TF401192: “The source branch has been modified since the last merge attempt” (fix)

You try to complete a pull request in Azure DevOps and get Operation failed: TF401192: The source branch has been modified since the last merge attempt. (409). It looks alarming, but nothing is broken: the pull request changed while you were completing it. Here is what the error means and how to get past it in the browser, in VS Code, with the Azure CLI or with the REST API.

What TF401192 means

Before a pull request can be completed, Azure DevOps makes a test merge of the source branch into the target branch. That merge is based on one specific commit of the source branch, the last merge source commit. When you complete the pull request, your browser or tool tells Azure DevOps which commit it expects. If the source branch has a newer commit by then, Azure DevOps refuses with 409 Conflict and TF401192, so it never completes a merge you haven't seen.

Common reasons the source branch moved on:

• Someone (or you, from another machine) pushed to the source branch while the pull request page was open.
• A pipeline pushes to the source branch, for example a version bump, formatting or a generated file, during build validation.
• You just resolved merge conflicts in the browser. That creates a new commit on the source branch, and Azure DevOps has to merge again.
• Your view is simply stale: a VS Code extension, script or browser tab still holds the pull request as it was a while ago.

Fix it in the browser

1. Reload the pull request page.
2. Wait until Checking for merge conflicts is gone and the merge has succeeded. If it never finishes, see pull request stuck on “Checking for merge conflicts”.
3. Look at the new commits on the Commits tab, so you know what you are about to merge.
4. Choose Complete again. If the button now says Set auto-complete, the new push reset a policy (for example approvals or build validation); see can't complete a pull request after resolving conflicts.

If the new merge has conflicts, resolve them first: see how to resolve merge conflicts in an Azure DevOps pull request.

Fix it in VS Code or another tool

Extensions and tools that complete pull requests for you use the same check. Refresh the pull request in the tool (or close and reopen it) so it picks up the latest source commit, wait for the merge to finish, and complete it again. If the tool keeps failing, complete the pull request once in the browser.

Fix it in a script or the REST API

Completing a pull request through the REST API is a PATCH on the pull request with the status and the source commit you expect:

PATCH https://dev.azure.com/{org}/{project}/_apis/git/repositories/{repo}/pullrequests/{id}?api-version=7.1

{
  "status": "completed",
  "lastMergeSourceCommit": { "commitId": "<current lastMergeSourceCommit.commitId>" }
}

TF401192 means the commitId you sent is no longer the latest. Read the pull request again with a GET, wait until mergeStatus is succeeded, and send the new lastMergeSourceCommit.commitId. In a script, do this in a loop with a short delay and a maximum number of tries, so a pipeline that pushes once doesn't fail your run. The same applies to az repos pr update --status completed: run it again once the merge has finished.

It keeps happening

Then something keeps pushing to the source branch. Check the branch's Pushes (Repos → Pushes) or the pull request's Commits tab for who or what pushed last. Usually it is a pipeline that commits during build validation. Let that pipeline push only after the pull request is merged, or set Set auto-complete on the pull request: Azure DevOps then completes it itself once the latest merge and all policies have passed, so you don't race the pushes.

Related guides

• Can't complete an Azure DevOps pull request after resolving conflicts
• Azure DevOps pull request stuck on “Checking for merge conflicts”
• How to resolve merge conflicts in an Azure DevOps pull request
• How to update a pull request with the target branch in Azure DevOps
• Merge Conflict Resolver: features, pricing and free plan