Implementing Immutability in Mercurial for Stable, Traceable Code

The codebase was breaking in ways no one could trace. Changes seemed to bleed into places they had no right to be. The cause was clear: mutability without discipline.

Mercurial is fast, distributed, and trusted by teams shipping critical software. But without immutability, its history can still be altered, rewritten, or polluted. Immutability in Mercurial means designing your workflow, hooks, and repository policies so commits and tags cannot be changed once written. This eliminates hidden merges, prevents history rewrites, and guarantees that every hash points to the same exact data forever.

Implementing immutability in Mercurial starts with server-side enforcement. Disable --force pushes. Require signed commits. Use hooks like pretxnchangegroup to reject incoming changesets that rewrite public history. Store repositories on filesystems with write protections for .hg/store. Audit pushes with scripts that compare incoming changes against known public states.

Immutable workflows also need clear branching rules. Mark default branches read-only once released. Use feature branches for development, merge into main through a controlled gate, and lock main after merge. Combine this with continuous integration checks that fail on history divergence.

The payoff is stability. No more silent overwrites. No more hunting down who rewrote a release commit. Every version in Mercurial becomes permanent, traceable, and reliable.

If you want to see immutability enforced without cobbling together scripts, try it now on hoop.dev. Build the repo, push the rules live, and watch it run in minutes.