How to review changes against a base branch with Codex

Reviewing a branch against its base branch isolates the exact changes that would land in a merge or pull request. Catching risky diffs early reduces regressions, shortens human review cycles, and keeps releases predictable.

The Codex CLI can run a non-interactive review over a Git diff and return prioritized findings. The --base option sets the comparison point so the review focuses on differences from a target branch such as main, develop, or a release branch.

Accurate results depend on the base branch ref being available locally and up to date, especially when comparing against a remote-tracking branch like origin/main. Keeping unrelated working tree changes out of the diff also helps ensure the findings map cleanly back to the branch being reviewed.

1. Confirm the current directory is a Git repository.

   $ git rev-parse --show-toplevel
   /home/user/projects/example-repo

2. Confirm the working tree has no uncommitted changes.

   $ git status --porcelain

   No output indicates there are no staged or unstaged changes.

3. Fetch the latest remote refs so the base branch comparison is current.

   $ git fetch --prune

   No output indicates the remote is already up to date.

4. List commits that differ from the base branch to confirm the review scope.

   $ git log --oneline origin/main..HEAD
   e5ae13c Add audit logging flag
   eacaa6a Add API client timeout helper

5. Review changes compared to the main branch.

   $ codex review --base main
   The changes add two small utility functions and a type definition without introducing any apparent functional or integration issues.

   Use --base with the branch that will receive the merge, such as develop or release/1.2.

6. Inspect the pull request-style diff summary for the same comparison range.

   $ git diff --stat origin/main...HEAD
    src/api/client.ts | 7 +++++++
    src/api/types.ts  | 4 ++++
    2 files changed, 11 insertions(+)