diff --git a/evalharness/model/runner.py b/evalharness/model/runner.py
index 1e9ca83..c81df9f 100644
--- a/evalharness/model/runner.py
+++ b/evalharness/model/runner.py
@@ -317,7 +317,15 @@ async def generate_predictions(
                 _progress(progress, done_count, len(samples), t0, total_usage)
             return pred
 
-        messages = ([ChatMessage(role='user', content=assemble(sample))]
+        # assemble() tokenizes for the max_input_tokens truncation -- on
+        # long-context benches that is SECONDS of CPU per sample (2M-token
+        # docs), and running it inline FROZE the whole event loop: heartbeat,
+        # gate probes and every other request serialized behind one encode.
+        # Thread it: the loop stays live and encodes parallelize (the Rust
+        # fast tokenizer releases the GIL).
+        text = await asyncio.to_thread(assemble, sample) \
+            if isinstance(sample.input, str) else None
+        messages = ([ChatMessage(role='user', content=text)]
                     if isinstance(sample.input, str) else list(sample.input))
         if not system and extra_system[0] and isinstance(sample.input, str):
             # renderer-provided SYSTEM contract (es lcb expert-programmer)