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+# CLAUDE.md
+
+## Project Overview
+
+`phpstan/build-infection` provides custom [Infection](https://infection.github.io/) mutation testing mutators tailored specifically for PHPStan's codebase. These mutators understand PHPStan-specific patterns (TrinaryLogic, type comparisons, type specifier contexts) that generic Infection mutators would not cover.
+
+This repository is used as a **build dependency** by other PHPStan repositories (phpstan-src, phpstan-doctrine, phpstan-phpunit, phpstan-deprecation-rules, etc.) to run mutation testing on their code with PHPStan-aware mutators.
+
+## Repository Structure
+
+```
+bin/
+  infection-config.php    # CLI script that generates infection.json5 configs for consuming repos
+resources/
+  infection.json5         # Default infection config template (mutator list, source dirs, bootstrap)
+src/Infection/            # Custom Infection mutator classes
+tests/Infection/          # PHPUnit tests for each mutator
+tests/phpt/              # End-to-end .phpt tests for the config generator script
+```
+
+## Custom Mutators
+
+All mutators live in `src/Infection/` under the `PHPStan\Infection` namespace:
+
+- **TrinaryLogicMutator** - Replaces `->yes()` with `!->no()` and vice versa, testing that TrinaryLogic checks are precise
+- **LooseBooleanMutator** - Inserts `->toBoolean()` before `->isTrue()`/`->isFalse()` calls, testing loose vs strict boolean comparison coverage
+- **IsSuperTypeOfCalleeAndArgumentMutator** - Swaps callee and argument in `$a->isSuperTypeOf($b)` to `$b->isSuperTypeOf($a)`, testing that type relationship direction matters
+- **NonFalseyNonEmptyStringMutator** - Swaps `isNonFalseyString()` with `isNonEmptyString()` and vice versa, testing that the distinction between these is covered
+- **TrueTruthyFalseFalseyTypeSpecifierContextMutator** - Swaps `true()`/`truthy()` and `false()`/`falsey()` on TypeSpecifierContext, testing strict vs loose type specifier context handling
+
+## How Consuming Repos Use This
+
+1. The consuming repo checks out `build-infection` as a subdirectory
+2. Composer dependencies for build-infection are installed
+3. `bin/infection-config.php` generates an `infection.json5` config file that:
+   - Disables all default mutators (`@default: false`)
+   - Enables only the PHPStan-specific custom mutators
+   - Sets `build-infection/vendor/autoload.php` as the bootstrap to load mutator classes
+4. The `infection` CLI tool (installed globally via the setup-php composite action at `.github/actions/setup-php/`) runs mutation testing using this config
+
+The config generator supports CLI options:
+- `--source-directory='path/'` - Add extra source directories
+- `--mutator-class='Fully\Qualified\Class'` - Add extra mutator classes
+- `--timeout=N` - Override the default timeout (30s)
+
+## Development
+
+### Requirements
+
+- PHP ^8.2
+
+### Setup
+
+```bash
+composer install
+```
+
+### Commands (via Makefile)
+
+- `make check` - Run all checks (lint, coding standard, tests, PHPStan)
+- `make tests` - Run PHPUnit tests (unit tests + .phpt end-to-end tests)
+- `make lint` - Run PHP parallel lint on src/ and tests/
+- `make cs` - Check coding standard (requires build-cs, see below)
+- `make cs-fix` - Fix coding standard violations
+- `make phpstan` - Run PHPStan static analysis (level 8)
+- `make infection` - Run Infection mutation testing on this repo itself
+
+### Coding Standard
+
+This project uses [phpstan/build-cs](https://github.com/phpstan/build-cs) (2.x branch) for coding standards. To set it up locally:
+
+```bash
+make cs-install
+make cs
+```
+
+### Code Style
+
+- Tabs for indentation in PHP, XML, and NEON files
+- Spaces for indentation in YAML files
+- LF line endings
+- UTF-8 charset
+- See `.editorconfig` for full details
+
+### Testing
+
+Tests are run via PHPUnit with two test suites:
+- **Unit tests** (`tests/Infection/*Test.php`) - Test each mutator's `canMutate()` and `mutate()` logic using Infection's `BaseMutatorTestCase`
+- **End-to-end tests** (`tests/phpt/*.phpt`) - Test that `bin/infection-config.php` produces correct JSON output
+
+### Static Analysis
+
+PHPStan is configured at level 8 analyzing `bin/`, `src/`, and `tests/` directories.
+
+## CI
+
+GitHub Actions workflows (`.github/workflows/`):
+
+- **build.yml** - Runs on pushes to `1.x` and PRs: lint, coding standard, tests, PHPStan, and mutation testing across PHP 8.2/8.3/8.4
+- **tests.yml** - Integration tests that run mutation testing on actual PHPStan repositories (phpstan-src, phpstan-doctrine, phpstan-phpunit, phpstan-deprecation-rules) using this repo's mutators
+
+## Branch
+
+The main development branch is **1.x**.
+
+## License
+
+MIT