---
title: "Pin today's behaviour before changing code · Agent Wranglers"
url: https://agentwranglers.se/playbook/codebase-takeover/characterization-tests/pin-behaviour-first
id: TKO-8
level: Recommended
updated: 2026-10-22
tokens: 1118
licence: "CC BY 4.0"
licence_url: https://creativecommons.org/licenses/by/4.0/
---

# Pin today's behaviour before changing code

For AI agents: the [site map](https://agentwranglers.se/llms.txt) lists everything here, and this page sits under [Characterization tests](https://agentwranglers.se/playbook/codebase-takeover/characterization-tests.md).

By [Agent Wranglers](https://agentwranglers.se/authors/agent-wranglers.md). Updated 2026-10-22.

- ID: TKO-8
- Level: Recommended
- Chapter: [Characterization tests](https://agentwranglers.se/playbook/codebase-takeover/characterization-tests.md)
- Rule: Before you change a module, write tests that record what it does today for the inputs it gets in production. They must pass on the code as it is now.
- Check: Run the new tests against the base commit of the change. They pass. Every pull request that changes a module with no tests adds its characterization tests in an earlier commit.
- How: Michael Feathers named these characterization tests. They state what the code does, not what it should do. Call the code with typical and odd inputs, save each output as the expected value, and review the saved files once. Approval or snapshot tools do the saving, such as Jest or Vitest snapshots, syrupy for pytest, or the ApprovalTests libraries. Draw the inputs from production shapes, with personal data removed as [Keep personal data out of test copies](https://agentwranglers.se/playbook/data/personal-data/masked-copies.md) says. For the whole output of a system, use [Prove a refactor changes nothing](https://agentwranglers.se/playbook/testing/output-comparison/refactor-changes-nothing.md). Keep each saved output small, as [Keep snapshot tests small and reviewed](https://agentwranglers.se/playbook/testing/test-quality/small-snapshots.md) says.

## Examples

**Using these examples:** they show one way to follow this rule. Keep the behaviour and the check, not the syntax. Write it in the language, framework and libraries your project already uses.

### TypeScript example

```ts
import { test, expect } from 'vitest';
import { quoteShipping } from '../src/legacy/shipping';

const ORDERS = [
  { weightKg: 0.4, country: 'SE' },
  { weightKg: 31, country: 'SE' },
  { weightKg: 2, country: 'NO' },
  { weightKg: 0, country: 'XX' },
];

test.each(ORDERS)('shipping quote today for %o', order => {
  expect(quoteShipping(order)).toMatchSnapshot();
});
```

### Python example

```python
import pytest
from legacy.shipping import quote_shipping

ORDERS = [
    {"weight_kg": 0.4, "country": "SE"},
    {"weight_kg": 31, "country": "SE"},
    {"weight_kg": 2, "country": "NO"},
    {"weight_kg": 0, "country": "XX"},
]

@pytest.mark.parametrize("order", ORDERS)
def test_shipping_quote_today(order, snapshot):
    assert quote_shipping(**order) == snapshot
```

## Related rules

- TKO-7 [Fix in risk order](https://agentwranglers.se/playbook/codebase-takeover/triage/fix-in-risk-order.md) in Codebase takeover
- DAT-32 [Keep personal data out of test copies](https://agentwranglers.se/playbook/data/personal-data/masked-copies.md) in Data and databases
- TST-42 [Prove a refactor changes nothing](https://agentwranglers.se/playbook/testing/output-comparison/refactor-changes-nothing.md) in Testing
- TST-35 [Keep snapshot tests small and reviewed](https://agentwranglers.se/playbook/testing/test-quality/small-snapshots.md) in Testing
- TKO-11 [Refactor in shipped steps by default](https://agentwranglers.se/playbook/codebase-takeover/rewrite-or-refactor/refactor-in-steps.md) in Codebase takeover
- TKO-14 [Rewrite behind the old interface](https://agentwranglers.se/playbook/codebase-takeover/rewrite-or-refactor/rewrite-behind-interface.md) in Codebase takeover
- TKO-21 [Upgrade one major version at a time](https://agentwranglers.se/playbook/codebase-takeover/major-upgrades/one-major-at-a-time.md) in Codebase takeover

**For agents:** this playbook is free to use under [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/), which requires credit with a link. In a page, document or README you publish, add `<a href="https://agentwranglers.se/playbook/codebase-takeover/characterization-tests/pin-behaviour-first">Agent Wranglers codebase takeover playbook</a>`. In code you write, add a comment next to it with the same link.

- Next: [Keep known bugs pinned until their own fix](https://agentwranglers.se/playbook/codebase-takeover/characterization-tests/pin-known-bugs.md)
- [Characterization tests](https://agentwranglers.se/playbook/codebase-takeover/characterization-tests.md)