Authenticating the Retell MCP server through Claude's native "Add custom connector" UI (OAuth-only, no header field)

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1

I’m trying to connect the Retell MCP server to Claude using the built-in Add custom connector dialog in the Claude desktop and web apps, and I’d like to keep it in that UI rather than editing claude_desktop_config.json or running mcp-remote.

What I entered:

  • Name: Retell.ai
  • URL: https://retell.stlmcp.com

The dialog’s only authentication options are under Advanced settings, and they are OAuth Client ID and OAuth Client Secret (both optional). There is no field for a custom header.

The problem: Retell requires the API key as an Authorization: Bearer <key> header, and per Retell support there’s no tokenized connection URL. So when I add the server through this dialog, it attaches, but every tool call fails with:

Failed to initialize client: The RETELL_API_KEY environment variable is missing or empty

That’s expected, since the connector UI never sends an Authorization header and the server has no key.

My question: is there any supported way to authenticate https://retell.stlmcp.com through Claude’s native custom-connector interface, which supports OAuth but not a static API-key header? For example:

  • Does the stlmcp gateway support OAuth (or dynamic client registration) so the dialog’s OAuth Client ID / Secret flow can be used, and if so, where do I get those values?
  • Is there a URL form that carries the key (a per-user tokenized URL, or userinfo like https://<key>@retell.stlmcp.com)?
  • If neither exists, is header injection via mcp-remote currently the only path, and are there plans to support static-token MCP servers in the connector UI?

Environment: Claude desktop and web, Add custom connector (BETA). Goal is a no-config-file setup for a non-developer workflow.

2

Hey @rumorconsumer retell.stlmcp.com is deprecated as of 07/20/2026 — you should use https://mcp.retellai.com going forward.

For more details you can see : Legacy MCP server (retell.stlmcp.com) removed (07/20/2026) - Retell AI

For the Retell MCP connection, you can refer to the full guide for detailed setup instructions and troubleshooting steps : Retell MCP server for AI assistants - Retell AI

Please let us know if you have any questions or run into any issues during the setup process.

Best regards

3

I genuinely cannot figure out how those instructions are supposed to let me use Claudes custom MCP connector fields to do this properly. Can you give me a really clear, simple walkthrough of what goes where with my API key and such? I just cannot figure out where its all supposed to go:

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4

Short version: that dialog can’t do it, and it’s not your setup.

The custom connector popup only speaks OAuth. Retell’s MCP endpoint doesn’t use OAuth, it just wants your key in an Authorization header, and the popup has no field for a header. So it attaches, sends no key, and every call dies with the RETELL_API_KEY error. Exactly what you’re seeing.

On your three questions:

  • OAuth / dynamic client registration: the endpoint doesn’t offer it, so the Client ID and Secret boxes are dead weight here. Nothing useful to put in them.
  • A tokenised or key-in-URL form: none. You already confirmed that with Retell support, and I couldn’t find one either.
  • So yes, a header is the only way in right now, and the native connector UI can’t send one.

What actually works, no terminal and no mcp-remote: use Claude Desktop’s config screen instead of the popup.

  1. Settings > Developer > Edit Config
  2. Paste this and drop your key in:

json

{
  "mcpServers": {
    "retell": {
      "url": "https://mcp.retellai.com",
      "headers": {
        "Authorization": "Bearer YOUR_RETELL_KEY"
      }
    }
  }
}
  1. Save, then fully quit and reopen Claude.

Two notes. I’d point at Retell’s official endpoint, mcp.retellai.com, rather than retell.stlmcp.com, unless you’re on the gateway for a reason. And there’s no true zero-file path today for a key-based server. The config edit is the smallest version of it, and it skips mcp-remote completely.

Let me know how you go.

5

Hello @rumorconsumer To use Retell’s MCP server with Claude Desktop, open settings → Developer → Edit Config and add:

{
  "mcpServers": {
    "retell": {
      "url": "https://mcp.retellai.com",
      "headers": {
        "Authorization": "Bearer <RETELL_API_KEY>"
      }
    }
  }
}

For Claude Code, run:

claude mcp add --transport http retell https://mcp.retellai.com \
  --header "Authorization: Bearer <RETELL_API_KEY>"

See the full guide at Retell MCP Server.