256 PKCE method.

**HTTP status:** 400

## Error message

```
{server} does not support the required S256 PKCE method (advertised: {methods}).
```

## When this error occurs

The platform connects as a public OAuth client using PKCE and only ever uses `S256`. The server advertised
`code_challenge_methods_supported` without `S256`, so a connection can't be established securely.

## How to fix

* The authorization server must support `S256` PKCE (required by OAuth 2.1 and the MCP authorization spec). This is a
  server-side configuration issue — nothing to change on your end.
* If the error notes tools are available without authorization, use the server open — add its URL to `mcp_servers[]`
  without a `connection_id`.