Agent returns "Bad Request - Header Field Too Long" (HTTP 400) when trying to add to MS teams

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Posted on by JK-04052223-0 Microsoft Employee

Hi,

My Copilot Studio agent was working fine for over a week in both Teams and M365 Copilot (added through channels). Today, after few changes were made, I uninstalled the agent from Teams and tried reinstall it again. Then started getting issues.

Current status:
- M365 Copilot link (m365.cloud.microsoft/chat/?titleId=...) — Working...
- Teams deep link (teams.microsoft.com/l/app/?titleId=...) — STILL returning "Bad Request - Header Field Too Long" (HTTP 400)

The Teams link appears to route through Classic Teams which is now deprecated, and returns a "Bad Request - Header Field Too Long" error. After uninstalling the agent from Teams, it no longer shows up when trying to add it back via channels.

What I've tried:
- Cleared browser cookies for all Microsoft domains
- Opened in InPrivate/Incognito window
- Signed out and signed back in
- Verified the agent works fine via M365 Copilot chat

Questions:
1. Is the Teams deep link format (teams.microsoft.com/l/app/?titleId=...) still valid, or does it need to be updated for New Teams?
2. After uninstalling a Copilot Studio agent from Teams, is there a propagation delay before it can be reinstalled?

The agent works in M365 Copilot, so this seems to be Teams-specific. Any guidance appreciated. Thanks!

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Publish & channel management

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11manish 3,333 on at

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No. The teams.microsoft.com/l/app/?titleId=... format is legacy and unreliable in the new Teams. Use the Apps catalog instead.

Yes. After uninstalling or republishing, there can be a delay (minutes to hours) before the agent becomes available again in Teams.

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JK-04052223-0 Microsoft Employee on at

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Thanks, Manish, for the response.

It's been over 12 hours now and the agent still doesn't show up in the Teams app catalog when I search for it. The only workaround I found is downloading the manifest ZIP from Copilot Studio and manually uploading it through "Manage your apps" → "Upload a custom app" — which does work, but that shouldn't be the expected behavior.

The bigger concern is that I need to share this agent with others for UAT. Right now, the only option I have is the M365 Copilot link, which works fine, but Teams also should be an option for our testers. Having each person manually upload a ZIP isn't scalable for UAT.

If this is a known issue after environment-level changes, I'd appreciate any pointers. Also want to understand if the manual ZIP upload breaks every time the agent gets republished — because that would mean re-uploading after every update during testing.
Any help would be great. Thanks!

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