diff --git a/control-pipeline/services/decomposition_pass.py b/control-pipeline/services/decomposition_pass.py
index 5b9a75a..c4a44f1 100644
--- a/control-pipeline/services/decomposition_pass.py
+++ b/control-pipeline/services/decomposition_pass.py
@@ -3489,10 +3489,10 @@ class DecompositionPass:
                     source_ref=ctrl["source_ref"],
                 )
 
-            # Control IDs in custom_id for result mapping
-            ids_str = "+".join(c["control_id"] for c in batch)
+            # Numeric index as custom_id (max 64 chars, alphanumeric only)
+            batch_idx = i // batch_size
             requests.append({
-                "custom_id": f"p0a_{ids_str}",
+                "custom_id": f"p0a_b{batch_idx:05d}",
                 "params": {
                     "model": ANTHROPIC_MODEL,
                     "max_tokens": max(8192, len(batch) * 2000),