diff --git a/TODO.md b/TODO.md
index 23a1f8f..bf44a06 100644
--- a/TODO.md
+++ b/TODO.md
@@ -19,6 +19,8 @@ Rationale, Design, TODO, License, Author) if any are still missing.
 
 # Completed Steps
 
+- 2026-10-01: `script/install-precommit` asks git for the repository's git
+  directory, so `make hooks` also works where `.git` is a file (closes #129).
 - 2026-10-01: wildcard CORS now applies only to the public routes, not to
   `/metrics`, and allows only the methods they serve (closes #100).
 - 2026-10-01: `internal/state` and `internal/watcher` no longer export test-only
diff --git a/script/install-precommit b/script/install-precommit
index 723bae1..c005c5e 100755
--- a/script/install-precommit
+++ b/script/install-precommit
@@ -7,7 +7,20 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    hook=".git/hooks/pre-commit"
+    # Stop if this directory is not the top of its own git checkout, for
+    # example a copy inside another repository, whose hook must not be
+    # replaced.
+    if [ "$(git rev-parse --show-toplevel)" != "$ROOT" ]; then
+        echo "install-precommit: $ROOT is not the top of a git checkout" >&2
+        exit 1
+    fi
+    # Ask git for the repository's own git directory: .git is a file, not
+    # a directory, in some checkouts (for example a clone made with
+    # --separate-git-dir). core.hooksPath is deliberately not followed, so
+    # the hook is never written outside this repository.
+    hooks="$(git rev-parse --git-common-dir)/hooks"
+    mkdir -p "$hooks"
+    hook="$hooks/pre-commit"
     printf '#!/bin/sh\nset -e\nscript/precommit\n' > "$hook"
     chmod +x "$hook"
     echo "pre-commit hook installed: runs script/precommit"