chisel-checker 0.1.1

Opinionated architecture constraint checker

chisel

Opinionated architecture constraint checker for FastAPI backends. Designed to catch the patterns agents get wrong — use it as a pre-commit hook to block violations, or run it interactively to steer an agent while it's working.

It pairs with agent skills: the skill teaches the agent the right pattern, the checker enforces it deterministically. Run chisel explain <rule-id> to get fix guidance the agent can consume directly.

Installation

pip install chisel

Quick start

chisel check ./backend              # check a project
chisel rules                        # list all ~55 rules grouped by category
chisel explain structural:print-banned  # detailed fix guidance
chisel check . --json               # machine-readable output for agents
chisel check . --no-strict          # skip src-layout enforcement

Commands

Command	Description
chisel check [path]	Scan a project for architectural violations
chisel check . --json	Output violations as structured JSON
chisel check . --strict/--no-strict	Toggle src-layout and build-config enforcement
chisel rules	List all rules, grouped by category
chisel rules --json	Machine-readable rule listing
chisel explain <rule-id>	Detailed description + fix guidance for a rule
chisel explain <category>	All rules in a category (e.g. structural)

What gets checked

Category	What it enforces
Import Boundaries	Layer-based import restrictions — services can't import SQLAlchemy, routes can't import services, HTTPException only in error_handlers.py, concrete services only in factory.py
Structural Invariants	No getattr/setattr, no % string formatting, no f-strings in logger calls, no print() in src/, @dataclass uses slots=True, AppFactory has zero logic, match/case only in error handlers
Complexity	Controller method ≤30 LoC, route handler ≤20 LoC, factory cyclomatic complexity = 1, app.py ≤50 LoC
Concurrency	asyncio.gather banned unconditionally — use asyncio.TaskGroup
Error Flow	AppError subclasses never contain HTTP status codes; status codes decided only in error_handlers.py
Project Structure	pyproject.toml only (no setup.py), env vars read at startup only via AppConfig, no .py files at project root
Test Structure	Test files only under tests/unit/, tests/integration/, tests/e2e/; one assert per test; no mocking libraries; test names describe invariants; pytest.mark.skip needs a reason; no TestClient outside e2e

Exceptions

Create a chisel-exceptions.toml at your project root:

[[exceptions]]
files = ["src/legacy/*.py", "src/cli/main.py"]
rules = ["structural:print-banned"]
reason = "CLI requires stdout output"

* matches all rules. A category prefix like structural matches all rules in that category.

Inline suppression per line: # noqa: rule-id — reason.

Severity tiers

Tier	Behaviour
ERROR	Blocks commit, no override. Import boundary violations, asyncio.gather, getattr/setattr, f-string in logger, print() in src/.
WARNING	Blocks commit, suppressible with a reason comment. Controller/route LoC thresholds, test coverage gaps.
INFO	Logged, non-blocking.

How it works

models/         Pure data — Violation, Severity, FileInfo, ProjectInfo
services/       One service per rule category, self-describing via describe_rules()
repositories/   File discovery, import graph analysis (grimp)
controllers/    Orchestrates all services, filters exceptions, suppresses via noqa
factory.py      Zero-logic DI — wires all services into the controller
cli/main.py     Typer-based CLI — check, rules, explain
reporter.py     Coloured terminal output via rich

Adding a rule requires one new check method in a service + one describe_rules() entry. The CLI discovers it automatically.

Development

pip install -e ".[dev]"
pytest tests/ -q

The checker is self-validating: chisel check . --strict produces zero violations on its own source.