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name: rebuild description: Rebuild Ray from source — determines the right build mode based on what changed

Rebuild Ray

Canonical build docs: doc/source/ray-contribute/development.md Use the user's configured Python from CLAUDE.local.md, or fall back to which python. Update this skill if any changes are detected in development.rst.

Determine what to rebuild

1. Check what files changed: git diff --name-only HEAD (or ask the user)
2. Pick the appropriate build mode:
   • Only .py files changed (no .h, .cc, .pyx, .pxd) → Python-only rebuild (not required if installed with pip install -e . — editable mode auto-picks up Python changes)
   • .h, .cc, .pyx, .pxd files changed → C++/Cython rebuild
   • Dashboard files changed (python/ray/dashboard/client/) → Dashboard rebuild first
   • First time / clean build → Full source build

Running Python tests that exercise C++ changes: When a Python test depends on C++ or Cython code (raylet, GCS, core_worker, etc.), run the C++/Cython rebuild below before running the test. Editable Python installs do not rebuild the compiled extensions, so stale binaries will run otherwise.

Python-only rebuild

For changes to Tune, RLlib, Autoscaler, and most Python files. Two approaches depending on how Ray was installed:

If installed with pip install -e . (editable/full source build)

No rebuild needed — Python changes are picked up automatically.

If installed from nightly wheel + setup-dev.py

1. Ensure a nightly wheel is installed (see doc/source/ray-contribute/development.md for the correct wheel URL for the user's platform and Python version)
2. Link local files: python python/ray/setup-dev.py
3. Can skip specific dirs: python python/ray/setup-dev.py -y --skip _private dashboard

Warning: With setup-dev.py, don't run pip uninstall ray or pip install -U. Instead: rm -rf <site-packages>/ray, reinstall wheel, re-run setup-dev.py.

C++/Cython rebuild (after initial full build)

Full package rebuild

bazel run //:gen_ray_pkg

Build individual modules (faster)

bazel build //src/ray/raylet:raylet          # Raylet only
bazel build //src/ray/gcs:gcs_server         # GCS server only
bazel build //src/ray/core_worker:core_worker # Core worker only
bazel build //:ray_pkg                       # All C++ and Cython

Build variants

• bazel run -c fastbuild //:gen_ray_pkg — fast build (less optimization)
• bazel run -c dbg //:gen_ray_pkg — debug build with symbols (for gdb)
• bazel run -c opt //:gen_ray_pkg — optimized build

Make permanent in ~/.bazelrc: build --compilation_mode=fastbuild

Full source build (first time)

1. If dashboard changed: cd python/ray/dashboard/client && npm ci && npm run build && cd -
2. Install deps: cd python/ && pip install -r requirements.txt
3. Build: pip install -e . --verbose

Resource-constrained builds

Add to ~/.bazelrc if machine runs out of memory:

build --local_ram_resources=HOST_RAM*.5 --local_cpu_resources=4
build --disk_cache=~/bazel-cache

Worktree notes

• Verify build paths: bazel info execution_root
• If build fails, check .bazelversion matches installed Bazel
• Each worktree may need its own pip install -e .
• If in a worktree and bazel targets fail, ensure symlinks are correct