# Perch vs ESLint: check behavior beyond JavaScript syntax rules

Compare Perch’s saved requirement checks with ESLint and choose a workflow for recurring coding-agent mistakes.

Source: https://perchscan.com/compare/perch-vs-eslint/
Published: 2026-10-14

Perch lets you state a behavioral requirement without building a JavaScript rule plugin. The same rule workflow works across supported languages, including the Python refund example below.

## Check a saved rule on the current method

This is the same requirement and command used in the [captured refund example](/examples/plain-english-rules/):

```yaml
- name: refund-once
  where: checkout.py
  each: method
  ensure: Refunded orders cannot be refunded again.
```

The deliberately broken method calls the payment provider without checking the order status:

```python
def refund(order, payment):
    payment.refund(order.total)
    order.status = "refunded"
    return order
```

Run the focused check from the repository root:

```sh
perch check checkout.py::refund --rules refund-once --json
```

In the [September 20, 2026 capture](/demos/refund-capture.json), the unguarded refund was flagged at 80% confidence and exited 3. Adding `if order.status == "refunded": return order` before the payment call made the check pass and exit 0. The capture uses `refund.py`; use `checkout.py` when adapting the public [order-service source at commit 4af67af](https://github.com/lakeday-org/perch/blob/4af67af46d76ac99398d61c7ad0a45441760d4dc/test/fixtures/order-service/checkout.py).

Save the rule with the code, then use the same command to check the next agent edit. Add regression tests for concurrency and payment-provider idempotency in the application.


## Compare the review workflow

| Workflow | Perch | ESLint |
| --- | --- | --- |
| Custom-rule authoring | Plain-English requirement | JavaScript rule module and AST visitors |
| Language workflow | Same CLI across Perch’s supported languages | JavaScript ecosystem with parsers and plugins |
| Best use | Repeated behavioral corrections | Deterministic syntax, style, and AST requirements |

## Write behavioral rules without a rule plugin

Perch lets you write the application requirement in English and check it against the relevant code. Save “refunded orders cannot be refunded again” as a rule without implementing an AST visitor. Use the same authoring and checking workflow across supported languages, including the Python example below.

## How ESLint handles review

ESLint’s [core concepts](https://eslint.org/docs/latest/use/core-concepts) describe configurable lint rules. Its [custom-rule documentation](https://eslint.org/docs/latest/extend/custom-rules) shows JavaScript modules with metadata and AST visitor functions. Plugins and parsers extend its capabilities.

## Add Perch to your agent workflow

Follow the [Perch quick start](https://docs.perchscan.com/install/), then save the requirement with the code. Install the skill for the coding agent you use:

```sh
perch setup claude-code
# Or: perch setup codex
# Or: perch setup cursor
```

The Perch skill instructs the agent to check the named rule after each relevant edit, inspect the finding, repair the code, and recheck. Keep the rule and regression test beside the application so the next session can use the same requirement.

With your base reference fetched, add `perch scan --since origin/main --filter type=lint` beside your tests in CI. [CI setup](https://docs.perchscan.com/ci/) explains the changed-code scan. Gated findings exit 3; exits 1 and 2 mean the check did not complete.

## Compare cost and result limits

Perch charges 10¢ per 1,000 rule checks, with cached checks free. [The pricing comparison](/#pricing) estimates about 50¢ for a mid-sized pull request with about 150 methods read, including surrounding code. Usage follows the selected questions and their code context.

Unchanged requests reuse cached answers for free; fresh model judgments can vary. Perch’s CLI is MIT licensed. Model analysis sends relevant source to the configured service.

Competitor documentation reviewed October 14, 2026. [All comparisons](/compare/) · [Perch rule syntax](https://docs.perchscan.com/rules/)