crozier

An ultra-fast, memory-efficient OpenAPI-to-SDK code generator written in Rust + minijinja. crozier reproduces Fern's generated SDKs byte-for-byte (with generator-identifying comments aside), driven by nothing but an OpenAPI document and a couple of naming flags — no per-project config, no generators written in the target language.

Python is the only target today; more will follow.

Status: early. crozier generates the Python type layer (pydantic models, enums, unions/aliases) plus version.py/py.typed, and byte-matches Fern's output for those files. See docs/matching.md for exactly what is matched and the roadmap.

Install

From PyPI (fastest — a prebuilt binary, no Rust toolchain): crozier ships as platform wheels that wrap the compiled binary and expose it as a console script, so any Python installer puts it on your PATH in seconds:

pip install crozier      # or: pipx install crozier
uvx crozier --help       # run once without installing

Platforms without a prebuilt wheel fall back to the source distribution, which builds from Rust (a toolchain is needed there).

From crates.io:

cargo install crozier --locked

From the install script (prebuilt binary from GitHub Releases, verified):

curl -fsSL https://raw.githubusercontent.com/nickderobertis/crozier/main/scripts/install.sh | sh

It detects your platform, downloads the matching archive, and verifies it against a trust root independent of where it was downloaded — a Sigstore build-provenance attestation when a verifier (cosign, pip install sigstore, or gh) is present, else the canonical SHA-256 checksum. Pin a version or install location with sh -s -- --version v0.1.0 --to ~/.local/bin.

From source (latest main):

cargo install --git https://github.com/nickderobertis/crozier --locked

From a release archive (manual): each release publishes per-platform archives named crozier-<tag>-<target>.tar.gz with a matching .sha256 and a .sigstore.json provenance bundle, for targets x86_64/aarch64 Linux, x86_64/aarch64 macOS, and x86_64 Windows. Download the archive for your platform from the Releases page, verify the checksum (or the attestation), and put the crozier binary on your PATH.

Usage

crozier generate \
  --spec path/to/openapi.yml \
  --output ./generated \
  --package-name my_api \
  --project-name my-api

• --spec — the OpenAPI 3.x document (.yml, .yaml, or .json).
• --output — directory to write the SDK into.
• --package-name — the Python import package (the directory under src/). Defaults to a snake_case of the API title.
• --project-name — the distribution name recorded in version.py. Defaults to the package name.
• --client-class-name — the name of the generated root client class (Fern's client_class_name). Defaults to {PascalCase(package_name)}Api.
• --audience — repeatable x-crozier-audiences filter; generate only the operations carrying a matching audience (or none), plus the schemas they reference. Omit to generate the whole API.

crozier exits 0 on success (with a one-line summary on stderr) and 1 on any error, printing the exact problem and a suggested fix.

Development

The command surface is a small set of just recipes:

just bootstrap   # set up from a clean clone (toolchain + dev tools)
just check       # full gate: fmt, clippy -D warnings, tests + e2e + coverage, deny, machete, doc
just test        # fast tests with coverage enforced (95%)
just test-e2e    # drive the compiled binary and byte-compare against fixtures
just format      # rustfmt in place
just upgrade     # cargo update, then re-run the gate

See AGENTS.md for the durable contributor guide and docs/matching.md for the byte-matching strategy.

License and attribution

crozier is licensed under Apache-2.0. It is an independent, clean-room implementation — it reproduces Fern's generated output format (the project's explicit goal) and does not copy Fern's generator source.

The test fixtures under tests/fixtures/ are Fern's own output and OpenAPI test specs, used under Apache-2.0 with attribution and a statement of changes; see NOTICE and licenses/fern-APACHE-2.0.txt.