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docs(mcp): clarify hosted client compatibility #164

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20 changes: 20 additions & 0 deletions cal-ai/mcp-server.mdx
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Expand Up @@ -67,6 +67,26 @@ The quickest way to get started is the hosted server at `mcp.cal.com`. When you
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## Hosted server compatibility

The hosted server uses OAuth 2.1 for remote MCP clients. Before connecting, confirm that your client supports:

- remote Streamable HTTP MCP connections;
- an OAuth 2.1 authorisation flow; and
- a redirect URI accepted by the hosted deployment.

The hosted deployment validates redirect URIs. A client that uses a custom scheme instead of an `http` or `https` callback, or an external HTTPS callback host that has not been approved, may receive an `invalid redirect_uri` error during registration or authorisation.

### Troubleshooting hosted authentication

1. Confirm that the MCP URL is exactly `https://mcp.cal.com/mcp`.
2. If registration or authorisation returns `invalid redirect_uri`, inspect the callback URI generated by your client.
3. For local clients, use a supported loopback callback such as `localhost`, `127.0.0.1` or `::1`.
4. For an external HTTPS callback, confirm that the callback host has been approved for the hosted deployment.
5. If your client cannot use an accepted callback URI, use the self-hosted setup below if the client supports stdio and API-key authentication.

The MCP URL alone does not determine whether a client can complete OAuth. The client’s redirect URI and the hosted deployment’s redirect policy must also be compatible.

## Connect using a self-hosted server

If you prefer to run the server locally, you can use an API key instead of the OAuth flow. This option requires Node.js 18 or later.
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