I'm building a Telegram chatbot in n8n that uses OpenAI to process both text and voice messages. Text messages work fine, but for voice messages, the AI Agent node throws a "No prompt specified" error even though the transcription exists

Questions
1

Hello,

I’m currently developing a conversational agent on n8n, connected to Telegram, with the goal of receiving messages (text or voice) and automatically replying via the AI Agent node.

:white_check_mark: What works:

When I send a text message via Telegram:

  • The message is successfully captured,
  • It is passed to the AI Agent node,
  • The prompt is recognized ({{ $json.message.text }}), and the response is correctly sent back via Telegram.

:x: Issue encountered:

When I send a voice message, I get the following error from the AI Agent node:

Problem in node 'AI Agent'
No prompt specified

:mag: Details:

  • The voice message is successfully downloaded using Telegram -> Get File.
  • The transcription is correctly performed using OpenAI - Transcribe Recording, and the text field is present in the output.
  • I use {{ $json.text }} in the Prompt (User Message) field of the AI Agent node.
  • However, the "No prompt specified" error appears only with voice messages.
  • The text field is clearly visible in the output JSON of the previous node.

:wrench: What I’ve tried:

  • Added a Switch to handle text and voice messages separately.
  • Used {{ $json.text }} for voice and {{ $json.message.text }} for text.
  • Added a Set node to unify both into a common field like prompt = {{ $json.text }}.
  • Set "language": "fr" in the transcription node to force French.

:bulb: What I need:

I want the AI Agent node to dynamically handle either case:

  • Use message.text when the message is a text,
  • Use text (transcription result) when it’s a voice message,
  • And generate a response in both scenarios without triggering the "No prompt specified" error.

Any suggestions or working setups are welcome. Thanks in advance!

—Preformatted text

2

Not clean, but probably works

connect both node, json.text and json.message.text

then in your ai prompt field. use like :

// your input here
{{  $json.text ?? json.message.text }}