Using Changesets in a polyglot monorepo

One of the nice things about working in a smaller business is you can enjoy using things that don’t need to scale to extreme sizes.
One example in the software world is monorepos.
While monorepos can scale well (see Google, Facebook, and others), doing so requires special tooling and infrastructure.
With plain git, you can only go so far.
While you can use it, it has meaningful advantages, like being able to make atomic changes that affect many parts of the system in a single commit, which eliminates whole classes of compatibility and integration issues.
You can always split a monorepo later (see git-filter-repo).

So, suppose you’re a small-to-medium team using a monorepo.
Let’s go further and say that this monorepo stores all your company’s code, meaning it spans many different programming languages—it’s a polyglot monorepo.
What tool can you use to manage versioning in a consistent way?

I argue that changesets is a solid choice, even if it’s primarily focused on the JavaScript/TypeScript ecosystem.

Background

For any versioning tool, you are typically looking for how to:

• define what content appears in the changelog/release notes
• influence the version numbers of the packages
• automate the commits doing the actual metadata bumps and tagging
• automate the builds that happen in response

changesets assumes per-package semantic versioning (i.e., all packages have their own version).
In addition, each package has its own CHANGELOG.md.

The changesets team also has a GitHub Action, changesets/action which importantly allows specifying custom scripts for the version and publish commands.
That customization is what gives changesets support for polyglot repositories.

In changesets, engineers commit “changeset” files to the repository that define what content ends up in changelogs, and what packages versions are bumped (i.e., major, minor, patch).

See the changesets documentation for more details.

Implementing an automated release process on GitHub

I’m a fan of just.
I also really like uv scripts.
The example below uses both.

I’m also going to assume you are in a enterprise setting where all your monorepo is private, not open-source.

Repository setup

My recommended organization (at least at time of writing) is something like the following.

.
├── .changeset
│   ├── config.json
│   └── README.md
├── contrib
│   └── utils
├── docker
│   └── Dockerfile
├── docs
│   ├── package.json
│   ├── pnpm-lock.yaml
│   ├── ...
│   └── pnpm-workspace.yaml
├── Justfile
├── package-lock.json
├── package.json
├── packages
│   ├── python-one
│   │   ├── ...
│   │   └── package.json
│   ├── rust-one
│   │   ├── ...
│   │   └── package.json
│   └── rust-two
│   │   ├── ...
│   │   └── package.json
├── pnpm-workspace.yaml
└── third-party

Put all packages in a packages/ directory, no matter what language they are.
I also enjoy having documentation as code, so let’s say you have a docs/ directory, too, and that your docs is written in a javascript-based frontend (like Starlight), for the purposes of highlighting a nuance later.

Changeset configuration

With this setup, you can configure changesets with a proxy pnpm workspace at the root with all your packages.

# pnpm-workspace.yaml
packages:
  - "packages/**"

And, declare your changesets dependencies:

// package.json
💬

You should now also update your .gitignore:

node_modules/

Because changesets is built for JavaScript, we also need “proxy” package.json files for all of our packages; changesets uses these to perform version bumps.

These can be as simple as:

// packages/python-one/package.json
⚡

With this setup, note how we are intentionally trying to exclude our internal docs/ as a pnpm workspace member—we only want to version packages.
To do so, declare the docs/ directory its own pnpm workspace, otherwise it will try and combine the docs/ dependencies into the root package-lock.json.
This can be as simple as:

# docs/pnpm-workspace.pyml
packages: []

Next, we can add our changeset configuration:

// .changeset/config.json
{
  "$schema": "https://unpkg.com/@changesets/[email protected]/schema.json",
  "changelog": "@changesets/changelog-git",
  "commit": false,
  "fixed": [],
  "linked": [],
  "access": "restricted",
  "baseBranch": "main",
  "updateInternalDependencies": "patch",
  "ignore": [],
  "privatePackages": 💬,
  "___experimentalUnsafeOptions_WILL_CHANGE_IN_PATCH": {
    "onlyUpdatePeerDependentsWhenOutOfRange": true
  }
}

Automating releases with GitHub

The glue to create polyglot versioning PRs

Next, we want to automate our releases.
That is, generating the changelog PRs, bumping package metadata, pushing tags, and triggering builds on those tags.

Let’s start with our GitHub Workflow definition, and unpack the scripts it calls.

name: Release

on:
  push:
    branches:
      - main

concurrency: ${{ github.workflow }}-${{ github.ref }}

permissions:
  contents: write
  pull-requests: write

jobs:
  release:
    name: Release
    runs-on: ubuntu-latest
    outputs:
      published: ${{ steps.changesets.outputs.published }}
    steps:
      - uses: actions/checkout@v6
      - uses: actions/setup-node@v4
        with:
          cache: npm
      - uses: astral-sh/setup-uv@v7
      - uses: taiki-e/install-action@just
      - run: npm install
      - name: Create Release Pull Request or Tag
        id: changesets
        uses: changesets/action@v1
        with:
          version: just version
          publish: npx @changesets/cli publish
          # I like conventional commits
          commit: "chore(release): version packages"
          title: "chore(release): version packages"
        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

  docker:
    needs: [release]
    if: needs.release.outputs.published == 'true'
    uses: ./.github/workflows/docker.yml
    secrets: inherit

Setting version: just version is the key to polyglot support.

# Version packages based on changesets
[doc('Consume changesets: bump versions, update changelogs, sync native version files.')]
[group('release')]
version:
    npx @changesets/cli version
    uv run --script contrib/utils/sync-versions.py

The meat of the glue for polyglot support then, is how you implement sync-versions.py.

The key bit here is we rely on changesets to bump the versions in package.json for us when we call npx @changesets/cli version, but then it is up to us to propagate that version to the respective language’s metadata appropriately.

Here is an example that uses pretty naive parsing.
You can write something similar (or better!) for the languages you use.

#!/usr/bin/env -S uv run --script
#
# /// script
# requires-python = ">=3.12"
# dependencies = []
# ///
#
# Sync versions from package.json files (updated by changesets) to native
# package manifests (Cargo.toml, pyproject.toml, etc.).

import json
import re
import subprocess
from enum import Enum, auto
from pathlib import Path

PACKAGES_DIR = Path(__file__).resolve().parent.parent.parent / "packages"


class SyncResult(Enum):
    NOT_FOUND = auto()
    UP_TO_DATE = auto()
    UPDATED = auto()


def read_package_json(pkg_dir: Path) -> dict | None:
    """Read and parse a package.json file."""
    pkg_json = pkg_dir / "package.json"
    if not pkg_json.exists():
        return None
    return json.loads(pkg_json.read_text())


def update_cargo_toml(pkg_dir: Path, version: str) -> SyncResult:
    """Update version in [package] section of Cargo.toml."""
    cargo_toml = pkg_dir / "Cargo.toml"
    if not cargo_toml.exists():
        return SyncResult.NOT_FOUND

    lines = cargo_toml.read_text().splitlines(keepends=True)
    in_package_section = False

    for i, line in enumerate(lines):
        stripped = line.strip()

        # Track which TOML section we're in
        if stripped.startswith("["):
            in_package_section = stripped == "[package]"
            continue

        if in_package_section and stripped.startswith("version"):
            new_line = re.sub(
                r'^(\s*version\s*=\s*")([^"]+)(")',
                rf"\g<1>{version}\3",
                line,
            )
            if new_line != line:
                lines[i] = new_line
                cargo_toml.write_text("".join(lines))
                rel = cargo_toml.relative_to(PACKAGES_DIR.parent)
                print(f"  Updated {rel}")
                return SyncResult.UPDATED
            return SyncResult.UP_TO_DATE

    return SyncResult.UP_TO_DATE


def update_pyproject_toml(pkg_dir: Path, version: str) -> SyncResult:
    """Update version in [project] section of pyproject.toml."""
    pyproject = pkg_dir / "pyproject.toml"
    if not pyproject.exists():
        return SyncResult.NOT_FOUND

    lines = pyproject.read_text().splitlines(keepends=True)
    in_project_section = False

    for i, line in enumerate(lines):
        stripped = line.strip()

        # Track which TOML section we're in
        if stripped.startswith("["):
            in_project_section = stripped == "[project]"
            continue

        if in_project_section and stripped.startswith("version"):
            new_line = re.sub(
                r'^(\s*version\s*=\s*")([^"]+)(")',
                rf"\g<1>{version}\3",
                line,
            )
            if new_line != line:
                lines[i] = new_line
                pyproject.write_text("".join(lines))
                rel = pyproject.relative_to(PACKAGES_DIR.parent)
                print(f"  Updated {rel}")
                return SyncResult.UPDATED
            return SyncResult.UP_TO_DATE

    return SyncResult.UP_TO_DATE

def refresh_lockfiles() -> None:
    """Refresh all lockfiles under the repo to match updated versions."""
    repo_root = PACKAGES_DIR.parent

    print("Refreshing lockfiles...")

    # Cargo.lock — root workspace + any standalone crate lockfiles
    cargo_locks = sorted(
        set(repo_root.glob("Cargo.lock")) | set(PACKAGES_DIR.rglob("Cargo.lock"))
    )
    for cargo_lock in cargo_locks:
        lock_dir = cargo_lock.parent
        rel = lock_dir.relative_to(repo_root) or Path(".")
        print(f"  cargo update --workspace in {rel}")
        subprocess.run(["cargo", "update", "--workspace"], cwd=lock_dir, check=True)

    # uv.lock — Python packages
    for uv_lock in sorted(PACKAGES_DIR.rglob("uv.lock")):
        lock_dir = uv_lock.parent
        print(f"  uv lock in {lock_dir.relative_to(repo_root)}")
        subprocess.run(["uv", "lock"], cwd=lock_dir, check=True)

def main() -> None:
    print("Syncing versions from package.json to native manifests...")
    print()

    updated = 0
    for pkg_json in sorted(PACKAGES_DIR.rglob("package.json")):
        pkg_dir = pkg_json.parent
        pkg_data = read_package_json(pkg_dir)
        if pkg_data is None:
            continue

        version = pkg_data.get("version")
        if version is None:
            continue

        name = pkg_data.get("name", pkg_dir.name)
        print(f"{name} @ {version}")

        results = [
            update_cargo_toml(pkg_dir, version),
            update_pyproject_toml(pkg_dir, version),
        ]

        if any(r == SyncResult.UPDATED for r in results):
            updated += 1
        elif all(r == SyncResult.NOT_FOUND for r in results):
            print("  (no native manifest found)")
        else:
            print("  (already up to date)")

    print()
    print(f"Synced {updated} package(s).")
    print()

    refresh_lockfiles()
    print()
    print("Done.")


if __name__ == "__main__":
    main()

Reacting to package tags

In the standard changesets flow, you will now have a pull request on GitHub with the appropriate CHANGELOG.md updates, as well as the metadata updates for all the relevant packages.

Once that is merged, the very same action will run, realize all the .changeset files are consumed, and push tags.

"privatePackages": {
    "version": true,
    "tag": true
}

Typically, you’ll then want to react to these pushed tags.
For example, to build new docker images.

For that, rather than using an on.push.tags trigger as you would reasonably assume, you probably want a workflow_call.
See the tip earlier in the post for why.

on:
  workflow_call: {}
  workflow_dispatch:
    inputs:
      dry_run:
        description: 'Build images without pushing to GHCR'
        required: false
        type: boolean
        default: false
      no_cache:
        description: 'Force a build without using the cache'
        required: false
        type: boolean
        default: false

Summary

changesets can manage per-package semantic versioning and changelogs in polyglot monorepos today, even without direct native support for multiple languages.
The trick is to treat JavaScript package manifests as the canonical source of version bumps, and then syncing those bumps via your own scripts to the language-native manifests.

A few gotchas exist (like explicitly making independent pnpm-workspace.yaml files for subdirectories you want to be independent, or using a separate personal access token to push tags), but none are blockers to being able to benefit from changesets’s convenient workflow.

I used to suggest versioning monorepos with a single global version using a tool like semantic-release.
Since trying changesets, I’m sold on the benefits of letting people write commit messages for future internal engineers while also adding a separate changelog note for end users.
These are often two distinct audiences, and relying on a single conventional commit to serve both is often suboptimal.