Source: https://codereviewguide.fyi/perch-vs-diffen/

Perch vs Diffen · reviewed 2026-10-14

# Diffen alternative for checking an uncommitted repair

Perch lets the agent check its repair before opening another pull request. The selected English requirement stays in the repository and can be rerun on the next edit with free unchanged cache reuse.

`perch check` reads working files; `perch scan` reads Git HEAD. Relevant source is sent to TypeSafe’s hosted models. The MIT CLI does not run model inference offline.

[See the rule catch a regression](https://codereviewguide.fyi/perch-vs-diffen/#check)[Install the Perch CLI](https://github.com/lakeday-org/perch/blob/main/docs/install.md)

## Perch

~50¢

per mid-sized pull request · estimate

- 10¢ per 1,000 fresh rule checks
- Cached checks free
- No seat fees · $5 free usage credit
- MIT CLI · English project rules

**Diffen**Pro: $19/month for 30 merge-request reviews and 5 repositories. Pro+: $49/month with fair-use unlimited review.

Perch estimate: approximately 150 methods read, based on September–October 2026 check density. [Compare billing assumptions](https://codereviewguide.fyi/perch-vs-diffen/#pricing).

**Run it locally**Check uncommitted files

**Keep the rule**Version it with the code

**Read each method**Within the selected parsed scope

**Pay for checks**No per-developer subscription

## Catch the fix an agent removes later.

A saved requirement makes a recurring mistake checkable. In this Perch capture from September 20, 2026, the same rule flags a missing guard and passes after the guard is restored.

### The regression

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

1 check, 1 broken. Refunded orders cannot be refunded again.

### The guarded method

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

1 check, nothing to report.

Source: [public order-service checkout.py at the linked commit](https://github.com/lakeday-org/perch/blob/4af67af46d76ac99398d61c7ad0a45441760d4dc/test/fixtures/order-service/checkout.py). The captured standalone file is refund.py. [Perch command output and timestamp](https://codereviewguide.fyi/perch-vs-diffen/refund-capture.json). This saved-rule demonstration does not measure Diffen's detection rate.

## Perch vs Diffen: what your team runs.

Perch turns project behavior into focused checks that agents can run before committing. Use these differences to choose the review workflow and billing model your team needs.

Published Perch capabilities and Diffen documentation, reviewed 2026-10-14

| Capability | Perch | Diffen |
| --- | --- | --- |
| Billing | 10¢ per 1,000 fresh rule checks. Cached checks free. No seat fees. | Pro: $19/month for 30 merge-request reviews and 5 repositories. Pro+: $49/month with fair-use unlimited review.[Vendor documentation ↗](https://diffen.ai/) |
| Before commit | perch check reads the files on disk, including uncommitted edits. | GitHub app and PR patch workflows; local CLI was not verified.[Vendor documentation ↗](https://diffen.ai/) |
| Custom requirements | English ensure rules in perch.yaml: project, file or method scope. No paid custom-rule tier. | Compliance.md instructions on Pro.[Vendor documentation ↗](https://diffen.ai/) |
| Coverage | Each parsed non-test application method in the selected scan scope is read, with caller and callee context. No method-count cap. | Agentic PR review, patch generation and test execution.[Vendor documentation ↗](https://diffen.ai/) |
| Open source | MIT-licensed CLI. Hosted model analysis is a paid service. | Commercial review service.[Vendor documentation ↗](https://diffen.ai/) |

## Diffen alternative for checking an uncommitted repair

Perch lets the agent check a fix while it is still local. Save the recurring requirement once, run it against the edited method, and retain it for later PRs. Method and caller/callee context support requirements that depend on more than changed lines.

## Review before committing

Diffen: GitHub app and PR patch workflows; local CLI was not verified. [Diffen workflow documentation](https://diffen.ai/).

Perch reads the current working file with `perch check`. For a known requirement, select its rule name and method instead of requesting another broad review. A local CLI describes where the command runs; Perch sends relevant source to hosted TypeSafe models for analysis.

## How do saved requirements differ?

Diffen: Compliance.md instructions on Pro. [Diffen rule documentation](https://diffen.ai/).

Perch stores English `ensure` rules in `perch.yaml` or `.perch/rules/`. Project, file and method scopes determine what is checked. Run the named check after an edit and after the repair; a rule finding can fail the command and a CI gate. [Perch rules](https://github.com/lakeday-org/perch/blob/main/docs/rules.md) · [Working-file checks](https://github.com/lakeday-org/perch/blob/main/docs/check.md).

## What does the review bill depend on?

[Compare Diffen's billing unit and Perch's usage estimate](https://reviewpricing.fyi/diffen/)

## Is the CLI open source and offline?

Diffen: Commercial review service. [Diffen product or license source](https://diffen.ai/).

Perch’s CLI is MIT-licensed; hosted model inference is a separate paid service included in its published check rate. It does not run the model offline. Unchanged requests can reuse cached answers for free. Fresh judgments can vary, and a passing requirement check does not prove application correctness. [Perch MIT license](https://github.com/lakeday-org/perch/blob/main/LICENSE).

## When does Diffen fit the workflow?

Diffen provides agentic PR review, patches and test execution. Its capped plans and trial do not establish a 100-PR bill, and a local working-tree CLI was not verified.

## Estimate the review bill

[Perch and Diffen: pricing, allowances and workload calculator](https://reviewpricing.fyi/diffen/)

## Make the agent check its next edit.

### Install the CLI

```
npm install -g @lakeday/perch
```

[Perch installation and quick start](https://github.com/lakeday-org/perch/blob/main/docs/install.md)

### Save a behavior rule

```
perch rules add refund-once \
  --where refund.py --each method \
  --ensure "Refunded orders cannot be refunded again."
```

The captured rule checks the requirement shown above. [Rule syntax and selectors](https://github.com/lakeday-org/perch/blob/main/docs/rules.md)

### Check the working file

```
perch check refund.py::refund --rules refund-once
```

Run the check after an edit and after the fix. [Working-tree check documentation](https://github.com/lakeday-org/perch/blob/main/docs/check.md)

Use [perch scan](https://github.com/lakeday-org/perch/blob/main/docs/scan.md) for repository code at Git HEAD and [changed-code CI checks](https://github.com/lakeday-org/perch/blob/main/docs/ci.md) for pull requests. New model judgments can vary. Repeatability comes from cached answers for unchanged requests; a pass does not prove application correctness.

## Keep your project requirements checkable.

Choose Perch when you want an MIT CLI, English behavior rules and usage pricing that stays independent of team size. Give your coding agent a saved check it can run after every relevant edit.

[Install Perch](https://github.com/lakeday-org/perch/blob/main/docs/install.md)

## Sources and price details

Published by Perch. Vendor facts were reviewed on 2026-10-14; plans and allowances can change. Vendor marks belong to their owners.

- [diffen.ai/](https://diffen.ai/)

Perch sources and example arithmetic

- [Public CLI source](https://github.com/lakeday-org/perch) · [MIT license](https://github.com/lakeday-org/perch/blob/main/LICENSE)
- [English rules](https://github.com/lakeday-org/perch/blob/main/docs/rules.md) · [Working-tree checks](https://github.com/lakeday-org/perch/blob/main/docs/check.md) · [Method scope and context](https://github.com/lakeday-org/perch/blob/main/docs/scanning.md)
- [Published Perch rate and estimate assumptions](https://codereviewguide.fyi/perch-vs-diffen/pricing-evidence.json)
- [September 20, 2026 refund-rule capture](https://codereviewguide.fyi/perch-vs-diffen/refund-capture.json)

**Code Review Guide**

Tools & workflows.

Published by Perch · © 2026 Verglas LLC. Code Review Guide is a Perch publication.

Perch’s MIT license covers the CLI; hosted model analysis is a paid service. Relevant source is sent to the configured model service.

[Perch CLI MIT license](https://github.com/lakeday-org/perch/blob/main/LICENSE) · [Article text](https://codereviewguide.fyi/perch-vs-diffen/index.md) · [Resource index](https://codereviewguide.fyi/llms.txt)