# Find defects in your coding agent's changes

Choose defect, security, and saved-rule checks when reviewing generated code, then inspect the agent's changes against a base commit.

Source: https://perchscan.com/examples/scan-types/
Published: 2026-09-20
Updated: 2026-10-23

![A Perch scan highlights checkout, cart, inventory, and token-generation problems in an example project.](/demos/project-scan.gif)

*Scan for checkout, cart, inventory, and token generation issues in an example project.*

In this example, we scan a small order service. Perch found 19 issues in just 5 files that make up this project. (We planted the bugs.) The GIF shows 4, and the [output](/demos/scan-capture.json) captures the full scan.

## Scan for just one type of issue

By default `perch scan` will scan for `defect`, `security`, and any custom rules you have added in `perch.yaml`. You can use the `--filter type=` option to change the types of issues it will scan for. The possible types are:

| Type | Checks | Example |
| --- | --- | --- |
| `defect` | Bug and defect patterns, like incorrect behavior, ordering, missing values, error handling, etc. | [Checkout accepts an incomplete cart](/examples/defect-scans/) |
| `security` | Security issues, like missing authorization, injection, unsafe input handling, and more. See all the [security classes](https://docs.perchscan.com/scan/). | [Order cancellation without an owner check](/examples/security-scans/) |
| `lint` | Custom rules written in `perch.yaml`. | [Refunds and other behavior requirements](/examples/plain-english-rules/) |
| `docs` | Checks that methods are documented. | [Comments that match the implementation](/examples/code-comment-rules/) |
| `refactor` | Checks how well a method is structured. Enable to get help reviewing code for complex or duplicated methods. | |

Once you've completed the [Getting Started](https://docs.perchscan.com/install/) steps for a git repository, you can run a scan for one type at a time:

```sh
perch scan --filter type=defect
perch scan --filter type=security
perch scan --filter type=lint
```

Note that each of these is a separate scan. Also note that `perch scan` will only scan code committed to git, since it scans at `HEAD`. If you'd like to check uncommitted code, you can run `perch check path::method` or `perch check path` to check a file against your lint rules.

If you'd like to scan for docs or refactoring issues, you can enable those as well:

```sh
perch scan --filter type=docs
perch scan --filter type=refactor
```

You can also change the default types Perch will scan for by specifying `scan_types` as a map in `perch.yaml`:

```yaml
scan_types: [defect, security, lint, docs]
rules:
  - name: errors-reach-callers
    where: "src/**/*.py"
    each: method
    ensure: |
      Errors should be reported to the caller. The method should not catch an
      error and return a default value with the same shape as a success result.
```

Note that the [docs](/examples/documentation-rules/) above are actually custom lint rules that check for markdown. To check for documentation on your methods, use the built-in [`docs` type](/examples/code-comment-rules/). See the [documentation rules](/examples/documentation-rules/) for examples.

You can also filter the issues listed in `perch issues`, which will not make any model requests and will only show results Perch has already found and saved locally:

```sh
perch issues --filter type=security
perch issues --filter type=defect --min 80
```

Note that you cannot use `perch issues` to add additional scan types, since it does not make any requests to the model.

You can scan just the changes in a branch (assuming you've fetched the main branch to `origin/main`) like so:

```sh
perch scan --since origin/main --filter type=defect,security
```

Perch will exit with exit code `3` if it finds any results that fail. It exits with `1` if it is unable to run, or `2` if the command is invalid.

See the [scan reference](https://docs.perchscan.com/scan/) for a list of all the built-in questions Perch will ask when you run a scan, and the [rules reference](https://docs.perchscan.com/rules/) for more on how to configure `scan_types` and other rule options like thresholds and gates.