Title: MCP tools work in Test LLM (text) but are NEVER invoked during live phone calls

Support Help
1

Hi Retell team,

I have a single-prompt voice agent connected to an n8n MCP server (Google Calendar tools). It works perfectly in the Test LLM (text) playground, but on real inbound phone calls the MCP tools are never invoked. I’ve done extensive debugging and isolated the issue to Retell’s voice mode. Details below.

-– SETUP —

  • Agent: “Single-Prompt Luisy Barber Shop” (agent_35f650896fff185ecd93f26f96)
  • LLM: GPT 4.1, Spanish (Latin America)
  • MCP server (n8n, Streamable HTTP): https://n8n2ver-n8n.zvnyaa.easypanel.host/mcp/eba3c265-2899-43ae-ab70-ee0f009fb777
  • Tools exposed by the MCP: Get_many_events, Create_an_event, Delete/Get/Update event (Google Calendar)
  • The MCP connection IS listed in the agent’s MCPs section, with the correct URL.

-– WHAT WORKS —

  • In Test LLM (text), the agent correctly invokes Get_many_events and Create_an_event, and the event is created in Google Calendar (status: confirmed). End to end works in text.
  • A built-in function (obtener_fecha_y_hora_actual / get current time) IS invoked normally during voice calls.
  • Querying the n8n MCP production URL directly returns HTTP 400 to an incomplete handshake, i.e. the endpoint is alive and listening.

-– THE PROBLEM (voice calls) —

  • On real inbound phone calls, the agent SAYS its filler line (“let me check the calendar…”) but NEVER invokes Get_many_events or Create_an_event.
  • n8n logs show ZERO executions during these calls — the request never leaves Retell toward the MCP.
  • The built-in time function fires, but the MCP (calendar) tools do not. So it’s specific to the MCP tools in voice mode.

-– FAILED CALL IDs (all inbound, same agent) —

  • call_31f1c8eeb7ecf569228…821
  • call_8bb7aa7a75e966686e0…d6e (please pull full IDs from the agent’s call history around Jun 25-26, 2026)

-– WHAT I’VE ALREADY RULED OUT —

  • MCP URL is correct (identical to the working n8n production URL).
  • Agent re-published (version went from V250 to V315) — no change.
  • MCP timeout raised from 1000ms to 30000ms — no change.
  • Prompt verified; tools execute fine in Test LLM with the same prompt.
  • n8n side fully healthy (5 calendar tools connected, production endpoint responding).

-– QUESTION —
Why are MCP tools invoked in Test LLM but never invoked during live voice calls, with no request reaching the MCP server? Is there a setting that controls MCP tool availability in voice mode vs text mode, or is this a known issue? Happy to provide call recordings, transcripts, or anything else you need.

Thank you!

#function-calling, #n8n

2

Hey @lmorello Thanks for the detail writeup , I have escalated this to the team for a review.

Thank You

3

Hi @Shah-Fazal, thank you for escalating this to the team — I really appreciate it. I’ll stay tuned for any update from them. In the meantime, if it would help the review, I’m happy to provide more details, logs, or screenshots. Thanks again!

4

Hey @lmorello Can you share the call ids?

5

Hi @Shah-Fazal, sure — here’s the call ID from my most recent test call: call_e62ce872ce2700a23f5a117bb41. This is a call where the appointment-creation tool should have been invoked but wasn’t. Let me know if you need any more call IDs or additional details. Thanks!

6

@lmorello Noted, escalated with the team

7

Hey @lmorello In this call, the agent is using version v250, while the latest version is v316.

When testing in the playground chat, it uses the latest version, but during the phone call, the agent is assigned to an older version that does not have the MCP configured correctly.

Thank You

8

Shah, thank you for the diagnosis. I tried changing the phone number from v143 to v317 (latest), but the live phone test still failed with the exact same symptom — the agent never invokes the MCP tools, just keeps ‘verifying’ without creating the appointment. So the issue is NOT the old version. The problem must be elsewhere in the MCP configuration itself. Can you help identify what else could prevent the MCP tools from firing during a live call?

9

Hey @lmorello can you share call id for the new test.

10

call_eaa91f31fdf85a07df7e18c2a86

11

Hey @lmorello Using v250

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