Unable to post messages using @mention tags in Teams

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Posted on by CU11060307-0

Around June 11, 2026, a workflow that had been running on a scheduled basis stopped working. After checking, it appears to fail at the point where it retrieves a Teams @mention tag and attempts to post a message that includes it.

The error JSON (specifically the cause of the error) is shown below:

{

"error": {

"code": "BotRequestFailed",

"message": "Request to the Bot Framework failed with error: '{\"error\":{\"code\":\"BadArgument\",\"message\":\"Decoded tag id has invalid format\"}}'."

}
}

Regarding the message content, the tag ID is not manually entered. Instead, the @mention retrieved in a previous step via the GUI is inserted into the message, so there should be no issue with the input itself. In fact, the tag ID obtained in the earlier step and the ID that is rendered as <atTag> at the time of posting are identical.

Despite this, the error above occurs.

Is anyone else experiencing the same issue?
Also, does anyone know how to resolve this?

For reference, I have confirmed that user mentions (not tag mentions) work without any issues.

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Building flows

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Vish WR 3,748 on at

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@CU11060307-0

Since it broke with no change on your side, and user mentions still work while only tag mentions fail, this looks like a service-side change in the Teams connector / Bot Framework rather than your flow. "Decoded tag id has invalid format" means the API is rejecting the tag ID format at post time.

Worth trying:

Re-fetch the tag token fresh in the same run (delete and re-add the "Get @mention token for a tag" step) so it regenerates with the current format — a stale or recreated tag ID throws exactly this.

Confirm the tag still exists with the same members.

If it persists, check the M365 admin health dashboard and raise a Microsoft support ticket . As a temporary workaround, switch to individual user mentions since those still work.

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chiaraalina 2,425 Super User 2026 Season 1 on at

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Hi @CU11060307-0

It works for me.

This is my flow:

How does your flow look like?

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NB-11061454-0 2 on at

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I am getting the same when using Posta Adaptive card and wait for a response.

I tried almost everything and i can't seem to get it work

The mention user is working fine

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X11061413V 2 on at

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I have the same issue on 2 different flows that use the (at)mention tags!

5 tags are active in a team and my flow chooses the correct one based on dynamic content. 1 tag always fails no matter what and the other 4 fail about 15% of the time.

The 4 tags that run successfully 85% of the time are encoded differently and that results in a success or failure of the Post message action (the same tag might generate an (at)mention output ending in "...YlhmWUg" which runs successfully, but the next time the same tag is mentioned it generates an (at)mention output ending in "...YlhmWUg%3D" which fails. But then a different tag might generate one ending in "%3D" or "%3D%3D" and it runs successfully. So I can't confirm that has anything to do with the "=" being encoded in a certain way.

But the 1 tag that always fails always generates the same (at)mention output.

The errors are always either:

- Request to the Bot framework failed with error: '{\"error\":{\"code\":\"BadArgument\",\"message\":\"Mentioned Tag with id M...GhMeA does not exist in current Team\"}}'.

- Request to the Bot framework failed with error: '{\"error\":{\"code\":\"BadArgument\",\"message\":\"Decoded tag id has invalid format\"}}'.

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