diff --git a/examples/speculative_decoding/scripts/ar_validate.py b/examples/speculative_decoding/scripts/ar_validate.py
index 5699c480b7b..d8c28af48d7 100644
--- a/examples/speculative_decoding/scripts/ar_validate.py
+++ b/examples/speculative_decoding/scripts/ar_validate.py
@@ -123,7 +123,24 @@ def main():
         accelerator.device,
     )
 
-    if results and accelerator.is_main_process:
+    # validate_ar() clamps to len(ds), so report what was actually attempted rather than the
+    # requested --num_samples, which can be larger than the dataset. A non-positive count means
+    # nothing ran at all -- distinct from "everything ran and failed", so say so separately.
+    attempted = min(args.num_samples, len(ds))
+    if attempted <= 0:
+        raise ValueError(
+            f"No samples to validate: --num_samples={args.num_samples} with a dataset of "
+            f"{len(ds)} prompts. Pass a positive --num_samples."
+        )
+
+    if not results:
+        raise RuntimeError(
+            f"AR validation produced no results: all {attempted} samples failed. "
+            "See the per-sample WARNING lines above for the underlying error. "
+            "Exiting non-zero so this is not mistaken for a successful validation."
+        )
+
+    if accelerator.is_main_process:
         all_ars = [ar for _, ar in results]
         avg_ar = sum(all_ars) / len(all_ars)
         print(f"\n==== AR Validation Results (osl={args.osl}, steps={args.steps}) ====")