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Caching

Dependency caching

uv uses aggressive caching to avoid re-downloading (and re-building) dependencies that have already been accessed in prior runs.

The specifics of uv's caching semantics vary based on the nature of the dependency:

  • For registry dependencies (like those downloaded from PyPI), uv respects HTTP caching headers.
  • For direct URL dependencies, uv respects HTTP caching headers, and also caches based on the URL itself.
  • For Git dependencies, uv caches based on the fully-resolved Git commit hash. As such, uv pip compile will pin Git dependencies to a specific commit hash when writing the resolved dependency set.
  • For local dependencies, uv caches based on the last-modified time of the source archive (i.e., the local .whl or .tar.gz file). For directories, uv caches based on the last-modified time of the pyproject.toml, setup.py, or setup.cfg file.

If you're running into caching issues, uv includes a few escape hatches:

  • To force uv to revalidate cached data for all dependencies, pass --refresh to any command (e.g., uv sync --refresh or uv pip install --refresh ...).
  • To force uv to revalidate cached data for a specific dependency pass --refresh-package to any command (e.g., uv sync --refresh-package flask or uv pip install --refresh-package flask ...).
  • To force uv to ignore existing installed versions, pass --reinstall to any installation command (e.g., uv sync --reinstall or uv pip install --reinstall ...).

As a special case, uv will always rebuild and reinstall any local directory dependencies passed explicitly on the command-line (e.g., uv pip install .).

Dynamic metadata

By default, uv will only rebuild and reinstall local directory dependencies (e.g., editables) if the pyproject.toml, setup.py, or setup.cfg file in the directory root has changed, or if a src directory is added or removed. This is a heuristic and, in some cases, may lead to fewer re-installs than desired.

To incorporate additional information into the cache key for a given package, you can add cache key entries under tool.uv.cache-keys, which covers both file paths and Git commit hashes. Setting tool.uv.cache-keys will replace defaults, so any necessary files (like pyproject.toml) should still be included in the user-defined cache keys.

For example, if a project specifies dependencies in pyproject.toml but uses setuptools-scm to manage its version, and should thus be rebuilt whenever the commit hash or dependencies change, you can add the following to the project's pyproject.toml:

pyproject.toml
[tool.uv]
cache-keys = [{ file = "pyproject.toml" }, { git = { commit = true } }]

If your dynamic metadata incorporates information from the set of Git tags, you can expand the cache key to include the tags:

pyproject.toml
[tool.uv]
cache-keys = [{ file = "pyproject.toml" }, { git = { commit = true, tags = true } }]

Similarly, if a project reads from a requirements.txt to populate its dependencies, you can add the following to the project's pyproject.toml:

pyproject.toml
[tool.uv]
cache-keys = [{ file = "pyproject.toml" }, { file = "requirements.txt" }]

Globs are supported for file keys, following the syntax of the glob crate. For example, to invalidate the cache whenever a .toml file in the project directory or any of its subdirectories is modified, use the following:

pyproject.toml
[tool.uv]
cache-keys = [{ file = "**/*.toml" }]

Note

The use of globs can be expensive, as uv may need to walk the filesystem to determine whether any files have changed. This may, in turn, requiring traversal of large or deeply nested directories.

Similarly, if a project relies on an environment variable, you can add the following to the project's pyproject.toml to invalidate the cache whenever the environment variable changes:

pyproject.toml
[tool.uv]
cache-keys = [{ file = "pyproject.toml" }, { env = "MY_ENV_VAR" }]

Finally, to invalidate a project whenever a specific directory (like src) is created or removed, add the following to the project's pyproject.toml:

pyproject.toml
[tool.uv]
cache-keys = [{ file = "pyproject.toml" }, { dir = "src" }]

Note that the dir key will only track changes to the directory itself, and not arbitrary changes within the directory.

As an escape hatch, if a project uses dynamic metadata that isn't covered by tool.uv.cache-keys, you can instruct uv to always rebuild and reinstall it by adding the project to the tool.uv.reinstall-package list:

pyproject.toml
[tool.uv]
reinstall-package = ["my-package"]

This will force uv to rebuild and reinstall my-package on every run, regardless of whether the package's pyproject.toml, setup.py, or setup.cfg file has changed.

Cache safety

It's safe to run multiple uv commands concurrently, even against the same virtual environment. uv's cache is designed to be thread-safe and append-only, and thus robust to multiple concurrent readers and writers. uv applies a file-based lock to the target virtual environment when installing, to avoid concurrent modifications across processes.

Note that it's not safe to modify the uv cache (e.g., uv cache clean) while other uv commands are running, and never safe to modify the cache directly (e.g., by removing a file or directory).

Clearing the cache

uv provides a few different mechanisms for removing entries from the cache:

  • uv cache clean removes all cache entries from the cache directory, clearing it out entirely.
  • uv cache clean ruff removes all cache entries for the ruff package, useful for invalidating the cache for a single or finite set of packages.
  • uv cache prune removes all unused cache entries. For example, the cache directory may contain entries created in previous uv versions that are no longer necessary and can be safely removed. uv cache prune is safe to run periodically, to keep the cache directory clean.

Caching in continuous integration

It's common to cache package installation artifacts in continuous integration environments (like GitHub Actions or GitLab CI) to speed up subsequent runs.

By default, uv caches both the wheels that it builds from source and the pre-built wheels that it downloads directly, to enable high-performance package installation.

However, in continuous integration environments, persisting pre-built wheels may be undesirable. With uv, it turns out that it's often faster to omit pre-built wheels from the cache (and instead re-download them from the registry on each run). On the other hand, caching wheels that are built from source tends to be worthwhile, since the wheel building process can be expensive, especially for extension modules.

To support this caching strategy, uv provides a uv cache prune --ci command, which removes all pre-built wheels and unzipped source distributions from the cache, but retains any wheels that were built from source. We recommend running uv cache prune --ci at the end of your continuous integration job to ensure maximum cache efficiency. For an example, see the GitHub integration guide.

Cache directory

uv determines the cache directory according to, in order:

  1. A temporary cache directory, if --no-cache was requested.
  2. The specific cache directory specified via --cache-dir, UV_CACHE_DIR, or tool.uv.cache-dir.
  3. A system-appropriate cache directory, e.g., $XDG_CACHE_HOME/uv or $HOME/.cache/uv on Unix and %LOCALAPPDATA%\uv\cache on Windows

Note

uv always requires a cache directory. When --no-cache is requested, uv will still use a temporary cache for sharing data within that single invocation.

In most cases, --refresh should be used instead of --no-cache — as it will update the cache for subsequent operations but not read from the cache.

It is important for performance for the cache directory to be located on the same file system as the Python environment uv is operating on. Otherwise, uv will not be able to link files from the cache into the environment and will instead need to fallback to slow copy operations.

Cache versioning

The uv cache is composed of a number of buckets (e.g., a bucket for wheels, a bucket for source distributions, a bucket for Git repositories, and so on). Each bucket is versioned, such that if a release contains a breaking change to the cache format, uv will not attempt to read from or write to an incompatible cache bucket.

For example, uv 0.4.13 included a breaking change to the core metadata bucket. As such, the bucket version was increased from v12 to v13. Within a cache version, changes are guaranteed to be both forwards- and backwards-compatible.

Since changes in the cache format are accompanied by changes in the cache version, multiple versions of uv can safely read and write to the same cache directory. However, if the cache version changed between a given pair of uv releases, then those releases may not be able to share the same underlying cache entries.

For example, it's safe to use a single shared cache for uv 0.4.12 and uv 0.4.13, though the cache itself may contain duplicate entries in the core metadata bucket due to the change in cache version.