mailmap-checker

A pre-commit hook that detects unmapped Git identities by comparing your .mailmap against the full commit history. It groups authors and committers by email address and email local-part so duplicates are caught even across domain changes.

Follows the gitmailmap specification: all four mapping formats are supported, and both names and emails are matched case-insensitively.

How it works

The checker scans git log for all unique author and committer identities and groups them using two rules:

Rule 1 — Same email (case-insensitive)

Identities that share the exact same email address are the same person.

Alice Johnson <alice@acme.com>
alice.j       <alice@acme.com>   ← same email, grouped together

Rule 2 — Same email local-part (different domain)

Identities whose email local-part (the part before @) and name match are likely the same person who changed companies or used a different address. Local-parts shorter than 8 characters are automatically skipped to reduce false positives.

Alice Johnson <alice.johnson@acme.com>
Alice Johnson <alice.johnson@oldcorp.com>    ← same local-part + name, grouped

Once groups are built, the checker looks for identities that are not mapped in .mailmap. If a group has more than one identity and any of them is missing from the file, the hook fails and reports the gap.

Example

Given these identities in git history and an empty .mailmap:

Alice Johnson <alice.johnson@acme.com>
Alice Johnson <alice.johnson@oldcorp.com>
alice.j       <alice.johnson@acme.com>

mailmap-checker check detects the gap:

  Canonical: Alice Johnson <alice.johnson@acme.com> (42 commits)
    - Alice Johnson <alice.johnson@oldcorp.com> (15 commits)
    - alice.j <alice.johnson@acme.com> (3 commits)

Found 2 unmapped identities in 1 group (canonical chosen by name heuristic
— prefers names that start with a letter and contain a space
(e.g. 'Jane Doe' over 'jdoe')).

Tip: Use --by-commit-count to choose canonical by highest commit count.

mailmap-checker fix --dry-run suggests entries to add:

Suggested .mailmap entries (canonical chosen by name heuristic):

  Alice Johnson <alice.johnson@acme.com> Alice Johnson <alice.johnson@oldcorp.com>
  Alice Johnson <alice.johnson@acme.com> alice.j <alice.johnson@acme.com>

Disabling local-part matching

If Rule 2 produces false positives on very large repositories, disable it with --no-local-part-matching.

Mailmap source resolution

Just like Git itself, mailmap-checker reads and merges entries from multiple sources:

1. --mailmap <path> — explicit path (highest priority)
2. mailmap.file Git config — git config mailmap.file (used when no explicit --mailmap is given)
3. .mailmap in the repository root (default fallback)
4. mailmap.blob Git config — git config mailmap.blob (e.g. HEAD:.mailmap, read from a Git object)

Entries from all applicable sources are merged before checking. This means a project that stores mappings in a committed blob, a separate file, or the default .mailmap will all be handled correctly.

Shared emails (different people, same address)

When two different people share the same email address (e.g. a generic workstation account), the checker automatically detects this from the .mailmap file. If the same email is mapped to different canonical identities, the checker knows they are different people and does not group them together.

# .mailmap — two different people share user@workstation.local
Alice Johnson <alice@acme.com> admin <user@workstation.local>
Bob Smith <bob@acme.com> Bob Smith <user@workstation.local>

Installation

Pre-commit hook (recommended)

# .pre-commit-config.yaml
repos:
  - repo: https://github.com/cansarigol/mailmap-checker
    rev: ""  # run: pre-commit autoupdate
    hooks:
      - id: mailmap-check

Then run pre-commit autoupdate to pin the latest release.

Available hooks

Hook ID	Description
mailmap-check	Fail if any identity is missing from .mailmap
mailmap-fix	Automatically add missing entries to .mailmap
mailmap-fix-dry-run	Preview suggested entries without modifying the file
mailmap-normalize	Deduplicate, collapse to Format 1, and sort entries

Standalone

pip install mailmap-checker

Usage

check

Scan all Git authors and committers and exit non-zero if any identity is missing from .mailmap.

mailmap-checker check

init

Create a .mailmap file (if it does not exist) and run a full check.

mailmap-checker init

fix

Preview or apply suggested .mailmap entries. New entries are inserted in sorted order and the existing blank-line style of the file is preserved (separator-style files keep separators; compact files stay compact).

The canonical identity for each group is chosen by a name heuristic — preferring names that look like real person names (e.g. Alice Johnson) over usernames (alicej), git config artifacts (--global), or handle-style names (@username). Use --by-commit-count to choose the identity with the most commits instead.

# Preview
mailmap-checker fix --dry-run

# Apply
mailmap-checker fix

# Choose canonical by commit count
mailmap-checker fix --by-commit-count

normalize

Deduplicate, collapse same-email aliases to Format 1 (Proper Name <email>), and sort entries alphabetically. Does not require git — operates only on the .mailmap file.

# Preview
mailmap-checker normalize --dry-run

# Apply
mailmap-checker normalize

Common options

Flag	Description
--mailmap <path>	Custom .mailmap file path (default: git config mailmap.file, then .mailmap)
--git-dir <path>	Path to git repository (default: current directory)
--no-local-part-matching	Disable grouping by email local-part across domains
--by-commit-count	Choose canonical identity by highest commit count instead of name heuristic

Contributing

git clone https://github.com/cansarigol/mailmap-checker.git
cd mailmap-checker
uv sync
uv run poe setup   # installs pre-commit and commit-msg hooks
uv run poe check    # lint + security + tests

Commits must follow Conventional Commits with a required scope (e.g. feat(cli): add --verbose flag).

License

MIT