diff --git a/AGENTS.md b/AGENTS.md
index e5552dc..7bef9d3 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -23,8 +23,8 @@ A pre-commit hook repo that auto-installs and runs `clang-format` and `clang-tid
 
 | File | Purpose |
 |------|---------|
-| `pyproject.toml` | Build config, deps (`pip>=26.1`), dev deps, entry points |
-| `cpp_linter_hooks/util.py` | Installed-wheel check, PyPI version resolution + pip install (prefix matching, offline fallback) |
+| `pyproject.toml` | Build config, deps (`pip>=26.1`, `filelock>=3.16`), dev deps, entry points |
+| `cpp_linter_hooks/util.py` | Installed-wheel check, PyPI version resolution + pip install (prefix matching, offline fallback), all under a `filelock` lock on the hook environment so parallel hook processes install once |
 | `cpp_linter_hooks/clang_format.py` | clang-format wrapper (`--verbose`, `--dry-run`) |
 | `cpp_linter_hooks/clang_tidy.py` | clang-tidy wrapper (`--compile-commands`, missing-database policy, `--jobs`, `--fix`, `--all-lines`, `-quiet` + status-line filter, `--checks` repair, error hints) |
 | `cpp_linter_hooks/changed_lines.py` | Added line ranges per file from `git diff` (staged changes, or `PRE_COMMIT_FROM_REF...PRE_COMMIT_TO_REF`) |
diff --git a/cpp_linter_hooks/util.py b/cpp_linter_hooks/util.py
index dac3565..d129cb2 100644
--- a/cpp_linter_hooks/util.py
+++ b/cpp_linter_hooks/util.py
@@ -1,5 +1,6 @@
 """Shared helpers for resolving and installing clang tool wheels."""
 
+import contextlib
 import json
 import logging
 import os
@@ -162,7 +163,9 @@ def _installed_wheel_version(tool: str) -> str | None:
     installed on an earlier run) into the hook's own environment. The version
     comes from the wheel's ``<name>-<version>.dist-info`` directory name, which
     needs neither the network nor a subprocess, and avoids parsing the large
-    METADATA file the way ``importlib.metadata.version`` does.
+    METADATA file the way ``importlib.metadata.version`` does. A wheel counts
+    only once its ``RECORD`` file exists: pip writes it last, so a wheel another
+    hook process is still installing is not used half-written.
     """
     prefix = f"{re.sub(r'[-_.]+', '_', tool).lower()}-"
     suffix = ".dist-info"
@@ -175,7 +178,11 @@ def _installed_wheel_version(tool: str) -> str | None:
             continue
         for name in names:
             lower = name.lower()
-            if lower.startswith(prefix) and lower.endswith(suffix):
+            if (
+                lower.startswith(prefix)
+                and lower.endswith(suffix)
+                and os.path.isfile(os.path.join(site_dir, name, "RECORD"))
+            ):
                 return name[len(prefix) : -len(suffix)]
     return None
 
@@ -207,6 +214,38 @@ def _find_installed_wheel(tool: str, version: str | None) -> tuple[Path, str] |
     return Path(path), installed
 
 
+# Seconds to wait for another hook process to finish installing.
+_INSTALL_LOCK_TIMEOUT = 300
+
+
+@contextlib.contextmanager
+def _install_lock():
+    """Let one hook process at a time install tools into this environment.
+
+    pre-commit and prek run the hook on batches of files in parallel. Without
+    the lock, every batch runs ``pip install`` into the same directory at once.
+    If the lock is not free within 5 minutes, or the file system does not support
+    locks (some NFS mounts), the hook runs unlocked, as before. The limit also
+    covers filelock 3.x, which keeps retrying when the file system refuses the
+    lock, and a stale lock file left by a crashed process.
+    filelock is imported here because it is only needed to install, and
+    importing it takes longer than a whole hook run that finds the tool.
+    """
+    from filelock import FileLock
+
+    lock = FileLock(os.path.join(sys.prefix, ".cpp-linter-hooks-install.lock"))
+    try:
+        lock.acquire(timeout=_INSTALL_LOCK_TIMEOUT)
+    except (OSError, NotImplementedError) as exc:  # filelock.Timeout is an OSError
+        LOG.warning("Installing without a lock: %s", exc)
+        lock = None
+    try:
+        yield
+    finally:
+        if lock is not None:
+            lock.release()
+
+
 def _install_tool(tool: str, version: str) -> Path | None:
     """Install a tool using pip, logging output on failure."""
     result = subprocess.run(
@@ -233,17 +272,30 @@ def resolve_install_with_diagnostics(
     ``additional_dependencies`` are kept and the hook works offline.
     Otherwise the version is resolved dynamically from PyPI — no hardcoded
     list is maintained in-tree.
+
+    Installing runs under :func:`_install_lock`, so when several hook processes
+    start at once, one installs and the others wait and then use its wheel.
     """
     installed_wheel = _find_installed_wheel(tool, version)
-    if installed_wheel is not None:
-        path, installed = installed_wheel
-        if verbose:
-            print(
-                f"Using installed {tool} Python wheel version {installed}",
-                file=sys.stderr,
-            )
-        return path, None
+    if installed_wheel is None:
+        with _install_lock():
+            # Another hook process may have installed it while this one waited.
+            installed_wheel = _find_installed_wheel(tool, version)
+            if installed_wheel is None:
+                return _resolve_and_install(tool, version, verbose)
+    path, installed = installed_wheel
+    if verbose:
+        print(
+            f"Using installed {tool} Python wheel version {installed}",
+            file=sys.stderr,
+        )
+    return path, None
 
+
+def _resolve_and_install(
+    tool: str, version: str | None, verbose: bool
+) -> tuple[Path | None, str | None]:
+    """Resolve *version* on PyPI and install it, unless it is already on PATH."""
     user_version, error = _resolve_version_from_pypi(tool, version)
     if error is not None:
         return None, error
diff --git a/pyproject.toml b/pyproject.toml
index 09c542d..89225d7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -31,6 +31,7 @@ classifiers = [
     "Topic :: Software Development :: Build Tools",
 ]
 dependencies = [
+    "filelock>=3.16", # One hook process at a time installs clang tools
     "pip>=26.1", # Required to install clang-format and clang-tidy
 ]
 dynamic = ["version"]
diff --git a/tests/test_util.py b/tests/test_util.py
index 11d0d0c..563141b 100644
--- a/tests/test_util.py
+++ b/tests/test_util.py
@@ -1,10 +1,12 @@
 """Tests for cpp_linter_hooks.util -- dynamic PyPI version resolution."""
 
+import errno
 import json
 import logging
 import subprocess
 import sys
 import sysconfig
+import textwrap
 from pathlib import Path
 from unittest.mock import patch
 
@@ -15,6 +17,7 @@
     _env_paths,
     _find_installed_wheel,
     _get_pypi_versions,
+    _install_lock,
     _install_tool,
     _installed_wheel_version,
     _is_version_installed,
@@ -740,6 +743,12 @@ def test_env_paths_match_sysconfig():
     assert _env_paths() == sysconfig.get_paths()
 
 
+def _installed(dist_info: Path) -> None:
+    """Create a dist-info directory the way pip leaves it after an install."""
+    dist_info.mkdir(parents=True)
+    (dist_info / "RECORD").touch()
+
+
 def test_installed_wheel_version_reads_dist_info_name(tmp_path):
     for name in (
         "clang_tidy_extra-1.0.dist-info",
@@ -747,7 +756,7 @@ def test_installed_wheel_version_reads_dist_info_name(tmp_path):
         "clang_tidy",
         "Clang_Tidy-21.1.6.dist-info",
     ):
-        (tmp_path / name).mkdir()
+        _installed(tmp_path / name)
     with patch("cpp_linter_hooks.util._env_paths", return_value=_paths(tmp_path)):
         assert _installed_wheel_version("clang-tidy") == "21.1.6"
         assert _installed_wheel_version("clang-format") == "22.1.8"
@@ -756,14 +765,21 @@ def test_installed_wheel_version_reads_dist_info_name(tmp_path):
 def test_installed_wheel_version_checks_platlib(tmp_path):
     purelib, platlib = tmp_path / "purelib", tmp_path / "platlib"
     purelib.mkdir()
-    (platlib / "clang_tidy-21.1.6.dist-info").mkdir(parents=True)
+    _installed(platlib / "clang_tidy-21.1.6.dist-info")
     paths = {"purelib": str(purelib), "platlib": str(platlib), "scripts": ""}
     with patch("cpp_linter_hooks.util._env_paths", return_value=paths):
         assert _installed_wheel_version("clang-tidy") == "21.1.6"
 
 
-def test_installed_wheel_version_missing_distribution(tmp_path):
+def test_installed_wheel_version_ignores_unfinished_install(tmp_path):
+    """pip writes RECORD last; without it another process is still installing."""
     (tmp_path / "clang_format-22.1.8.dist-info").mkdir()
+    with patch("cpp_linter_hooks.util._env_paths", return_value=_paths(tmp_path)):
+        assert _installed_wheel_version("clang-format") is None
+
+
+def test_installed_wheel_version_missing_distribution(tmp_path):
+    _installed(tmp_path / "clang_format-22.1.8.dist-info")
     with patch("cpp_linter_hooks.util._env_paths", return_value=_paths(tmp_path)):
         assert _installed_wheel_version("clang-tidy") is None
     with patch(
@@ -912,3 +928,145 @@ def test_resolve_install_verbose_reports_installed_wheel(capsys):
         "Using installed clang-format Python wheel version 21.1.8"
         in capsys.readouterr().err
     )
+
+
+# ═══════════════════════════════════════════════════════════════════════
+# _install_lock
+# ═══════════════════════════════════════════════════════════════════════
+
+LOCK_WORKER = textwrap.dedent(
+    """
+    import os, sys, time
+    sys.prefix = sys.argv[1]
+    from cpp_linter_hooks.util import _install_lock
+
+    log = sys.argv[2]
+    with _install_lock():
+        with open(log, "a") as f:
+            f.write("start\\n")
+        time.sleep(0.2)
+        with open(log, "a") as f:
+            f.write("end\\n")
+    """
+)
+
+
+def test_install_lock_serializes_processes(tmp_path):
+    """Hook processes started at once take turns instead of installing together."""
+    log = tmp_path / "log.txt"
+    workers = [
+        subprocess.Popen(
+            [sys.executable, "-c", LOCK_WORKER, str(tmp_path), str(log)],
+        )
+        for _ in range(3)
+    ]
+    for worker in workers:
+        assert worker.wait(timeout=60) == 0
+
+    assert log.read_text().split() == ["start", "end"] * 3
+    assert (tmp_path / ".cpp-linter-hooks-install.lock").is_file()
+
+
+def test_install_lock_unlocks_on_error(tmp_path, monkeypatch):
+    monkeypatch.setattr(sys, "prefix", str(tmp_path))
+    with pytest.raises(RuntimeError), _install_lock():
+        _crash()
+    with _install_lock():
+        pass
+
+
+def _crash():
+    raise RuntimeError("pip crashed")
+
+
+@pytest.mark.parametrize(
+    "error",
+    [
+        OSError(errno.ENOLCK, "No locks available"),
+        NotImplementedError("FileSystem does not appear to support flock"),
+        TimeoutError("The file lock could not be acquired"),
+    ],
+)
+def test_install_lock_runs_unlocked_without_lock_support(
+    tmp_path, monkeypatch, caplog, error
+):
+    """File systems without lock support (e.g. some NFS mounts) run unlocked."""
+    monkeypatch.setattr(sys, "prefix", str(tmp_path))
+    with (
+        patch("filelock.FileLock.acquire", side_effect=error),
+        caplog.at_level(logging.DEBUG, logger="cpp_linter_hooks.util"),
+        _install_lock(),
+    ):
+        entered = True
+    assert entered
+    assert "Installing without a lock" in caplog.text
+
+
+def test_install_lock_waits_a_limited_time(tmp_path, monkeypatch):
+    monkeypatch.setattr(sys, "prefix", str(tmp_path))
+    with (
+        patch("filelock.FileLock.acquire") as acquire,
+        patch("filelock.FileLock.release"),
+        _install_lock(),
+    ):
+        pass
+    acquire.assert_called_once_with(timeout=300)
+
+
+def test_resolve_install_runs_under_install_lock():
+    events = []
+
+    class Lock:
+        def __enter__(self):
+            events.append("lock")
+
+        def __exit__(self, *exc):
+            events.append("unlock")
+
+    def install(tool, version):
+        events.append("install")
+        return Path("/venv/bin/clang-format")
+
+    with (
+        patch("cpp_linter_hooks.util._install_lock", return_value=Lock()),
+        patch("shutil.which", return_value=None),
+        patch(
+            "cpp_linter_hooks.util._get_pypi_versions", side_effect=_pypi_side_effect
+        ),
+        patch("cpp_linter_hooks.util._install_tool", side_effect=install),
+    ):
+        result = resolve_install_with_diagnostics("clang-format", "20")
+
+    assert result == (Path("/venv/bin/clang-format"), None)
+    assert events == ["lock", "install", "unlock"]
+
+
+def test_resolve_install_skips_lock_when_installed():
+    with (
+        patch(
+            "cpp_linter_hooks.util._find_installed_wheel",
+            return_value=(Path("/venv/bin/clang-format"), "20.1.8"),
+        ),
+        patch("cpp_linter_hooks.util._install_lock") as lock,
+    ):
+        result = resolve_install_with_diagnostics("clang-format", "20")
+
+    assert result == (Path("/venv/bin/clang-format"), None)
+    lock.assert_not_called()
+
+
+def test_resolve_install_uses_wheel_installed_while_waiting():
+    """Another hook process installed the wheel while this one waited for the lock."""
+    with (
+        patch(
+            "cpp_linter_hooks.util._find_installed_wheel",
+            side_effect=[None, (Path("/venv/bin/clang-format"), "20.1.8")],
+        ),
+        patch("cpp_linter_hooks.util._get_pypi_versions") as pypi,
+        patch("cpp_linter_hooks.util._install_tool") as install,
+    ):
+        result = resolve_install_with_diagnostics("clang-format", "20")
+
+    assert result == (Path("/venv/bin/clang-format"), None)
+    pypi.assert_not_called()
+    install.assert_not_called()
diff --git a/uv.lock b/uv.lock
index 27d611b..6c34416 100644
--- a/uv.lock
+++ b/uv.lock
@@ -152,6 +152,7 @@ wheels = [
 name = "cpp-linter-hooks"
 source = { editable = "." }
 dependencies = [
+    { name = "filelock" },
     { name = "pip" },
 ]
 
@@ -164,7 +165,10 @@ dev = [
 ]
 
 [package.metadata]
-requires-dist = [{ name = "pip", specifier = ">=26.1" }]
+requires-dist = [
+    { name = "filelock", specifier = ">=3.16" },
+    { name = "pip", specifier = ">=26.1" },
+]
 
 [package.metadata.requires-dev]
 dev = [