Show which files conflict in the pull request panel

[thomas-anst]

Problem

When a pull request conflicts with its base branch, the PR panel shows that it is conflicting and offers Resolve conflicts. It does not show which files conflict, or what conflicts in them. To find out, the user has to check out the branch and merge locally. GitHub does not show this either.

Seeing the conflicts first tells the user how big they are and whether to hand them to an agent or fix them by hand.

Proposal

A Merge conflicts section in the PR panel's summary tab. It sits below Reviewers and Labels, above Description. It shows only while the PR is open and the host reports it as conflicting.

1. List the conflicting files.
2. View a conflict: a click on a file expands its conflict regions, read-only. Each region has 3 lines of context and real line numbers. The base-branch side and the PR side are told apart, and the markers are labelled with the branch names.
3. Resolve from a file: a button starts the existing Resolve conflicts hand-off. The prompt now lists the conflicting files and names the one to start with. The global Resolve conflicts button also passes the list.

Editing in the panel is out of scope. The editor works on files on disk in a thread's worktree, and the existing Check out + agent flow already covers that.

Approach

GitHub, GitLab and Forgejo expose a conflict flag but no file list, so the server computes it from the project's local checkout:

• Two new reads in packages/contracts:

  • pullRequests.conflicts takes a PullRequestRef and returns { paths, truncated } | null.
  • pullRequests.conflictFile takes a ref and a path and returns { contents } | null.

  Both are separate from detail, so the pull request watch never pays for them. The client asks for them only when the detail says conflicting, and for a file only when it is expanded.

• The server looks for the checkout remote whose URL matches the PR's repository, so origin being a fork is no problem. If no remote matches, nothing is computed.

• It fetches the base branch and the PR head (refs/pull/<n>/head, or refs/merge-requests/<n>/head on GitLab) into private per-PR refs, refs/t3/pull-request-conflicts/<n>/{base,head}. The fetch uses an empty --refmap and never prompts for credentials. The user's remote-tracking branches never move, and concurrent reads cannot swap heads through FETCH_HEAD.

• It then runs git merge-tree --write-tree --name-only --no-messages -z, which needs git ≥ 2.38 and never touches the worktree or index. A file's conflict comes from the merged tree that command writes, and only listed paths can be read. Results are cached for 30 s and capped at 100 paths, and the prompt says when the list is truncated.

• Any failure (no matching remote, shallow clone, older git, network) returns null and is logged. The section then stays hidden, and the host's flag and the existing actions are unchanged.

• Scope: web and desktop. Mobile has no PR detail view. Bitbucket and Azure keep the section hidden until their head refs are wired. MCP exposure is out of scope for now, but the logic lives in service methods.

Two structural points need your call: two new WebSocket methods in packages/contracts, and a new dependency from PullRequestService on GitVcsDriver. GitVcsDriver is already provided to it in server.ts.

Status

A working implementation is on thomas-anst/t3code@feat/pr-panel-merge-conflicts. It has focused tests, including real-git tests for the merge, the private-ref fetch and reading a conflicted file. It has also been checked by hand against real conflicting PRs.

If you agree with the direction and scope, I will open the PR with before/after screenshots and link it here. I can also split it into two PRs: the file list first, then viewing and per-file resolve.

---

Drafted with Claude Code (Claude Opus 5.5).