What The Diff alternative for checking behavior after edits

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.

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

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

Source: public order-service checkout.py at the linked commit. The captured standalone file is refund.py. Perch command output and timestamp. This saved-rule demonstration does not measure What The Diff's detection rate.

Perch vs What The Diff: 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.

What The Diff alternative for checking behavior after edits

Perch checks a saved behavior requirement against the selected code, including uncommitted files. Use it when a readable PR description is only the start: the agent also needs to verify that a guard, call contract or write order survives the next edit.

Review before committing

What The Diff: GitHub and GitLab integrations; dedicated CLI review was not documented. What The Diff workflow documentation.

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?

What The Diff: Token limits and excluded file patterns; semantic rule format not documented. What The Diff rule documentation.

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 · Working-file checks.

What does the review bill depend on?

Compare What The Diff's billing unit and Perch's usage estimate

Is the CLI open source and offline?

What The Diff: Commercial service; qualifying OSS sponsorship. What The Diff product or license source.

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.

When does What The Diff fit the workflow?

What The Diff provides PR descriptions, diff review and inline refactoring. Token-pack capacity and Git integration differ from a named local check; a dedicated local review CLI was not documented.

Estimate the review bill

Perch and What The Diff: pricing, allowances and workload calculator

Make the agent check its next edit.

npm install -g @lakeday/perch

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

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

Use perch scan for repository code at Git HEAD and changed-code CI checks for pull requests. New model judgments can vary. Repeatability comes from cached answers for unchanged requests; a pass does not prove application correctness.