Tagging copilot in an existing PR doesn't work

[nnellanspdl]

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Copilot Feature Area

Copilot Coding Agent

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For a given repo, CCA works fine in 2 out of 3 scenarios. For example, if I manually create a brand new Coding Agent task through the Agent panel, then it works fine. CCA will open a new branch, make its commits, and then open a draft PR. Also, it works fine when I assign an Issue to CCA, it is able to do what it needs to do.

However, on that same repo, if I go to an existing Pull Request, tag @copilot and then ask it to make updates, then this does NOT work. Copilot reacts to my comment with the eyes emoji. But, then after that nothings happens. CCA does not open a branch, it does not start a new CCA task.

I'm not sure why, as normal CCA tasks works fine, just not from an existing PR.

[parniayzdin]

Hello there. This actually isn’t expected behaviour. Copilot Coding Agent is supposed to respond to an @copilot mention on an existing pull request by starting a task and opening a child pull request that uses the existing PR’s branch as its base. Since it already works for you when you start a task from the agents panel or by assigning an issue, your org and repo are most likely configured correctly.

Make sure your @copilot message is a submitted top-level comment or review (not just a draft), that the PR is in a repository where Copilot Coding Agent is enabled, and that the repository isn’t using managed user accounts, where some Copilot features are restricted. After you post the comment, check the agents panel or your Copilot task list to see whether a task briefly starts and then fails. If you still see only the reaction and no new task or child PR, this is likely a bug rather than a configuration problem, so I believe the next step is to open a ticket with GitHub Support and include a link to one of the affected PRs and the time of your @copilot comment so they can look at the logs.

Hope this helps!

[yassine-krichen]

Right now, Copilot Coding Agent can’t reliably trigger a task from comments on an existing PR.
The eyes emoji just means “Copilot saw your comment,” not that it can actually act on it. For CCA to start a task automatically, a few conditions need to be met:

What does work

• Creating a task directly from the Copilot panel.
• Assigning an Issue to Copilot.

Both of these create a fresh task context that Copilot controls end-to-end, which is why they behave normally.

What doesn’t work (by design, currently)

• Tagging @copilot in an existing PR and asking it to perform updates.

CCA doesn’t yet support taking over an already-open PR and treating it as a task. It won’t create a task, won’t open a working branch, and won’t push commits. The system reacts to the mention but cannot act on it.

Workaround

If you need Copilot to modify the code:

1. Create a Copilot task manually via the Agent panel, or
2. Create/assign an Issue and let Copilot open its own PR.

This gives it the controlled environment it expects.

Bottom line

Your repo isn’t misconfigured. This scenario just isn’t supported yet.

[nnellanspdl]

Your repo isn’t misconfigured. This scenario just isn’t supported yet.

Are you sure about that? Source: https://docs.github.com/en/enterprise-cloud@latest/copilot/how-tos/use-copilot-agents/coding-agent/make-changes-to-an-existing-pr

[yassine-krichen]

Thanks for the clarification. After digging deeper into how Copilot handles this workflow, here’s what’s actually going on:

You’re right that the docs state Copilot can modify existing PRs. That feature definitely exists. The issue is that modifying an existing PR has a separate set of constraints that don’t apply when you create a brand-new Copilot task or assign an issue. In those two cases, Copilot owns the environment and can freely create its own branch, so everything works normally.

When you ask Copilot to update a PR that already exists, it doesn’t get that freedom. It has to inherit the exact conditions of that PR: the branch it’s based on, the protection rules tied to it, repository permissions, whether it’s from a fork, and any workflows that restrict automation. If anything in that environment blocks automated writes, Copilot will respond to the comment (the 👀) but won’t start a task.

If you already verified all of the usual constraints (PR isn’t from a fork, no strict branch protection, you have write access, and Agents are enabled for the repo), then you’re very likely hitting a real GitHub-side bug. This specific path is much more brittle than the other two Copilot entry points, so bugs happen here more often than they should.

So your repo isn’t misconfigured, and the feature does exist. It’s just that the “modify an existing PR” flow has extra requirements under the hood, and if all of them are satisfied but the agent still doesn’t start, the failure is probably on GitHub’s side rather than yours.

[GowsiSM]

This behavior suggests there might be a specific configuration or permission issue that's blocking CCA from operating on existing PRs, even though it works for new tasks. Here are the most likely reasons:

1. Branch Protection Rules
   The existing PR's branch might have protection rules that prevent CCA from pushing to it or creating a child branch
   Check if there are required status checks, required approvals, or restrictions on who can push to the branch

2. Missing Permissions for the CCA Bot
   While CCA has permissions for new branches/tasks, it might lack specific permissions to interact with protected branches or existing PR contexts

3. PR State Limitations
   The existing PR might be in a state that CCA can't work with (e.g., closed, merged, or in a conflicted state)
   CCA typically requires the PR to be open and mergeable

Quick Debugging Steps:
Check the PR's branch protection settings
Verify the PR is open and has no merge conflicts
Look in your GitHub Actions logs for any failed CCA attempts

Try the @copilot mention on a different, simpler PR in the same repo to isolate the issue

The fact that you get the "👀" reaction confirms CCA is receiving the trigger, but something is preventing the execution phase. This is likely a permissions or policy barrier rather than a complete feature failure.

[nnellanspdl]

I figured this out, but it still seems like a bug, and I still opened a support ticket with GitHub.

We use repository Rulesets to restrict every one of our commit messages. We require all of our commit messages to match a given RegEx pattern.

In order to get CCA working, I have added Copilot Coding Agent as a bypass to this Ruleset. As stated before, this works great for new CCA tasks, and for CCA tasks assigned from an Issue. However, this bypass does not work for CCA tasks assigned from an existing PR. This doesn't seem right to me, and that's what I opened my support ticket about.

I did some testing, temporarily disabled my Ruleset, and now CCA works just fine when assigning from an existing PR.

[yassine-krichen]

Thanks for confirming the root cause. That definitely sounds like a bug. If the agent can bypass the ruleset for new tasks and issue-based tasks, it should behave the same way when invoked from an existing PR. The inconsistent behavior basically proves it isn’t applying the bypass in that specific workflow.

Good call on opening the support ticket. This one’s on GitHub’s side.