diff --git a/TODO.md b/TODO.md
index 8af2f80..614432b 100644
--- a/TODO.md
+++ b/TODO.md
@@ -20,6 +20,8 @@ https://git.eeqj.de/sneak/dnswatcher/issues/104
 
 # 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: `TODO.md` brought up to date: open issues listed by URL, every
   Completed Steps entry cut to at most two lines (closes #146).
 - 2026-10-01: wildcard CORS now applies only to the public routes, not to
@@ -106,7 +108,5 @@ https://git.eeqj.de/sneak/dnswatcher/issues/104
 - README accuracy sweep: https://git.eeqj.de/sneak/dnswatcher/issues/108
 - README sections required by policy:
   https://git.eeqj.de/sneak/dnswatcher/issues/173
-- `script/install-precommit` in a linked worktree:
-  https://git.eeqj.de/sneak/dnswatcher/issues/129
 - fixed root server order: https://git.eeqj.de/sneak/dnswatcher/issues/138
 - review toward 1.0: https://git.eeqj.de/sneak/dnswatcher/issues/144
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"