> ## Documentation Index
> Fetch the complete documentation index at: https://docs.fensu.dev/llms.txt
> Use this file to discover all available pages before exploring further.
# Quickstart
> Install Fensu, initialize a repository, and read your first faults in a few minutes.
This guide gets Fensu running against a Python repository and walks through
reading its output.
**Fastest wins.** On a **brand-new** repo, run [`fensu init`](/cli/init) and keep
everything on from the first commit. On a **small existing** repo, run
`fensu init` in your `pyproject.toml` directory, then drive the faults to zero
(a coding agent handles can handle these easily). That single pass is where Fensu pays off
most. Larger codebases need more judgment; see [Adopting Fensu](/adoption).
## 1. Install
```bash theme={null}
pip install fensu
fensu --version
```
Fensu needs Python 3.12 or newer.
## 2. Initialize
[`fensu init`](/cli/init) is the fastest way to start. It detects your layout,
writes a `fensu.toml` with the full ruleset enabled, runs a first check to show
current drift, and offers to install [agent skill files](/cli/skills):
```bash theme={null}
fensu init
```
If you accept the prompts, your repository is configured and you can start
checking right away. For an empty repository with no code yet, init scaffolds a
`src/` package and `tests/` directory; pass `--yes --name my_package` to run it
non-interactively.
Prefer to write configuration by hand? A minimal `fensu.toml` only needs your
product roots:
```toml theme={null}
roots = ["src/my_package"]
```
`tests` defaults to `["tests"]`, `tooling` is optional, and every rule family is on
by default. See [Configuration](/concepts/configuration) for scopes, selection, and
the full schema.
## 3. Check and read a fault
Run Fensu against the configured repository:
```bash theme={null}
fensu check
```
If everything conforms, Fensu prints a summary and exits `0`:
```text theme={null}
Found 0 faults
```
When it finds a problem, it prints a diagnostic for each fault and exits `1`. The
output points at the exact line, shows the source, and explains the fix:
```text theme={null}
FFL102 import 'my_package.payments.processing._helpers.charge_card' reaches into internal structure of 'my_package.payments'
--> src/my_package/orders/checkout/main/place_order.py:1:0
|
1 | from my_package.payments.processing._helpers.charge_card import charge_card
| ^
|
= help: Import from classes, models, types, constants, exceptions, or a thin main/ entry.
```
Every diagnostic has two parts: the `message` states the contract that was
violated, and the `= help:` line states the normal correction. The messages are
written to be read at the moment of violation, by a person or an agent, so the fix
is clear without opening the docs. Look up any code in full with
[`fensu rule `](/cli/rule).
## Next steps
The scopes, roles, and default layout Fensu enforces.
Every configuration key, threshold, and contract.
What each family (FFL, FFR, FFS, FFN, FFH, FFT, FFA) checks.
Rolling out on a new, small, or large repository.
Render a deterministic downstream call tree for any function.
Generate and verify agent guidance from your active rules.