diff --git a/src/extraction/robust_extractor.py b/src/extraction/robust_extractor.py
index 9119ece..cf06704 100644
--- a/src/extraction/robust_extractor.py
+++ b/src/extraction/robust_extractor.py
@@ -92,7 +92,24 @@ class RobustSynthIDExtractor:
             self.load_codebook(codebook_path)
     
     def load_codebook(self, path: str) -> None:
-        """Load pre-extracted codebook."""
+        """Load pre-extracted codebook. Supports .pkl files and auto-discovers
+        .pkl when given a .npz path (common user mistake)."""
+        if path.endswith('.npz'):
+            pkl_candidates = [
+                os.path.join(os.path.dirname(path), 'codebook', 'robust_codebook.pkl'),
+                os.path.join(os.path.dirname(path), 'robust_codebook.pkl'),
+                path.replace('.npz', '.pkl'),
+            ]
+            for pkl_path in pkl_candidates:
+                if os.path.exists(pkl_path):
+                    with open(pkl_path, 'rb') as f:
+                        self.codebook = pickle.load(f)
+                    return
+            raise FileNotFoundError(
+                f"Cannot load .npz as pickle codebook. "
+                f"Provide a .pkl file instead, e.g.: "
+                f"--detector artifacts/codebook/robust_codebook.pkl"
+            )
         with open(path, 'rb') as f:
             self.codebook = pickle.load(f)
     
@@ -100,7 +117,7 @@ class RobustSynthIDExtractor:
         """Save extracted codebook."""
         os.makedirs(os.path.dirname(path) if os.path.dirname(path) else '.', exist_ok=True)
         with open(path, 'wb') as f:
-            pickle.dump(self.codebook, f)
+            pickle.dump(self.codebook, f, protocol=4)
     
     # ================================================================
     # DENOISING METHODS
diff --git a/src/extraction/synthid_codebook_extractor.py b/src/extraction/synthid_codebook_extractor.py
index ae190fa..d88f146 100644
--- a/src/extraction/synthid_codebook_extractor.py
+++ b/src/extraction/synthid_codebook_extractor.py
@@ -201,9 +201,9 @@ def extract_codebook(image_dir, output_path, max_images=250, size=512):
     # Save codebook
     os.makedirs(os.path.dirname(output_path) if os.path.dirname(output_path) else '.', exist_ok=True)
     
-    # Save as pickle (includes numpy arrays)
+    # Save as pickle (includes numpy arrays, protocol=4 for compatibility)
     with open(output_path, 'wb') as f:
-        pickle.dump(codebook, f)
+        pickle.dump(codebook, f, protocol=4)
     
     # Save metadata as JSON
     json_path = output_path.replace('.pkl', '_meta.json')