Explicitly configure uv to use hard links

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - Updated pip from 25.3 to 26.0.1. ([#508](https://github.com/heroku/buildpacks-python/pull/508))
+- Explicitly configured uv to use hard links to maintain the behaviour of previous versions. ([#519](https://github.com/heroku/buildpacks-python/pull/519))
 
 ## [5.1.0] - 2026-02-26
 

src/layers/uv.rs

@@ -39,9 +39,9 @@ pub(crate) fn install_uv(
         },
     )?;
 
-    // Prevent uv from downloading/using its own Python installation:
-    // https://docs.astral.sh/uv/concepts/python-versions/#disabling-automatic-python-downloads
     let mut layer_env = LayerEnv::new()
+        // Prevent uv from downloading/using its own Python installation:
+        // https://docs.astral.sh/uv/concepts/python-versions/#disabling-automatic-python-downloads
         .chainable_insert(
             Scope::Build,
             ModificationBehavior::Override,
@@ -53,6 +53,14 @@ pub(crate) fn install_uv(
             ModificationBehavior::Override,
             "UV_PYTHON_DOWNLOADS",
             "never",
+        )
+        // Force uv to use hardlinks rather than the new default of reflinks, since the latter are
+        // significantly slower in some environments: https://github.com/astral-sh/uv/issues/18259
+        .chainable_insert(
+            Scope::Build,
+            ModificationBehavior::Override,
+            "UV_LINK_MODE",
+            "hardlink",
         );
 
     match layer.state {

tests/uv_test.rs

@@ -50,6 +50,7 @@ fn uv_basic_install_and_cache_reuse() {
                 PYTHONUNBUFFERED=1
                 SOURCE_DATE_EPOCH=315532801
                 UV_CACHE_DIR=/layers/heroku_python/uv-cache
+                UV_LINK_MODE=hardlink
                 UV_NO_MANAGED_PYTHON=1
                 UV_PROJECT_ENVIRONMENT=/layers/heroku_python/venv
                 UV_PYTHON_DOWNLOADS=never