diff --git a/ebuild/packages/cache.py b/ebuild/packages/cache.py
index 6f62df55..48045eae 100644
--- a/ebuild/packages/cache.py
+++ b/ebuild/packages/cache.py
@@ -66,6 +66,9 @@ def is_built(self, recipe: PackageRecipe) -> bool:
         if not marker.exists():
             return False
 
+        if not self.install_dir(recipe).is_dir():
+            return False
+
         try:
             data = json.loads(marker.read_text(encoding="utf-8"))
             return (
diff --git a/tests/ebuild/test_package_cache.py b/tests/ebuild/test_package_cache.py
new file mode 100644
index 00000000..061b2a31
--- /dev/null
+++ b/tests/ebuild/test_package_cache.py
@@ -0,0 +1,36 @@
+# SPDX-License-Identifier: MIT
+# Copyright (c) 2026 EoS Project
+
+"""Tests for ebuild.packages.cache.PackageCache."""
+
+import shutil
+
+from ebuild.packages.cache import PackageCache
+from ebuild.packages.recipe import PackageRecipe
+
+
+def make_recipe():
+    return PackageRecipe(
+        name="demo",
+        version="1.0.0",
+        url="https://example.com/demo.tar.gz",
+        checksum="sha256:" + "a" * 64,
+    )
+
+
+def test_is_built_is_false_when_install_dir_is_missing(tmp_path):
+    """Regression: a valid .built marker is not enough if install/ is gone.
+
+    Callers treat is_built() as "the cached install prefix is usable".
+    A leftover marker after a deleted install directory must not skip rebuild.
+    """
+    cache = PackageCache(tmp_path / "packages")
+    recipe = make_recipe()
+    cache.ensure_dirs(recipe)
+    cache.mark_built(recipe)
+
+    assert cache.is_built(recipe) is True
+
+    shutil.rmtree(cache.install_dir(recipe))
+
+    assert cache.is_built(recipe) is False