diff --git a/mcp_driver.py b/mcp_driver.py
index 1c6f621..8aab575 100644
--- a/mcp_driver.py
+++ b/mcp_driver.py
@@ -24,6 +24,10 @@ def shrink_media(source_path: str, output_dir: str, target_bytes: int = 2_000_00
 
     if not source.exists():
         return f"Error: Source file does not exist: {source}"
+    if not source.is_file():
+        return f"Error: Source path is not a file: {source}"
+    if target_bytes <= 0:
+        return "Error: target_bytes must be greater than 0."
 
     out_dir.mkdir(parents=True, exist_ok=True)
     root = source.parent
diff --git a/tests/test_mcp_driver.py b/tests/test_mcp_driver.py
index a0d7e31..6feba91 100644
--- a/tests/test_mcp_driver.py
+++ b/tests/test_mcp_driver.py
@@ -65,3 +65,21 @@ def test_shrink_media_handles_empty_result(self, mock_convert_file):
 
 if __name__ == '__main__':
     unittest.main()
+
+
+class MCPDriverValidationTests(unittest.TestCase):
+    """Trust-boundary input validation for the MCP shrink_media tool."""
+
+    def test_rejects_directory_source(self):
+        import tempfile
+        with tempfile.TemporaryDirectory() as temp_dir:
+            result = shrink_media(temp_dir, temp_dir + "/out")
+        self.assertIn("is not a file", result)
+
+    def test_rejects_nonpositive_target_bytes(self):
+        import tempfile
+        with tempfile.TemporaryDirectory() as temp_dir:
+            source = Path(temp_dir) / "s.wav"
+            source.touch()
+            result = shrink_media(str(source), str(Path(temp_dir) / "out"), 0)
+        self.assertIn("target_bytes must be greater than 0", result)