diff --git a/dsh/install.sh b/dsh/install.sh
index 6baab09..d95fd5c 100755
--- a/dsh/install.sh
+++ b/dsh/install.sh
@@ -412,6 +412,15 @@ ensure_pinned_clone() {
       echo "FAIL: move it away and run: $CLONE_HINT" >&2
       return 2
     fi
+    # A foreign git repository must never be mutated by fetch/checkout: a real
+    # superpowers clone always carries skills/using-superpowers/SKILL.md (I-1).
+    # The marker also holds for the moved-tag flow — skills are present in
+    # every pinned clone.
+    if [ ! -f "$dir/skills/using-superpowers/SKILL.md" ]; then
+      echo "FAIL: $dir is not a superpowers clone (no skills/using-superpowers/SKILL.md); left untouched" >&2
+      echo "FAIL: move it away or use --superpowers-from <dir>" >&2
+      return 2
+    fi
     # Online: refresh the pin. Fetching from $DSH_SUPERPOWERS_REPO (not from
     # the clone's origin) makes a repo change take effect on the next run
     # (T3.3); the force refspec updates the LOCAL tag as well as FETCH_HEAD, so
diff --git a/dsh/tests/install.test.mjs b/dsh/tests/install.test.mjs
index ae6b717..3f2d1ed 100644
--- a/dsh/tests/install.test.mjs
+++ b/dsh/tests/install.test.mjs
@@ -282,6 +282,40 @@ test('--skills removes a half-cloned dir after a failed fresh clone (RF5)', () =
   }
 })
 
+test('a foreign git repo in $DSH_HOME/superpowers is never touched (I-1)', () => {
+  const work = tempDir()
+  try {
+    const home = join(work, 'home')
+    const pinned = makePinnedRepo(work)
+    const foreign = join(home, '.dsh', 'superpowers')
+    mkdirSync(foreign, { recursive: true })
+    git(foreign, ['init', '-b', 'main'])
+    git(foreign, ['config', 'user.email', 'dsh-harness@example.invalid'])
+    git(foreign, ['config', 'user.name', 'DSH Harness Test'])
+    write(join(foreign, 'witness.txt'), 'do not touch\n')
+    git(foreign, ['add', 'witness.txt'])
+    git(foreign, ['commit', '-m', 'foreign repo'])
+    const headBefore = git(foreign, ['rev-parse', 'HEAD'])
+
+    const run = runInstaller(home, ['--skills'], { DSH_SUPERPOWERS_REPO: `file://${pinned}` })
+    const out = run.stdout + run.stderr
+    assert.equal(run.status, 1, out)
+    assert.match(out, /not a superpowers clone/)
+    assert.match(out, /move it away or use --superpowers-from/)
+    assert.equal(readFileSync(join(foreign, 'witness.txt'), 'utf8'), 'do not touch\n')
+    assert.equal(git(foreign, ['rev-parse', 'HEAD']), headBefore, 'HEAD must not move')
+    assert.equal(existsSync(join(foreign, 'skills')), false, 'no skills tree may appear')
+
+    // --check agrees: the foreign repo is not reported as a source.
+    const check = runInstaller(home, ['--check'], { DSH_SUPERPOWERS_REPO: `file://${pinned}` })
+    const checkOut = check.stdout + check.stderr
+    assert.equal(check.status, 0, checkOut)
+    assert.doesNotMatch(checkOut, /ok: superpowers source/)
+  } finally {
+    cleanup(work)
+  }
+})
+
 test('--skills reuses an on-ref clone when offline (WARN)', () => {
   const work = tempDir()
   try {