Add experimental uv-based PEX builder ([python].pex_builder)

docs/notes/2.32.x.md

@@ -106,6 +106,14 @@ Copying the `mypy` cache back to the ["named cache"](https://www.pantsbuild.org/
 
 The mypy subsystem now supports a new `cache_mode="none"` to disable mypy's caching entirely.  This is slower and intended as an "escape valve".  It is hoped that on the latest mypy with the above Pants side fixes it will not be necessary.
 
+A new **experimental** `[python].pex_builder` option allows using [uv](https://github.com/astral-sh/uv) to install
+dependencies when building PEX binaries via `pants package`. When set to `"uv"`, Pants creates a virtual environment
+with uv, then passes it to PEX via `--venv-repository` so PEX packages from the pre-populated venv instead of
+resolving with pip. When a PEX-native lockfile is available, uv installs the exact pinned versions from the lockfile
+with `--no-deps`, preserving reproducibility. This only applies to non-internal, non-cross-platform PEX builds with
+explicit requirement strings and a local Python interpreter; other builds silently fall back to pip.
+See [#20679](https://github.com/pantsbuild/pants/issues/20679) for background.
+
 The `runtime` field of [`aws_python_lambda_layer`](https://www.pantsbuild.org/2.32/reference/targets/python_aws_lambda_layer#runtime) or [`aws_python_lambda_function`](https://www.pantsbuild.org/2.32/reference/targets/python_aws_lambda_function#runtime) now has built-in complete platform configurations for x86-64 and arm64 Python 3.14. This provides stable support for Python 3.14 lambdas out of the box, allowing deleting manual `complete_platforms` configuration if any.
 
 The `grpc-python-plugin` tool now uses an updated `v1.73.1` plugin built from  <https://github.com/nhurden/protoc-gen-grpc-python-prebuilt]. This also brings `macos_arm64` support.

src/python/pants/backend/python/register.py

@@ -32,7 +32,7 @@
 from pants.backend.python.macros.python_artifact import PythonArtifact
 from pants.backend.python.macros.python_requirements import PythonRequirementsTargetGenerator
 from pants.backend.python.macros.uv_requirements import UvRequirementsTargetGenerator
-from pants.backend.python.subsystems import debugpy
+from pants.backend.python.subsystems import debugpy, uv
 from pants.backend.python.target_types import (
     PexBinariesGeneratorTarget,
     PexBinary,
@@ -70,6 +70,7 @@ def rules():
         # Subsystems
         *coverage_py.rules(),
         *debugpy.rules(),
+        *uv.rules(),
         # Util rules
         *ancestor_files.rules(),
         *dependency_inference_rules.rules(),

src/python/pants/backend/python/subsystems/setup.py

@@ -42,6 +42,12 @@ class LockfileGenerator(enum.Enum):
     POETRY = "poetry"
 
 
+@enum.unique
+class PexBuilder(enum.Enum):
+    pex = "pex"
+    uv = "uv"
+
+
 RESOLVE_OPTION_KEY__DEFAULT = "__default__"
 
 _T = TypeVar("_T")
@@ -312,6 +318,23 @@ def default_to_resolve_interpreter_constraints(self) -> bool:
         ),
         advanced=True,
     )
+    pex_builder = EnumOption(
+        default=PexBuilder.pex,
+        help=softwrap(
+            """
+            Which tool to use for installing dependencies when building PEX files.
+
+            - `pex` (default): Use pip via PEX.
+            - `uv` (experimental): Pre-install dependencies into a uv venv, then pass it
+              to PEX via `--venv-repository`. When a PEX-native lockfile is available,
+              uv installs the exact pinned versions with `--no-deps`.
+
+            Only applies to non-internal, non-cross-platform PEX builds. Other builds
+            silently fall back to pip.
+            """
+        ),
+        advanced=True,
+    )
     _resolves_to_interpreter_constraints = DictOption[list[str]](
         help=softwrap(
             """

src/python/pants/backend/python/subsystems/uv.py

@@ -0,0 +1,78 @@
+# Copyright 2026 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from pants.core.util_rules.external_tool import (
+    TemplatedExternalTool,
+    download_external_tool,
+)
+from pants.engine.fs import Digest
+from pants.engine.platform import Platform
+from pants.engine.rules import collect_rules, rule
+from pants.option.option_types import ArgsListOption
+from pants.util.strutil import softwrap
+
+
+class Uv(TemplatedExternalTool):
+    options_scope = "uv"
+    name = "uv"
+    help = "The uv Python package manager (https://github.com/astral-sh/uv)."
+
+    default_version = "0.6.14"
+    default_known_versions = [
+        "0.6.14|macos_x86_64|1d8ecb2eb3b68fb50e4249dc96ac9d2458dc24068848f04f4c5b42af2fd26552|16276555",
+        "0.6.14|macos_arm64|4ea4731010fbd1bc8e790e07f199f55a5c7c2c732e9b77f85e302b0bee61b756|15138933",
+        "0.6.14|linux_x86_64|0cac4df0cb3457b154f2039ae471e89cd4e15f3bd790bbb3cb0b8b40d940b93e|17032361",
+        "0.6.14|linux_arm64|94e22c4be44d205def456427639ca5ca1c1a9e29acc31808a7b28fdd5dcf7f17|15577079",
+    ]
+    version_constraints = ">=0.6.0,<1.0"
+
+    default_url_template = (
+        "https://github.com/astral-sh/uv/releases/download/{version}/uv-{platform}.tar.gz"
+    )
+    default_url_platform_mapping = {
+        "linux_arm64": "aarch64-unknown-linux-musl",
+        "linux_x86_64": "x86_64-unknown-linux-musl",
+        "macos_arm64": "aarch64-apple-darwin",
+        "macos_x86_64": "x86_64-apple-darwin",
+    }
+
+    def generate_exe(self, plat: Platform) -> str:
+        platform = self.default_url_platform_mapping[plat.value]
+        return f"./uv-{platform}/uv"
+
+    args_for_uv_pip_install = ArgsListOption(
+        tool_name="uv",
+        example="--index-strategy unsafe-first-match",
+        extra_help=softwrap(
+            """
+            Additional arguments to pass to `uv pip install` invocations.
+
+            Used when `[python].pex_builder = "uv"` to pass extra flags to the
+            `uv pip install` step (e.g. `--index-url`, `--extra-index-url`).
+            These are NOT passed to the `uv venv` step.
+            """
+        ),
+    )
+
+
+@dataclass(frozen=True)
+class DownloadedUv:
+    """The downloaded uv binary with user-configured args."""
+
+    digest: Digest
+    exe: str
+    args_for_uv_pip_install: tuple[str, ...]
+
+
+@rule
+async def download_uv_binary(uv: Uv, platform: Platform) -> DownloadedUv:
+    downloaded = await download_external_tool(uv.get_request(platform))
+    return DownloadedUv(digest=downloaded.digest, exe=downloaded.exe, args_for_uv_pip_install=tuple(uv.args_for_uv_pip_install))
+
+
+def rules():
+    return collect_rules()