Error in Custom Prompt – AIModelActionBadRequest (400)

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Posted on by TL-09090540-0

Hi everyone,

I’ve created an agent in Copilot Studio and added a topic flow where the goal is to generate a user story from meeting notes using a Custom Prompt. However, I’m consistently getting the following error

Error Message: The prompt 'Custom prompt 09/08/2025, 03:56:07 PM' ('0f4ec4e3-4029-4e3b-98d7-cc6cec0dfe10') failed to run with response code '400'.
Error Code: AIModelActionBadRequest
Conversation Id: cb93a68c-5417-47fc-a55e-f814a45f507f
Time (UTC): 2025-09-09T05:34:49.005Z

I’ve already tried:

Reducing the input size
Using plain text instead of JSON
Testing the prompt in Prompt Builder (which works fine)

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Jerry-IN 116 on at

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Error in Custom Prompt – AIModelActionBadRequest (400)

Hello TL-09090540-0,

This is a common issue. The fact that your prompt works in the builder but not in the topic almost always means the input data is the problem.

The AIModelActionBadRequest error usually happens because the variable you're feeding into your prompt (the meeting notes) is blank when the topic runs. The prompt gets an empty input and fails.

Here’s the quickest way to check:

In your topic, add a "Send a message" action right before the custom prompt action.

In this message, just display the variable that holds your meeting notes.

When you test it again, you'll likely see a blank message from the bot. That confirms the variable is empty, and you just need to trace back and see why it's not being filled correctly earlier in your flow.

Best Regards,
Jerald Felix

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