PATCH Update Retell LLM returns "MCP id cannot be empty or null" — same in API docs "Try it"

Support Help
1

Hi Retell team and community,

I’m getting a 400 error when calling PATCH /update-retell-llm/{llm_id}:

{
  "status": "error",
  "message": "MCP id cannot be empty or null"
}

Important: I see the same error when using the official documentation “Try it” flow for Update Retell LLM (not only from my app). That suggests it may be tied to how the request body is built or how the draft LLM is validated on the server, not only my integration.

What I understand from the docs and support chats:

  • The mcps[] array defines MCP servers (name, url, optional headers, etc.).

  • Tools with type: "mcp" inside general_tools or states[].tools must have a valid mcp_id that matches a mcps[].name.

  • If the stored draft still has bad MCP tool entries, validation can fail even when the mcps block in the request looks correct.

What I’ve tried:

  • GET the LLM first, then PATCH with cleaned general_tools / states (removing or fixing MCP tools with empty/invalid mcp_id).

  • Sending real mcps entries (no placeholder "<string>" values).

  • Clearing MCPs with "mcps": [] when appropriate.

Questions:

  1. Can you confirm whether “MCP id cannot be empty or null” always refers to mcp_id on MCP tools in general_tools / states[].tools, and not to a missing field inside mcps[] itself?

  2. Is there an official minimal JSON example for PATCH that is guaranteed to pass for an LLM that currently has multi-state + MCP, so we can diff against failing payloads?

  3. If “Try it” can return this 400 with the default/sample body, is that expected (e.g. sample LLM id / draft state), or should we report it as a docs/playground bug?

Environment: API key auth, updating an existing llm_id (not the agent id).

Thanks for any guidance — happy to share a redacted request body if useful.

2

Thank you for reaching out to Retell AI Support. We’ve received your ticket and our team will respond within 8 hours.

3

Hi @info19

Can you please share your organization ID and Agent ID?

Thank you!

4

org_cLtqW1wUZIYudrkT
agent_dcb9758254052b01eb1e04828e

5

Hello,

MCPs should have an id field on them. The mcp_id field on tools of that MCP should match the id field on the MCP.

Regards,
Retell Support Team