(In the following, use `sysctl -n hw.logicalcpu` instead of `nproc` on macOS)

## Building

To build Lean you should use `make -j$(nproc) -C build/release`.

The build uses `ccache`, and in a sandbox `ccache` may complain about read-only file systems.
Use `CCACHE_READONLY` and `CCACHE_TEMPDIR` instead of disabling ccache completely.

## Running Tests

See `tests/README.md` for full documentation. Quick reference:

```bash
# Full test suite (use after builds to verify correctness)
CTEST_PARALLEL_LEVEL="$(nproc)" CTEST_OUTPUT_ON_FAILURE=1 \
make -C build/release -j "$(nproc)" test

# Specific test by name (supports regex via ctest -R; double-quote special chars like |)
CTEST_PARALLEL_LEVEL="$(nproc)" CTEST_OUTPUT_ON_FAILURE=1 \
make -C build/release -j "$(nproc)" test ARGS="-R 'grind_ematch'"

# Multiple tests matching a pattern
CTEST_PARALLEL_LEVEL="$(nproc)" CTEST_OUTPUT_ON_FAILURE=1 \
make -C build/release -j "$(nproc)" test ARGS="-R 'treemap|phashmap'"

# Rerun only previously failed tests
CTEST_PARALLEL_LEVEL="$(nproc)" CTEST_OUTPUT_ON_FAILURE=1 \
make -C build/release -j "$(nproc)" test ARGS='--rerun-failed'

# Run a test manually without ctest (test pile: pass filename relative to the pile dir)
tests/with_stage1_test_env.sh tests/elab_bench/run_bench.sh cbv_decide.lean
tests/with_stage1_test_env.sh tests/elab/run_test.sh grind_indexmap.lean

# Build Lake and run a Lake test (pass directory name relative to `tests/lake`)
tests/lake/run_test.sh tests/ltar
```

## Benchmark vs Test Problem Sizes

Benchmarks are also run as tests. Use the `TEST_BENCH` environment variable (unset in tests, set to `1` in benchmarks) to scale problem sizes:

- In `compile_bench` `.init.sh` files: check `$TEST_BENCH` and set `TEST_ARGS` accordingly
- In `elab_bench` Lean files: use `(← IO.getEnv "TEST_BENCH") == some "1"` to switch between small (test) and large (bench) inputs

See `tests/README.md` for the full benchmark writing guide.

## Testing stage 2

When requested to test stage 2, build it as follows:
```
make -C build/release stage2 -j$(nproc)
```
Stage 2 is *not* automatically invalidated by changes to `src/` which allows for faster iteration
when fixing a specific file in the stage 2 build but for invalidating any files that already passed
the stage 2 build as well as for final validation,
```
make -C build/release/stage2 clean-stdlib
```
must be run manually before building.

To rebuild individual stage 2 modules without a full `make stage2`, use Lake directly:
```
cd build/release/stage2 && lake build Init.Prelude
```

To run tests in stage2, replace `-C build/release` from above with `-C build/release/stage2`.

## New features

When asked to implement new features:
* begin by reviewing existing relevant code and tests
* write comprehensive tests first (expecting that these will initially fail)
* and then iterate on the implementation until the tests pass.

## Success Criteria

*Never* report success on a task unless you have verified both a clean build without errors, and that the relevant tests pass.

## Build System Safety

**NEVER manually delete build directories** (build/, stage0/, stage1/, etc.) even when builds fail.
- ONLY use the project's documented build command: `make -j$(nproc) -C build/release`
- If a build is broken, ask the user before attempting any manual cleanup

## stage0 Is a Copy of src

**Never manually edit files under `stage0/`.** The `stage0/` directory is a snapshot of `src/` produced by `make update-stage0`. To change anything in stage0 (CMakeLists.txt, C++ source, etc.), edit the corresponding file in `src/` and let `update-stage0` propagate it.

## LSP and IDE Diagnostics

After rebuilding, LSP diagnostics may be stale until the user interacts with files. Trust command-line test results over IDE diagnostics.

## Update prompting when the user is frustrated

If the user expresses frustration with you, stop and ask them to help update this `.claude/CLAUDE.md` file with missing guidance.

## Creating pull requests

Follow the commit convention in `doc/dev/commit_convention.md`.

**Title format:** `<type>: <subject>` where type is one of: `feat`, `fix`, `doc`, `style`, `refactor`, `test`, `chore`, `perf`.
Subject should use imperative present tense ("add" not "added"), no capitalization, no trailing period.

**Body format:** The first paragraph must start with "This PR". This paragraph is automatically incorporated into release notes, so keep it short, focus on user-side impact, and avoid implementation-specific wording. Save the implementation details for a follow-up paragraph. Use imperative present tense. Do NOT use markdown headings (`## Summary`, `## Test plan`, etc.) in PR bodies.

Example:
```
feat: add optional binder limit to `mkPatternFromTheorem`

This PR adds a `num?` parameter to `mkPatternFromTheorem` to control how many
leading quantifiers are stripped when creating a pattern.
```

**Changelog labels:** Add one `changelog-*` label to categorize the PR for release notes:
- `changelog-language` - Language features and metaprograms
- `changelog-tactics` - User facing tactics
- `changelog-server` - Language server, widgets, and IDE extensions
- `changelog-pp` - Pretty printing
- `changelog-library` - Library
- `changelog-compiler` - Compiler, runtime, and FFI
- `changelog-lake` - Lake
- `changelog-doc` - Documentation
- `changelog-ffi` - FFI changes
- `changelog-other` - Other changes
- `changelog-no` - Do not include this PR in the release changelog

If you're unsure which label applies, it's fine to omit the label and let reviewers add it.

## Module System for `src/` Files

Files in `src/Lean/`, `src/Std/`, and `src/lake/Lake/` must have both `module` and `prelude` (CI enforces `^prelude$` on its own line). With `prelude`, nothing is auto-imported — you must explicitly import `Init.*` modules for standard library features. Check existing files in the same directory for the pattern, e.g.:

```lean
module

prelude
import Init.While  -- needed for while/repeat
import Init.Data.String.TakeDrop  -- needed for String.startsWith
public import Lean.Compiler.NameMangling  -- public if types are used in public signatures
```

Files outside these directories (e.g. `tests/`, `script/`) use just `module`.

## CI Log Retrieval

When CI jobs fail, investigate immediately - don't wait for other jobs to complete. Individual job logs are often available even while other jobs are still running. Try `gh run view <run-id> --log` or `gh run view <run-id> --log-failed`, or use `gh run view <run-id> --job=<job-id>` to target the specific failed job. Sleeping is fine when asked to monitor CI and no failures exist yet, but once any job fails, investigate that failure immediately.

## Copyright Headers

New files require a copyright header. To get the year right, always run `date +%Y` rather than relying on memory. The copyright holder should be the author or their current employer — check other recent files by the same author in the repository to determine the correct entity (e.g., "Lean FRO, LLC", "Amazon.com, Inc. or its affiliates").

Test files (in `tests/`) do not need copyright headers.

## Test Module Docstrings

Every test `.lean` file must include a module docstring (`/-! ... -/`) briefly explaining what the file tests. For regression tests, reference the issue (e.g. `#13599`). Place the docstring after the `import` block, if any.

## Rebasing vs PR base

When asked to "rebase a PR onto X", **only change the local branch base** — never change the PR's `--base` target on GitHub unless explicitly told to.

Common Lean4 case: rebasing onto `nightly-with-mathlib` is done to get a mathlib-tested snapshot for CI; the PR still targets `master`.