GitHub · Commits · REST API · Checked October 25, 2026

GitHub: why the comparison was refused

Two commits are compared with GET /repos/OWNER/REPO/compare/BASE...HEAD. Like a pull request's Files changed, it is a three-dot comparison: the files from the merge base (the newest commit both share) to the head, and the commits of git log BASE..HEAD. Branches and commits of forks in the same repository network can be compared; a fork's branch is written USERNAME:BRANCH. GitHub's documentation lists 200, 404, 422, 500 and 503 for it (checked October 25, 2026).

{"message": "No common ancestor between 03f7ef390fef7f30111e6a5bafc345f4389cbea8 and 5b4d124ec29c81900f0c34a89c7de23405dbd791."}

Server Error: Sorry, this diff is taking too long to generate. (HTTP 422)

GitHub's words and the reasons

No common ancestor between … and …. (404 in a public report from 2022): the two commits share no history, so there is no merge base to compare from; for example, one branch was started without a parent commit.
404 without that sentence: a branch or commit GitHub does not have in this repository or its network (deleted, never pushed, misspelled), or the repository is not there for this token, which is also how GitHub answers a private repository the token cannot see.
Sorry, this diff is taking too long to generate.: GitHub could not compute the comparison in time. Public reports met it as 422 on GitHub.com in 2026, including with per_page=1, and as 500 on a GitHub Enterprise Server in 2020. In one of them, CI tried three times, and the same comparison still gave the error when the report was written. For a request that GitHub ends after 10 seconds, see an API request timed out.
500 and 503 are also in GitHub's list (internal error, service unavailable).

Limits that cut a comparison short, with no error

250 commits: without per_page or page, the list stops at 250 commits, the last one being the newest of the whole comparison.
300 files: with paging, the changed files come on the first page only, up to 300 for the whole comparison; per_page goes up to 100 commits.
A pull request's own list (GET /repos/OWNER/REPO/pulls/NUMBER/files) goes further, to 3,000 files (see GitHub diff 406 "too_large").

The base commit a pull request keeps

GitHub's documentation says that when a pull request is opened, GitHub sets its base to the commit the base branch points to, and does not update that commit when the branch moves on.
Comparing that commit with the head directly is a two-dot comparison. The documentation says a two-dot comparison changes when the base branch is updated, even if the topic branch did not; a branch that has merged newer base commits then shows their changes as its own.
When a head branch is deleted after its pull request is merged, GitHub changes open pull requests that used it as their base to the merged pull request's base, the documentation says.

What to do

No common ancestor: there is nothing to compare from; merge or rebase the branch onto the base branch so the two share history.
Too long to generate: compare smaller steps, or read the changed files with git (git diff --name-only BASE...HEAD); one public report explored getting the file list from git instead.
More than 250 commits or 300 files: page through the commits; for the files, use the pull request's file list or git.
404: check that both commits are pushed to the repository or a fork in its network, that the branch still exists, and that the token can see the repository (see which permission to add).

GitHub pull requests in IntelliJ IDEA

Pull Request Inbox for GitHub, a plugin by Lesstep for JetBrains IDEs 2024.1 or later, compares the target branch with the pull request's head to find the merge base its diffs start from and whether the target has new commits (shown with an "Update the branch" link). When GitHub refuses that comparison, the diffs start from the base commit the pull request names instead, and the Files tab says so: changes merged in from the target since the pull request was opened may show as its own, and whether the target has moved on is not known. To mark the files that changed since you last looked at a pull request, it compares the head you saw with the new one; when GitHub refuses that, or lists 300 files or more, it cannot tell which files changed, so it clears your viewed marks for that pull request, says so on the Files tab, and marks none as "changed since you last looked". 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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