diff --git a/amplifier_core/loader.py b/amplifier_core/loader.py
index 7aff5a0..4e7d08f 100644
--- a/amplifier_core/loader.py
+++ b/amplifier_core/loader.py
@@ -412,6 +412,10 @@ def _find_package_dir(self, module_id: str, module_path: Path) -> Path | None:
         Returns:
             Path to the Python package directory, or None if not found
         """
+        # Guard: if module root doesn't exist, we can't find a package in it
+        if not module_path.exists():
+            return None
+
         # If the path itself has __init__.py, it's already a package
         if (module_path / "__init__.py").exists():
             return module_path
diff --git a/tests/test_loader.py b/tests/test_loader.py
new file mode 100644
index 0000000..4276cdc
--- /dev/null
+++ b/tests/test_loader.py
@@ -0,0 +1,113 @@
+"""
+Tests for ModuleLoader, specifically the _find_package_dir guard
+against missing module root directories.
+
+Regression test for the bug where calling _find_package_dir() on a
+non-existent path raised an opaque FileNotFoundError (from .iterdir())
+instead of returning None and letting _validate_module raise a clear
+ModuleValidationError.
+"""
+
+from pathlib import Path
+
+import pytest
+from amplifier_core.loader import ModuleLoader
+from amplifier_core.loader import ModuleValidationError
+
+
+class TestFindPackageDirMissingPath:
+    """Guard tests: _find_package_dir must never raise on a missing root."""
+
+    def test_returns_none_for_nonexistent_path(self):
+        """Core regression: a missing module root returns None, not FileNotFoundError."""
+        loader = ModuleLoader()
+        ghost = Path("/nonexistent/path/that/does/not/exist")
+
+        assert not ghost.exists(), "pre-condition: path must not exist"
+
+        result = loader._find_package_dir("provider-phantom", ghost)
+
+        assert result is None
+
+    def test_does_not_raise_file_not_found_error(self):
+        """Explicit anti-regression: the old behaviour was to propagate FileNotFoundError."""
+        loader = ModuleLoader()
+        ghost = Path("/nonexistent/amplifier-module-hooks-insight-blocks-ee85c27df4b5d3df")
+
+        try:
+            loader._find_package_dir("hook-insight-blocks", ghost)
+        except FileNotFoundError:
+            pytest.fail(
+                "_find_package_dir raised FileNotFoundError for a missing path; "
+                "the existence guard is missing or broken."
+            )
+
+    def test_hash_style_cache_path_returns_none(self, tmp_path):
+        """Paths that look like real cache dirs but don't exist also return None."""
+        loader = ModuleLoader()
+        # Mimic the actual path shape that triggered the original bug
+        cache_root = tmp_path / "cache"
+        missing_module = cache_root / "amplifier-module-hooks-insight-blocks-ee85c27df4b5d3df"
+        # Intentionally do NOT create missing_module on disk
+
+        result = loader._find_package_dir("hook-insight-blocks", missing_module)
+
+        assert result is None
+
+    def test_existing_empty_dir_returns_none(self, tmp_path):
+        """An existing but empty directory still returns None (no package inside)."""
+        loader = ModuleLoader()
+        empty_dir = tmp_path / "amplifier-module-empty"
+        empty_dir.mkdir()
+
+        result = loader._find_package_dir("provider-empty", empty_dir)
+
+        assert result is None
+
+    def test_existing_dir_with_package_returns_package(self, tmp_path):
+        """Happy-path: a correctly structured module directory returns the package dir."""
+        loader = ModuleLoader()
+        module_root = tmp_path / "amplifier-module-myprovider"
+        module_root.mkdir()
+        pkg_dir = module_root / "amplifier_module_myprovider"
+        pkg_dir.mkdir()
+        (pkg_dir / "__init__.py").write_text("# package\n")
+
+        result = loader._find_package_dir("myprovider", module_root)
+
+        assert result == pkg_dir
+
+
+class TestValidateModuleProducesClearError:
+    """
+    Integration: _validate_module must surface a ModuleValidationError
+    (not a raw FileNotFoundError) when the module root is missing.
+    """
+
+    @pytest.mark.asyncio
+    async def test_validate_module_raises_module_validation_error_not_file_not_found(self):
+        """
+        When the module root directory doesn't exist, _validate_module should
+        raise ModuleValidationError with an informative message — not the
+        opaque FileNotFoundError that was propagating before the guard was added.
+        """
+        loader = ModuleLoader()
+        ghost = Path("/nonexistent/amplifier-module-provider-ghost-aabbccdd")
+
+        with pytest.raises(ModuleValidationError) as exc_info:
+            await loader._validate_module("provider-ghost", ghost)
+
+        msg = str(exc_info.value)
+        assert "provider-ghost" in msg, f"Expected module id in error message, got: {msg!r}"
+        assert "no valid Python package" in msg, f"Expected clear reason in message, got: {msg!r}"
+
+    @pytest.mark.asyncio
+    async def test_validate_module_error_message_contains_path(self):
+        """The error message includes the missing path so operators can act on it."""
+        loader = ModuleLoader()
+        ghost = Path("/missing/amplifier-module-tool-calculator-deadbeef")
+
+        with pytest.raises(ModuleValidationError) as exc_info:
+            await loader._validate_module("tool-calculator", ghost)
+
+        assert str(ghost) in str(exc_info.value)