copilot code review failed: diff too large

TL;DR

Copilot code review caps the diff size it will analyze. Shrink the PR (split it, or mark generated files so they are excluded) and request the review again.

Error

Copilot code review: unable to review  -  diff exceeds the maximum size.

Steps

1. Check the diff size on the PR (files changed / additions+deletions). Expected: confirmation it exceeds the review limit.
2. Mark generated files as generated via .gitattributes (*.snap linguist-generated=true) so they are excluded. Expected: the reviewable diff shrinks.
3. If the human-written diff is still too large, split the PR into stacked smaller PRs. Expected: each PR fits the limit.
4. Re-request the Copilot review. Expected: the review posts.

When to use

• Copilot code review fails with a diff-too-large message.
• The PR contains generated or vendored files inflating the diff.

When not to use

• Copilot reviewed and you disagree with comments - reply to the comments.
• Copilot is not enabled for the repo; enable it first.

Tool compatibility

• GitHub Copilot code review.
• GitHub.

Variant phrasings

Diff large due to a lockfile

Lockfiles are the classic culprit - linguist-generated marking fixes it permanently.

Diff large due to vendored deps

Vendor directories should be gitignored or marked generated.

Why it happens

The review model has a fixed context window; beyond it, the tool cannot see the whole change, so it declines rather than reviewing blind. The cap also nudges PRs toward human-reviewable sizes.

Edge cases

• The size cap applies to the whole diff, not per file - one giant generated file can block review of everything else.
• Splitting purely to satisfy the bot is fine when each split is independently coherent.

Provenance

Resolved from the public thread: https://vectle.com/posts/pstcQ6AxuIqSpkiXNCFJ4jvA

Resolved from the public thread: https://vectle.com/posts/pstcQ6AxuIqSpkiXNCFJ4jvA

Thread query: copilot code review failed: diff too large

TL;DR: Copilot code review caps the diff size it will analyze. Shrink the PR (split it, or mark generated files so they are excluded) and request the review again.