GitHub · pull requests · merge API · Checked October 24, 2026

GitHub: "Pull Request is not mergeable" and "Head branch was modified"

When PUT /repos/OWNER/REPO/pulls/NUMBER/merge fails, GitHub's REST documentation names the statuses: 405 when the merge cannot be performed, 409 when you sent sha and the pull request's head no longer matches it, 403 when you may not do it, and 422 when the request fails validation or the endpoint has been spammed. GitHub's earlier API documentation showed the messages of the first two:

HTTP/1.1 405 Method Not Allowed

{"message": "Pull Request is not mergeable", "documentation_url": "…"}

HTTP/1.1 409 Conflict

{"message": "Head branch was modified. Review and try the merge again.", "documentation_url": "…"}

405 "Pull Request is not mergeable"

The message does not say why. These are the cases, from GitHub's documentation and from public reports:

GitHub has not finished checking it. GitHub's documentation: when a pull request is created or changed, GitHub works out in a background job whether it can be merged, and mergeable is null until the job is done. Public reports show the same message, from REST and from GraphQL, for a merge sent right after a push or a branch update, and the same pull request was mergeable a few seconds later.
It conflicts with the target branch. mergeable is false.
The branch's rules are not met yet. A branch protection rule or ruleset asks for approving reviews, passing status checks, or a branch that is up to date with the target before the merge.
It is a draft. A draft cannot be merged until it is marked ready for review.

Public reports also show 405 with Base branch was modified. Review and try the merge again. (the target branch moved): read the pull request again, then merge.

Where it stands

curl -H "Authorization: Bearer your-token" https://api.github.com/repos/OWNER/REPO/pulls/42

mergeable is true, false or null (still being worked out: read it again a moment later), and mergeable_state gives the state in a word, for example unknown. GraphQL documents the states: mergeable is MERGEABLE, CONFLICTING or UNKNOWN, and mergeStateStatus is one of these:

CLEAN: mergeable and the commit status passes. UNSTABLE: mergeable with a status that does not pass. HAS_HOOKS: mergeable with passing status and pre-receive hooks.
BLOCKED: the merge is blocked. BEHIND: the head branch is out of date.
DIRTY: the merge commit cannot be cleanly created. UNKNOWN: the state cannot be determined yet.

409 "Head branch was modified. Review and try the merge again."

You sent sha, the head you reviewed, and the branch has another head now. In public reports, a commit was pushed between reading the pull request and merging it: by a script's own step that brought the target branch in, or by a bot that updates dependencies. Sending the old sha again fails the same way. Read the pull request, look at the new commits, then merge with the new head.

The asynchronous merge API

GitHub's documentation now recommends PUT …/pulls/NUMBER/merge-async instead; the endpoint above does not support stacked pull requests or a merge queue. It answers 202 with an ID to fetch the result, 200 at once when the pull request is already merged or in a merge queue, 400 at once when it cannot be merged (closed, or still a draft), and 409 when another asynchronous merge request for it is already pending. Branch protection rules and rulesets are not checked at that first step.

What to do

405 right after a push: wait a few seconds, read the pull request until mergeable is not null, then merge again.
405 with conflicts: update the branch or resolve the conflicts in a clone and push.
405 from the branch's rules: get the reviews or the passing checks that the rules ask for, update the branch if it is behind, or turn on auto-merge so that GitHub merges once the rules are met.
409: review the new commits, then merge with the new head's sha.
403: the token or its account may not merge here; see "Resource not accessible by personal access token".

In IntelliJ IDEA: the buttons stay off, and a refused merge shows nothing

How the GitHub support built into IntelliJ IDEA merges a pull request, in its source (checked October 30, 2026; IntelliJ IDEA: GHPRReviewFlowViewModelImpl.kt, GHPRMergeabilityStateBuilder.kt, GHPullRequestMergeStateStatus.kt, GHPRDetailsServiceImpl.kt, GithubApiRequests.kt):

Merge… and Squash and Merge… are on only while GitHub's merge state is CLEAN, HAS_HOOKS or UNSTABLE (for an administrator, when the target branch has a branch protection rule, also BEHIND or BLOCKED), so never while it is UNKNOWN, DIRTY or DRAFT; Rebase is on only when GitHub says the pull request can be rebased. Each also needs the method to be allowed in the repository, write permission, and, when the target branch has a branch protection rule, that you can push to it.
It merges with the REST endpoint above, PUT /repos/OWNER/REPO/pulls/NUMBER/merge, not the asynchronous one, and sends the head it last loaded as sha: a commit pushed since then gets the 409 above. Merge… and Squash and Merge… first ask for the commit message.
When GitHub refuses the merge, with a 405, a 409 or anything else, nothing appears on screen: the source writes the error to the IDE's log, and showing it is marked there as still to do. The pull request is then loaded again, so the buttons follow its new state. For the reason, read the pull request with the curl above.

GitHub pull requests in IntelliJ IDEA

Pull Request Inbox for GitHub, a plugin by Lesstep for JetBrains IDEs 2024.1 or later, shows a pull request's merge checks before you merge, with the ones that block the merge first: approvals, requested changes, checks, conflicts, the target branch's rules not met yet, new commits on the target branch that the rules ask for, a draft, and whether your account may merge; while GitHub is still working it out, it says so. It merges the head commit you had on screen. When GitHub refuses, it says so: for 405, that GitHub cannot merge it now, with GitHub's words, and to see the merge checks; for 409, that the branch changed and to refresh. It can also turn on auto-merge and update the branch. The review inbox, diffs and conversations are free; approving, commenting and merging are Pro (US$1.50 a month, with a 30-day trial).

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