Anyone suddenly getting "request headers is too long" and UIFlowParentFlowFailed from PAD flow?

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Posted on by aerojean

Hi everyone

I'm suddenly getting an error in my Power Automate flow's "Run flow built with Power Automate for desktop" action. Below is the action's raw output "body" parameter:

"body": {
 "error": {
  "code": 400,
  "source": "flow-apim-unitedstates-002-eastus-01.azure-apim.net",
  "clientRequestId": "xxx",
  "message": "The response is not in a JSON format.",
  "innerError": "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01//EN\"\"http://www.w3.org/TR/html4/strict.dtd\">\r\n<HTML><HEAD><TITLE>Bad Request</TITLE>\r\n<META HTTP-EQUIV=\"Content-Type\" Content=\"text/html; charset=us-ascii\"></HEAD>\r\n<BODY>
<h2>Bad Request - Request Too Long</h2>\r\n<hr>
<p>HTTP Error 400. The size of the request headers is too long.</p>\r\n</BODY></HTML>\r\n"
 }
}

The flow has been working for months and then started doing this without any change done to it besides maybe re-authenticating connections. Even as the PAD flow does run and executes its actions the Power Automate action returns this error so it considers it as failed. Also worth noting that I checked the input arguments and all of it is under 1k characters and has not changed, so it is strange that in the innerError's HTML it's saying "size of request headers is too long".

The PAD flow runs under "Desktop Flow Runs" are marked as Succeeded status but with "UIFlowParentFlowFailed" error.

Is anyone else experiencing this or has seen this before? Any recommended actions? It started around February 13th for me.

Thank you

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Haque 3,229 on at

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Hi @aerojean,

Apparently the error says "The request headers that is sent to the API are exceeding the allowed size limit"! The error "HTTP Error 400. The size of the request headers is too long."

The probable causes are might be:

There is a high chance that the request includes very large cookies or authentication tokens.

There are too many or large custom headers.

The flow or environment has accumulated excessive header data (e.g., from repeated authentication or session tokens).

Possible actions you can take one by one:

Refresh the connection: Delete and recreate the connection used by the "Run flow built with Power Automate for desktop" action to reset authentication headers.

OR

Go for simplified flow: If your flow is very large and complex or has many nested calls, consider splitting it into smaller flows to reduce header size.

OR

Reduce payload size if possible: Although this error is about headers, sometimes large payloads can indirectly cause oversized headers (e.g., large authorization tokens).

OR

Clear or reduce cookies and tokens: If your flow or environment uses authentication tokens or cookies, try refreshing or regenerating them. Sometimes stale or oversized tokens cause this.

OR

Check for large or unnecessary headers:
Review your flow’s HTTP or connector actions to see if any headers are being set with large values or repeated unnecessarily.

Please let me know if these help.

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TT-26020214-0 2 on at

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I had the same issue, and I kept finding people like us where it just stopped working suddenly, throughout the years.

I eventually chased someone's idea about updating Connections. The desktop flow connection machine/machine group was set to "Select the machine or machine group to connect to", so I selected my group (of one desktop) and updated. This seemed to have fixed the issue, fingers crossed. The option still shows this as if I didn't save it, so perhaps that's expected behavior.

Anyway, I wonder if it's a licensing issue.

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aerojean 72 on at

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Thanks everyone for your answers. However, the issue solved itself as it is now running as it was before with no error and no change or action was done on my part.

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Haque 3,229 on at

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Hi @aerojean,

This is interesting - the problem came, the problem gone, giving a trouble in between. In my POV, it may reappaer - so it's alwasy better to have the root cause. This type of automagical problem sometime creates suffocaiton. Be vigilant!

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