diff --git a/cal-ai/mcp-server.mdx b/cal-ai/mcp-server.mdx
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--- a/cal-ai/mcp-server.mdx
+++ b/cal-ai/mcp-server.mdx
@@ -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.