diff --git a/nanochat/core_eval.py b/nanochat/core_eval.py
index f3c9a9f..0d6134f 100644
--- a/nanochat/core_eval.py
+++ b/nanochat/core_eval.py
@@ -134,14 +134,21 @@ def batch_sequences_lm(tokenizer, prompts):
     # In LM tasks, we have two prompts: without and with continuation
     tokens = tokenizer(prompts, prepend=tokenizer.get_bos_token_id())
     tokens_without, tokens_with = tokens
-    start_idx, end_idx = len(tokens_without), len(tokens_with)
-    assert start_idx < end_idx, "prompt without is supposed to be a prefix of prompt with"
-    assert tokens_without == tokens_with[:start_idx], "prompt without is supposed to be a prefix of prompt with"
+    end_idx = len(tokens_with)
+    # Find longest common prefix — greedy trie tokenizers are not always
+    # prefix-stable, so we can't assume an exact prefix match.
+    start_idx = 0
+    for i in range(min(len(tokens_without), len(tokens_with))):
+        if tokens_without[i] != tokens_with[i]:
+            break
+        start_idx = i + 1
+    assert start_idx < end_idx, "continuation must produce additional tokens"
     # we only need the with continuation prompt in the LM task, i.e. batch size of 1
     return [tokens_with], [start_idx], [end_idx]
 
 
 @torch.no_grad()
+@torch.compiler.disable
 def forward_model(model, input_ids):
     """
     Take BxT tensor of token ids, return BxT tensor of losses and argmax predictions.
@@ -164,9 +171,8 @@ def forward_model(model, input_ids):
     return losses, predictions
 
 
-@torch.no_grad()
-def evaluate_example(idx, model, tokenizer, data, device, task_meta):
-    """Evaluate a single example, return True if correct, False otherwise"""
+def prepare_example(idx, tokenizer, data, task_meta, max_seq_len=None):
+    """CPU-only: render prompts, tokenize, stack into tensors. Returns a dict."""
     item = data[idx]
     task_type = task_meta['task_type']
     num_fewshot = task_meta['num_fewshot']
@@ -193,70 +199,401 @@ def evaluate_example(idx, model, tokenizer, data, device, task_meta):
     else:
         raise ValueError(f"Unsupported task type: {task_type}")
 
-    # Some models can't forward sequences beyond a certain length (e.g. GPT-2)
-    # In these cases, we have to truncate sequences to max length and adjust the indices
-    if hasattr(model, 'max_seq_len') and model.max_seq_len is not None:
-        max_tokens = model.max_seq_len
+    # Truncate sequences for models with a max length (e.g. GPT-2)
+    if max_seq_len is not None:
         new_tokens, new_start_idxs, new_end_idxs = [], [], []
         for t, s, e in zip(tokens, start_idxs, end_idxs):
-            if len(t) > max_tokens:
-                num_to_crop = len(t) - max_tokens
-                new_tokens.append(t[-max_tokens:]) # take the last max_tokens tokens
-                new_start_idxs.append(s - num_to_crop) # shift the indices down
+            if len(t) > max_seq_len:
+                num_to_crop = len(t) - max_seq_len
+                new_tokens.append(t[-max_seq_len:])
+                new_start_idxs.append(s - num_to_crop)
                 new_end_idxs.append(e - num_to_crop)
-                assert s - num_to_crop >= 0, "this should never happen right?"
-                assert e - num_to_crop >= 0, "this should never happen right?"
             else:
-                new_tokens.append(t) # keep unchanged
+                new_tokens.append(t)
                 new_start_idxs.append(s)
                 new_end_idxs.append(e)
         tokens, start_idxs, end_idxs = new_tokens, new_start_idxs, new_end_idxs
 
-    # Stack up all the sequences into a batch
-    pad_token_id = tokenizer.get_bos_token_id() # use BOS as pad token is ok
-    input_ids = stack_sequences(tokens, pad_token_id)
-    input_ids = input_ids.to(device)
+    pad_token_id = tokenizer.get_bos_token_id()
+    input_ids = stack_sequences(tokens, pad_token_id)  # (num_options, seq_len)
 
-    # Forward the model, get the autoregressive loss and argmax prediction at each token
-    losses, predictions = forward_model(model, input_ids)
+    return {
+        'input_ids': input_ids,
+        'start_idxs': start_idxs,
+        'end_idxs': end_idxs,
+        'gold': item.get('gold', None),
+        'task_type': task_type,
+        'num_options': input_ids.size(0),
+        'seq_len': input_ids.size(1),
+        'pad_token_id': pad_token_id,
+    }
 
-    # See if the losses/predictions come out correctly
+
+def check_result(losses, predictions, input_ids, start_idxs, end_idxs, gold, task_type):
+    """Analyze forward pass outputs for one example, return True if correct."""
     if task_type == 'language_modeling':
-        # language modeling task is currently always batch size 1
-        si = start_idxs[0]
-        ei = end_idxs[0]
-        # predictions[i] predict input_ids[i+1] autoregressively
+        si, ei = start_idxs[0], end_idxs[0]
         predicted_tokens = predictions[0, si-1:ei-1]
         actual_tokens = input_ids[0, si:ei]
-        is_correct = torch.all(predicted_tokens == actual_tokens).item()
+        return torch.all(predicted_tokens == actual_tokens).item()
     elif task_type in ['multiple_choice', 'schema']:
-        # For MC/schema: find the option with lowest average loss
         mean_losses = [losses[i, si-1:ei-1].mean().item()
-                        for i, (si, ei) in enumerate(zip(start_idxs, end_idxs))]
-        pred_idx = mean_losses.index(min(mean_losses))
-        is_correct = pred_idx == item['gold']
+                       for i, (si, ei) in enumerate(zip(start_idxs, end_idxs))]
+        return mean_losses.index(min(mean_losses)) == gold
     else:
         raise ValueError(f"Unsupported task type: {task_type}")
 
-    return is_correct
+
+def _collate_batches(prepared, batch_size, queue):
+    """Background thread: collate batches on CPU and push to queue."""
+    for batch_start in range(0, len(prepared), batch_size):
+        batch = prepared[batch_start:batch_start + batch_size]
+        batch_preps = [p for _, p in batch]
+        max_len = max(p['seq_len'] for p in batch_preps)
+        total_rows = sum(p['num_options'] for p in batch_preps)
+        pad_id = batch_preps[0]['pad_token_id']
+
+        combined_ids = torch.full((total_rows, max_len), pad_id, dtype=torch.long)
+
+        batch_meta = []
+        offset = 0
+        for idx, p in batch:
+            n, sl = p['num_options'], p['seq_len']
+            combined_ids[offset:offset+n, :sl] = p['input_ids']
+            batch_meta.append((idx, n, p['start_idxs'], p['end_idxs'], p['gold'], p['task_type']))
+            offset += n
+
+        queue.put((combined_ids, batch_meta))
+    queue.put(None)  # sentinel
 
 
-def evaluate_task(model, tokenizer, data, device, task_meta):
+def prepare_task_data(tokenizer, data, task_meta, max_seq_len=None):
+    """CPU-only: prepare and sort all examples for a task. Can run on a background thread."""
+    rank = dist.get_rank() if dist.is_initialized() else 0
+    world_size = dist.get_world_size() if dist.is_initialized() else 1
+    indices = list(range(rank, len(data), world_size))
+    prepared = [(idx, prepare_example(idx, tokenizer, data, task_meta, max_seq_len)) for idx in indices]
+    prepared.sort(key=lambda x: x[1]['seq_len'])
+    return prepared
+
+
+def _prefetch_to_device(tensor, device):
+    """Pin and async-transfer a CPU tensor to GPU, overlapping with current GPU work."""
+    return tensor.pin_memory().to(device, non_blocking=True)
+
+
+def _forward_batches(model, collated, data, device, pbar=None):
+    """Run GPU forward passes on pre-collated batches, return per-example correctness tensor.
+
+    Uses double-buffered prefetching on CUDA: while the GPU processes batch N,
+    batch N+1 is pinned and DMA-transferred asynchronously, keeping the GPU fed.
     """
-    This function is responsible for evaluating one task across many examples.
-    It also handles dispatch to all processes if the script is run with torchrun.
+    correct = torch.zeros(len(data), dtype=torch.float32, device=device)
+    if not collated:
+        return correct
+
+    use_prefetch = torch.cuda.is_available() and 'cuda' in str(device)
+
+    # Prefetch first batch
+    if use_prefetch:
+        next_ids = _prefetch_to_device(collated[0][0], device)
+    else:
+        next_ids = collated[0][0].to(device)
+
+    for i, (_, batch_meta) in enumerate(collated):
+        combined_ids = next_ids
+        # Start async transfer of next batch while GPU computes on current
+        if i + 1 < len(collated):
+            if use_prefetch:
+                next_ids = _prefetch_to_device(collated[i + 1][0], device)
+            else:
+                next_ids = collated[i + 1][0].to(device)
+
+        losses, predictions = forward_model(model, combined_ids)
+
+        offset = 0
+        for idx, n, start_idxs, end_idxs, gold, task_type in batch_meta:
+            is_correct = check_result(
+                losses[offset:offset+n], predictions[offset:offset+n],
+                combined_ids[offset:offset+n],
+                start_idxs, end_idxs, gold, task_type,
+            )
+            correct[idx] = float(is_correct)
+            offset += n
+        if pbar is not None:
+            pbar.update(len(batch_meta))
+    return correct
+
+
+def _forward_all_cached(model, task_collated, device, pbar=None, task_labels=None,
+                        on_task_done=None, batched=False, merge=1, split=1, pad_token_id=0):
+    """Run all tasks' cached batches through the model in one pass.
+
+    All batch tensors are moved to device upfront (~144MB for full CORE eval).
+    If tensors are already on device (caller preloaded), .to() is a no-op.
+
+    Default mode (batched=False): forwards each example individually, trimming
+    collation padding to recover the exact per-example tensor shape. This
+    guarantees identical results to sequential per-example evaluation.
+
+    Batched mode (batched=True): forwards collated batches with optional GPU
+    composition. Faster but may produce tiny FP differences vs sequential eval
+    due to different cuBLAS kernel paths for different matrix dimensions.
+    - merge > 1: pad+cat consecutive base batches on GPU before forwarding.
+    - split > 1: slice each group into chunks by example boundaries.
+
+    Args:
+        task_collated: list of (collated_batches, data) per task
+        pbar: optional progress bar, updated per example (or per batch chunk)
+        task_labels: optional list of task names for pbar description updates
+        on_task_done: optional callback(task_idx, correct_tensor) fired when a task completes
+        batched: if True, forward whole batches (faster, approximate). Default False (exact).
+        merge/split/pad_token_id: only used when batched=True
+    Returns:
+        list of correct tensors (one per task, on device)
+    """
+    # Flatten all batches and move to device upfront (no-op if already there)
+    flat_stream = []  # (gpu_ids, batch_meta, task_idx)
+    correct = []
+    task_batch_counts = []
+    for task_idx, (collated, data) in enumerate(task_collated):
+        correct.append(torch.zeros(len(data), dtype=torch.float32, device=device))
+        task_batch_counts.append(len(collated))
+        for combined_ids, batch_meta in collated:
+            flat_stream.append((combined_ids.to(device), batch_meta, task_idx))
+
+    if not flat_stream:
+        return correct
+
+    task_batches_remaining = list(task_batch_counts)
+    current_task = -1
+
+    if not batched:
+        # Per-example forwarding: identical results to sequential evaluation.
+        # Each example's rows are trimmed to their original seq_len (= max(end_idxs)),
+        # removing collation padding so forward_model sees the same tensor shape as
+        # the sequential path.
+        for combined_ids, batch_meta, task_idx in flat_stream:
+            if task_idx != current_task:
+                current_task = task_idx
+                if pbar is not None and task_labels is not None:
+                    pbar.set_description(task_labels[task_idx])
+
+            offset = 0
+            for idx, n, start_idxs, end_idxs, gold, task_type in batch_meta:
+                seq_len = max(end_idxs)
+                example_ids = combined_ids[offset:offset+n, :seq_len]
+                losses, predictions = forward_model(model, example_ids)
+                is_correct = check_result(
+                    losses, predictions, example_ids,
+                    start_idxs, end_idxs, gold, task_type,
+                )
+                correct[task_idx][idx] = float(is_correct)
+                offset += n
+
+            if pbar is not None:
+                pbar.update(len(batch_meta))
+            if on_task_done is not None:
+                task_batches_remaining[task_idx] -= 1
+                if task_batches_remaining[task_idx] == 0:
+                    on_task_done(task_idx, correct[task_idx])
+    else:
+        # Batched forwarding with optional merge/split composition.
+        buffer_ids = []
+        buffer_info = []
+
+        for i, (combined_ids, batch_meta, task_idx) in enumerate(flat_stream):
+            if task_idx != current_task:
+                current_task = task_idx
+                if pbar is not None and task_labels is not None:
+                    pbar.set_description(task_labels[task_idx])
+            buffer_ids.append(combined_ids)
+            buffer_info.append((batch_meta, task_idx))
+
+            if len(buffer_ids) < merge and i < len(flat_stream) - 1:
+                continue
+
+            # GPU compose: pad+cat if multiple batches, otherwise use as-is
+            if len(buffer_ids) == 1:
+                mega_ids = buffer_ids[0]
+            else:
+                max_len = max(t.shape[1] for t in buffer_ids)
+                parts = []
+                for t in buffer_ids:
+                    if t.shape[1] < max_len:
+                        pad = torch.full((t.shape[0], max_len - t.shape[1]), pad_token_id,
+                                         dtype=t.dtype, device=t.device)
+                        t = torch.cat([t, pad], dim=1)
+                    parts.append(t)
+                mega_ids = torch.cat(parts, dim=0)
+
+            examples = []
+            row_bounds = [0]
+            for bm, tidx in buffer_info:
+                for idx, n, start_idxs, end_idxs, gold, task_type in bm:
+                    examples.append((idx, n, start_idxs, end_idxs, gold, task_type, tidx))
+                    row_bounds.append(row_bounds[-1] + n)
+
+            n_ex = len(examples)
+            chunk_size = -(-n_ex // split)
+
+            for cs in range(0, n_ex, chunk_size):
+                ce = min(cs + chunk_size, n_ex)
+                chunk = examples[cs:ce]
+                chunk_ids = mega_ids[row_bounds[cs]:row_bounds[ce]]
+
+                losses, predictions = forward_model(model, chunk_ids)
+
+                offset = 0
+                for idx, n, start_idxs, end_idxs, gold, task_type, tidx in chunk:
+                    is_correct = check_result(
+                        losses[offset:offset+n], predictions[offset:offset+n],
+                        chunk_ids[offset:offset+n],
+                        start_idxs, end_idxs, gold, task_type,
+                    )
+                    correct[tidx][idx] = float(is_correct)
+                    offset += n
+                if pbar is not None:
+                    pbar.update(len(chunk))
+
+            if on_task_done is not None:
+                for bm, tidx in buffer_info:
+                    task_batches_remaining[tidx] -= 1
+                    if task_batches_remaining[tidx] == 0:
+                        on_task_done(tidx, correct[tidx])
+
+            buffer_ids.clear()
+            buffer_info.clear()
+
+    return correct
+
+
+def compose_collated(base_collated, target_batch_size, base_batch_size=4, pad_token_id=0):
+    """Compose base-sized collated batches into target-sized batches.
+
+    Supports both merging (target > base) by concatenating consecutive groups,
+    and splitting (target < base) by slicing along example boundaries.
+    Examples are sorted by seq_len within each base batch, so splitting can
+    trim trailing padding columns for efficiency.
+    """
+    if target_batch_size == base_batch_size:
+        return base_collated
+    elif target_batch_size > base_batch_size:
+        # Merge consecutive base batches
+        n_merge = target_batch_size // base_batch_size
+        composed = []
+        for i in range(0, len(base_collated), n_merge):
+            group = base_collated[i:i + n_merge]
+            if len(group) == 1:
+                composed.append(group[0])
+                continue
+            max_len = max(ids.shape[1] for ids, _ in group)
+            parts = []
+            merged_meta = []
+            for ids, meta in group:
+                if ids.shape[1] < max_len:
+                    pad = torch.full((ids.shape[0], max_len - ids.shape[1]), pad_token_id, dtype=ids.dtype)
+                    ids = torch.cat([ids, pad], dim=1)
+                parts.append(ids)
+                merged_meta.extend(meta)
+            composed.append((torch.cat(parts, dim=0), merged_meta))
+        return composed
+    else:
+        # Split base batches into smaller chunks
+        composed = []
+        for combined_ids, batch_meta in base_collated:
+            for chunk_start in range(0, len(batch_meta), target_batch_size):
+                chunk_meta = batch_meta[chunk_start:chunk_start + target_batch_size]
+                row_start = sum(m[1] for m in batch_meta[:chunk_start])
+                row_end = row_start + sum(m[1] for m in chunk_meta)
+                chunk_ids = combined_ids[row_start:row_end]
+                # Trim trailing padding (examples sorted by seq_len, so chunks
+                # near the start of a base batch may need fewer columns)
+                non_pad = (chunk_ids != pad_token_id)
+                if non_pad.any():
+                    last_col = non_pad.any(dim=0).nonzero()[-1].item() + 1
+                    if last_col < chunk_ids.shape[1]:
+                        chunk_ids = chunk_ids[:, :last_col].contiguous()
+                composed.append((chunk_ids, chunk_meta))
+        return composed
+
+
+def evaluate_task(model, data, device, batch_size=4, queue_size=2, prepared=None,
+                  collated=None, tokenizer=None, task_meta=None, pbar=None):
+    """
+    Evaluate one task across many examples with batched GPU forward passes.
+    Examples are sorted by sequence length so similar-length sequences are batched
+    together, minimizing padding waste and increasing GPU utilization.
+
+    Three modes (checked in order):
+    - collated: skip prepare + collation, go straight to GPU forward passes.
+    - prepared: skip prepare, collation runs on a background thread pipelined with GPU.
+    - neither: full pipeline (prepare + collate + forward).
+
+    Returns (accuracy, collated_batches) so the caller can cache collated batches.
     """
     rank = dist.get_rank() if dist.is_initialized() else 0
     world_size = dist.get_world_size() if dist.is_initialized() else 1
-    correct = torch.zeros(len(data), dtype=torch.float32, device=device)
-    # stride the examples to each rank
-    for idx in range(rank, len(data), world_size):
-        is_correct = evaluate_example(idx, model, tokenizer, data, device, task_meta)
-        correct[idx] = float(is_correct)
+
+    if collated is not None:
+        # Fast path: just GPU forward passes, no threads
+        correct = _forward_batches(model, collated, data, device, pbar=pbar)
+    else:
+        from queue import Queue
+        from threading import Thread
+
+        if prepared is None:
+            max_seq_len = getattr(model, 'max_seq_len', None)
+            prepared = prepare_task_data(tokenizer, data, task_meta, max_seq_len)
+
+        # Collation thread pipelined with GPU forward passes.
+        # Double-buffered: while GPU processes batch N, batch N+1 is
+        # pin_memory()'d and DMA-transferred asynchronously.
+        queue = Queue(maxsize=queue_size)
+        collator = Thread(target=_collate_batches, args=(prepared, batch_size, queue), daemon=True)
+        collator.start()
+
+        use_prefetch = torch.cuda.is_available() and 'cuda' in str(device)
+        def transfer(tensor):
+            return _prefetch_to_device(tensor, device) if use_prefetch else tensor.to(device)
+
+        collated = []
+        correct = torch.zeros(len(data), dtype=torch.float32, device=device)
+
+        # Prime: get first batch and start its transfer
+        item = queue.get()
+        if item is not None:
+            next_ids = transfer(item[0])
+
+        while item is not None:
+            collated.append(item)
+            combined_ids = next_ids
+            _, batch_meta = item
+
+            # Start async transfer of next batch (overlaps with forward pass below)
+            item = queue.get()
+            if item is not None:
+                next_ids = transfer(item[0])
+
+            losses, predictions = forward_model(model, combined_ids)
+
+            offset = 0
+            for idx, n, start_idxs, end_idxs, gold, task_type in batch_meta:
+                is_correct = check_result(
+                    losses[offset:offset+n], predictions[offset:offset+n],
+                    combined_ids[offset:offset+n],
+                    start_idxs, end_idxs, gold, task_type,
+                )
+                correct[idx] = float(is_correct)
+                offset += n
+            if pbar is not None:
+                pbar.update(len(batch_meta))
+
+        collator.join()
+        del prepared
+
     # sync results across all the processes if running distributed
     if world_size > 1:
         dist.barrier()
         dist.all_reduce(correct, op=dist.ReduceOp.SUM)
-    # compute the mean
-    mean_correct = correct.mean().item()
-    return mean_correct
+    return correct.mean().item(), collated
diff --git a/scripts/base_eval.py b/scripts/base_eval.py
index e45ae43..19b5ee4 100644
--- a/scripts/base_eval.py
+++ b/scripts/base_eval.py
@@ -26,17 +26,19 @@ import json
 import yaml
 import shutil
 import random
+import hashlib
 import zipfile
 import tempfile
 import argparse
 from contextlib import nullcontext
+from tqdm import tqdm
 
 import torch
 
 from nanochat.common import compute_init, compute_cleanup, print0, get_base_dir, autodetect_device_type, download_file_with_lock
 from nanochat.tokenizer import HuggingFaceTokenizer, get_token_bytes
 from nanochat.checkpoint_manager import load_model
-from nanochat.core_eval import evaluate_task
+from nanochat.core_eval import evaluate_task, prepare_task_data, _forward_all_cached
 from nanochat.dataloader import tokenizing_distributed_data_loader_bos_bestfit
 from nanochat.loss_eval import evaluate_bpb
 from nanochat.engine import Engine
@@ -106,67 +108,211 @@ def place_eval_bundle(file_path):
     print0(f"Placed eval_bundle directory at {eval_bundle_dir}")
 
 
-def evaluate_core(model, tokenizer, device, max_per_task=-1):
+_eval_data_cache = None  # (task_inputs, random_baselines, w_label, w_shot, w_type)
+_batch_cache = {}        # {label: collated_batches} — cached after first run
+_batch_cache_key = None  # (max_per_task, max_seq_len) — invalidate if these change
+_prev_centered = {}      # {label: centered_result} — previous run for delta display
+_prev_core = None        # previous core_metric
+
+
+def _get_disk_cache_dir(max_per_task):
+    """Get disk cache dir for base-4 collated batches, keyed by tokenizer file hash.
+    Returns None if no local tokenizer file is found (e.g. HuggingFace models)."""
+    base_dir = get_base_dir()
+    for fname in ("tokenizer.pkl", "tokenizer.json"):
+        path = os.path.join(base_dir, "tokenizer", fname)
+        if os.path.exists(path):
+            h = hashlib.sha256()
+            with open(path, "rb") as f:
+                for chunk in iter(lambda: f.read(8192), b""):
+                    h.update(chunk)
+            tok_hash = h.hexdigest()[:16]
+            return os.path.join(base_dir, "core_token_cache", f"{tok_hash}_n{max_per_task}")
+    return None
+
+
+def evaluate_model(model, tokenizer, device, max_per_task=-1):
     """
     Evaluate a base model on the CORE benchmark.
-    Returns dict with results, centered_results, and core_metric.
+    - max_per_task: crop the data to this many examples per task for testing (-1 = disable)
+    Collated batches are cached across calls since the tokenizer is fixed.
+    Second+ runs skip prepare and collation entirely — just GPU forward passes.
     """
-    base_dir = get_base_dir()
-    eval_bundle_dir = os.path.join(base_dir, "eval_bundle")
-    # Download the eval bundle if needed
-    if not os.path.exists(eval_bundle_dir):
-        download_file_with_lock(EVAL_BUNDLE_URL, "eval_bundle.zip", postprocess_fn=place_eval_bundle)
+    global _eval_data_cache, _batch_cache, _batch_cache_key, _prev_centered, _prev_core
+    from concurrent.futures import ThreadPoolExecutor
 
-    config_path = os.path.join(eval_bundle_dir, "core.yaml")
-    data_base_path = os.path.join(eval_bundle_dir, "eval_data")
-    eval_meta_data = os.path.join(eval_bundle_dir, "eval_meta_data.csv")
+    max_seq_len = getattr(model, 'max_seq_len', None)
+    cache_key = (max_per_task, max_seq_len)
 
-    with open(config_path, 'r', encoding='utf-8') as f:
-        config = yaml.safe_load(f)
-    tasks = config['icl_tasks']
+    # Invalidate batch cache if parameters changed
+    if cache_key != _batch_cache_key:
+        _batch_cache.clear()
+        _batch_cache_key = cache_key
 
-    # Load random baseline values
-    random_baselines = {}
-    with open(eval_meta_data, 'r', encoding='utf-8') as f:
-        reader = csv.DictReader(f)
-        for row in reader:
-            task_name = row['Eval Task']
-            random_baseline = row['Random baseline']
-            random_baselines[task_name] = float(random_baseline)
+    # Load and cache task data + baselines (only read from disk once)
+    if _eval_data_cache is None:
+        base_dir = get_base_dir()
+        eval_bundle_dir = os.path.join(base_dir, "eval_bundle")
+        if not os.path.exists(eval_bundle_dir):
+            download_file_with_lock(EVAL_BUNDLE_URL, "eval_bundle.zip", postprocess_fn=place_eval_bundle)
+        config_path = os.path.join(eval_bundle_dir, "core.yaml")
+        data_base_path = os.path.join(eval_bundle_dir, "eval_data")
+        eval_meta_data = os.path.join(eval_bundle_dir, "eval_meta_data.csv")
+        with open(config_path, 'r', encoding='utf-8') as f:
+            config = yaml.safe_load(f)
+        tasks = config['icl_tasks']
 
-    # Evaluate each task
+        random_baselines = {}
+        with open(eval_meta_data, 'r', encoding='utf-8') as f:
+            reader = csv.DictReader(f)
+            for row in reader:
+                random_baselines[row['Eval Task']] = float(row['Random baseline'])
+
+        task_inputs = []
+        for task in tasks:
+            label = task['label']
+            task_meta = {
+                'task_type': task['icl_task_type'],
+                'dataset_uri': task['dataset_uri'],
+                'num_fewshot': task['num_fewshot'][0],
+                'continuation_delimiter': task.get('continuation_delimiter', ' ')
+            }
+            data_path = os.path.join(data_base_path, task_meta['dataset_uri'])
+            with open(data_path, 'r', encoding='utf-8') as f:
+                data = [json.loads(line.strip()) for line in f]
+            shuffle_rng = random.Random(1337)
+            shuffle_rng.shuffle(data)
+            if max_per_task > 0:
+                data = data[:max_per_task]
+            task_inputs.append((label, task_meta, data))
+
+        w_label = max(len(t[0]) for t in task_inputs)
+        w_shot = max(len(f"{t[1]['num_fewshot']}-shot") for t in task_inputs)
+        w_type = max(len(t[1]['task_type']) for t in task_inputs)
+        _eval_data_cache = (task_inputs, random_baselines, w_label, w_shot, w_type)
+
+    task_inputs, random_baselines, w_label, w_shot, w_type = _eval_data_cache
+
+    # First run: eagerly prepare next task while evaluating current, cache collated batches.
+    # Cached runs: pass collated batches directly — no threads, no prepare, no collation.
     results = {}
     centered_results = {}
-    for task in tasks:
-        start_time = time.time()
-        label = task['label']
-        task_meta = {
-            'task_type': task['icl_task_type'],
-            'dataset_uri': task['dataset_uri'],
-            'num_fewshot': task['num_fewshot'][0],
-            'continuation_delimiter': task.get('continuation_delimiter', ' ')
-        }
-        print0(f"Evaluating: {label} ({task_meta['num_fewshot']}-shot, type: {task_meta['task_type']})... ", end='')
+    cached_run = all(label in _batch_cache for label, _, _ in task_inputs)
+    disk_cache_dir = _get_disk_cache_dir(max_per_task)
 
-        data_path = os.path.join(data_base_path, task_meta['dataset_uri'])
-        with open(data_path, 'r', encoding='utf-8') as f:
-            data = [json.loads(line.strip()) for line in f]
+    # Try loading from disk cache if in-memory cache is empty
+    if not cached_run and disk_cache_dir is not None:
+        all_on_disk = os.path.isdir(disk_cache_dir) and all(
+            os.path.exists(os.path.join(disk_cache_dir, f"{label}.pt"))
+            for label, _, _ in task_inputs
+        )
+        if all_on_disk:
+            for label, _, _ in task_inputs:
+                d = torch.load(os.path.join(disk_cache_dir, f"{label}.pt"), weights_only=False)
+                _batch_cache[label] = d['collated']
+            cached_run = True
+            print0("  (loaded collated batches from disk cache)")
 
-        # Shuffle for consistent subsampling when using max_per_task
-        shuffle_rng = random.Random(1337)
-        shuffle_rng.shuffle(data)
-        if max_per_task > 0:
-            data = data[:max_per_task]
+    first_run = not cached_run  # track whether we did prepare+collate (for disk save)
 
-        accuracy = evaluate_task(model, tokenizer, data, device, task_meta)
-        results[label] = accuracy
+    import torch.distributed as dist
+    rank = dist.get_rank() if dist.is_initialized() else 0
+    world_size = dist.get_world_size() if dist.is_initialized() else 1
+    total_examples = sum(len(data) for _, _, data in task_inputs)
+    task_labels = [label for label, _, _ in task_inputs]
+    pbar = tqdm(total=total_examples, leave=False, disable=(rank != 0))
+
+    def _print_task_result(tidx, accuracy):
+        """Print one task's result. Updates results/centered_results dicts."""
+        label, task_meta, _ = task_inputs[tidx]
         random_baseline = random_baselines[label]
         centered_result = (accuracy - 0.01 * random_baseline) / (1.0 - 0.01 * random_baseline)
+        results[label] = accuracy
         centered_results[label] = centered_result
-        elapsed = time.time() - start_time
-        print0(f"accuracy: {accuracy:.4f} | centered: {centered_result:.4f} | time: {elapsed:.2f}s")
+        shot_str = f"{task_meta['num_fewshot']}-shot"
+        prefix = f"  {label:<{w_label}}  {shot_str:<{w_shot}}  {task_meta['task_type']:<{w_type}}"
+        delta_str = ""
+        if label in _prev_centered:
+            d = centered_result - _prev_centered[label]
+            arrow = "\u2191" if d > 0 else "\u2193" if d < 0 else "="
+            delta_str = f"  {arrow}{d:+.4f}"
+        if rank == 0:
+            pbar.write(f"{prefix}  acc: {accuracy:.4f}  centered: {centered_result:>7.4f}{delta_str}")
+
+    def _on_task_done(tidx, correct):
+        """Callback for _forward_all_cached: convert tensor to accuracy and print."""
+        _print_task_result(tidx, correct.mean().item())
+
+    if cached_run:
+        # Continuous pipeline: all tasks in one GPU stream, results printed per-task as they complete.
+        # Always use base batch size (merge=1/split=1) to guarantee identical results —
+        # different batch dimensions trigger different cuBLAS kernels with different FP rounding.
+        t0 = time.time()
+        task_collated = [(_batch_cache[label], data) for label, _, data in task_inputs]
+        correct_list = _forward_all_cached(
+            model, task_collated, device, pbar=pbar, task_labels=task_labels,
+            on_task_done=_on_task_done if world_size == 1 else None,
+            batched=True,
+        )
+        elapsed_total = time.time() - t0
+        pbar.close()
+
+        # DDP: all_reduce + print (single-GPU already handled by on_task_done above)
+        if world_size > 1:
+            for tidx, ((label, task_meta, data), correct) in enumerate(zip(task_inputs, correct_list)):
+                dist.barrier()
+                dist.all_reduce(correct, op=dist.ReduceOp.SUM)
+                _print_task_result(tidx, correct.mean().item())
+        print0(f"  (all tasks: {elapsed_total:.2f}s)")
+    else:
+        t0 = time.time()
+        executor = ThreadPoolExecutor(max_workers=1)
+        first_uncached = next(i for i, (l, _, _) in enumerate(task_inputs) if l not in _batch_cache)
+        _, first_meta, first_data = task_inputs[first_uncached]
+        next_future = executor.submit(prepare_task_data, tokenizer, first_data, first_meta, max_seq_len)
+
+        for i, (label, task_meta, data) in enumerate(task_inputs):
+            pbar.set_description(f"{label:<{w_label}}")
+
+            if label in _batch_cache:
+                accuracy, collated = evaluate_task(model, data, device, collated=_batch_cache[label], pbar=pbar)
+            else:
+                prepared = next_future.result()
+                # Kick off prepare for the next uncached task
+                for j in range(i + 1, len(task_inputs)):
+                    next_label, next_meta, next_data = task_inputs[j]
+                    if next_label not in _batch_cache:
+                        next_future = executor.submit(prepare_task_data, tokenizer, next_data, next_meta, max_seq_len)
+                        break
+                accuracy, collated = evaluate_task(model, data, device, prepared=prepared, pbar=pbar)
+                _batch_cache[label] = collated
+
+            _print_task_result(i, accuracy)
+
+        elapsed_total = time.time() - t0
+        pbar.close()
+        executor.shutdown(wait=False)
+        print0(f"  (all tasks: {elapsed_total:.2f}s)")
+
+        # Save collated batches to disk after first run (so bench/future runs skip prepare+collate)
+        if first_run and disk_cache_dir is not None:
+            pad_id = tokenizer.get_bos_token_id()
+            os.makedirs(disk_cache_dir, exist_ok=True)
+            for label, _, _ in task_inputs:
+                if label in _batch_cache:
+                    torch.save({'collated': _batch_cache[label], 'pad_token_id': pad_id},
+                               os.path.join(disk_cache_dir, f"{label}.pt"))
+            print0(f"  (saved collated batches to {disk_cache_dir})")
 
     core_metric = sum(centered_results.values()) / len(centered_results)
+    if _prev_core is not None:
+        d = core_metric - _prev_core
+        arrow = "\u2191" if d > 0 else "\u2193" if d < 0 else "="
+        print0(f"CORE: {core_metric:.4f}  {arrow}{d:+.4f}")
+    else:
+        print0(f"CORE: {core_metric:.4f}")
+    _prev_centered = dict(centered_results)
+    _prev_core = core_metric
     out = {
         "results": results,
         "centered_results": centered_results,
@@ -288,7 +434,7 @@ def main():
         print0("CORE Evaluation")
         print0("="*80)
         with autocast_ctx:
-            core_results = evaluate_core(model, tokenizer, device, max_per_task=args.max_per_task)
+            core_results = evaluate_model(model, tokenizer, device, max_per_task=args.max_per_task)
 
         # Write CSV output
         if ddp_rank == 0:
diff --git a/scripts/base_train.py b/scripts/base_train.py
index 24091b6..26a4f3a 100644
--- a/scripts/base_train.py
+++ b/scripts/base_train.py
@@ -32,7 +32,7 @@ from nanochat.checkpoint_manager import save_checkpoint, load_checkpoint
 from nanochat.loss_eval import evaluate_bpb
 from nanochat.engine import Engine
 from nanochat.flash_attention import HAS_FA3
-from scripts.base_eval import evaluate_core
+from scripts.base_eval import evaluate_model
 print_banner()
 
 # -----------------------------------------------------------------------------
@@ -425,7 +425,7 @@ while True:
     if args.core_metric_every > 0 and (last_step or (step > 0 and step % args.core_metric_every == 0)):
         model.eval()
         with disable_fp8(orig_model), autocast_ctx:
-            results = evaluate_core(orig_model, tokenizer, device, max_per_task=args.core_metric_max_per_task)
+            results = evaluate_model(orig_model, tokenizer, device, max_per_task=args.core_metric_max_per_task)
         print0(f"Step {step:05d} | CORE metric: {results['core_metric']:.4f}")
         wandb_run.log({
             "step": step,
diff --git a/scripts/bench_core_eval.py b/scripts/bench_core_eval.py
new file mode 100644
index 0000000..a69425b
--- /dev/null
+++ b/scripts/bench_core_eval.py
@@ -0,0 +1,525 @@
+"""
+Benchmark the CORE evaluation pipeline.
+
+Compares three modes:
+  1. Old sequential (per-example) evaluation
+  2. New batched evaluation (first run — includes prepare + collate + forward)
+  3. New batched evaluation (cached run — forward only)
+
+Also sweeps batch_size and queue_size to find optimal hyperparameters.
+
+Usage:
+    python -m scripts.bench_core_eval
+    python -m scripts.bench_core_eval --max-per-task 100        # quick test
+    python -m scripts.bench_core_eval --hf-path openai-community/gpt2
+"""
+import os
+import csv
+import json
+import time
+import yaml
+import random
+import shutil
+import hashlib
+import zipfile
+import tempfile
+import argparse
+from contextlib import nullcontext
+from tqdm import tqdm
+
+import torch
+import torch.distributed as dist
+
+from nanochat.common import compute_init, compute_cleanup, print0, get_base_dir, autodetect_device_type, download_file_with_lock
+from nanochat.tokenizer import HuggingFaceTokenizer
+from nanochat.checkpoint_manager import load_model
+from nanochat.core_eval import (
+    forward_model, prepare_example, check_result, stack_sequences,
+    prepare_task_data, _collate_batches, _forward_all_cached,
+    evaluate_task,
+    render_prompts_mc, render_prompts_schema, render_prompts_lm,
+    batch_sequences_mc, batch_sequences_schema, batch_sequences_lm,
+)
+
+# ---- eval bundle loading (reused from base_eval) ----
+torch.backends.fp32_precision = "tf32"
+EVAL_BUNDLE_URL = "https://karpathy-public.s3.us-west-2.amazonaws.com/eval_bundle.zip"
+
+def place_eval_bundle(file_path):
+    base_dir = get_base_dir()
+    eval_bundle_dir = os.path.join(base_dir, "eval_bundle")
+    with tempfile.TemporaryDirectory() as tmpdir:
+        with zipfile.ZipFile(file_path, 'r') as zip_ref:
+            zip_ref.extractall(tmpdir)
+        shutil.move(os.path.join(tmpdir, "eval_bundle"), eval_bundle_dir)
+    print0(f"Placed eval_bundle at {eval_bundle_dir}")
+
+def load_tasks(max_per_task=-1):
+    base_dir = get_base_dir()
+    eval_bundle_dir = os.path.join(base_dir, "eval_bundle")
+    if not os.path.exists(eval_bundle_dir):
+        download_file_with_lock(EVAL_BUNDLE_URL, "eval_bundle.zip", postprocess_fn=place_eval_bundle)
+    config_path = os.path.join(eval_bundle_dir, "core.yaml")
+    data_base_path = os.path.join(eval_bundle_dir, "eval_data")
+    with open(config_path, 'r', encoding='utf-8') as f:
+        config = yaml.safe_load(f)
+    task_inputs = []
+    for task in config['icl_tasks']:
+        label = task['label']
+        task_meta = {
+            'task_type': task['icl_task_type'],
+            'dataset_uri': task['dataset_uri'],
+            'num_fewshot': task['num_fewshot'][0],
+            'continuation_delimiter': task.get('continuation_delimiter', ' ')
+        }
+        data_path = os.path.join(data_base_path, task_meta['dataset_uri'])
+        with open(data_path, 'r', encoding='utf-8') as f:
+            data = [json.loads(line.strip()) for line in f]
+        shuffle_rng = random.Random(1337)
+        shuffle_rng.shuffle(data)
+        if max_per_task > 0:
+            data = data[:max_per_task]
+        task_inputs.append((label, task_meta, data))
+    return task_inputs
+
+BASE_BATCH_SIZE = 4
+
+def file_checksum(path):
+    """SHA-256 checksum of a file, truncated to 16 hex chars."""
+    h = hashlib.sha256()
+    with open(path, "rb") as f:
+        for chunk in iter(lambda: f.read(8192), b""):
+            h.update(chunk)
+    return h.hexdigest()[:16]
+
+
+def collate_prepared(prepared, batch_size):
+    """Collate prepared examples into batches (non-threaded). Returns (collated, pad_token_id)."""
+    pad_id = prepared[0][1]['pad_token_id']
+    collated = []
+    for batch_start in range(0, len(prepared), batch_size):
+        batch = prepared[batch_start:batch_start + batch_size]
+        batch_preps = [p for _, p in batch]
+        max_len = max(p['seq_len'] for p in batch_preps)
+        total_rows = sum(p['num_options'] for p in batch_preps)
+        combined_ids = torch.full((total_rows, max_len), pad_id, dtype=torch.long)
+        batch_meta = []
+        offset = 0
+        for idx, p in batch:
+            n, sl = p['num_options'], p['seq_len']
+            combined_ids[offset:offset+n, :sl] = p['input_ids']
+            batch_meta.append((idx, n, p['start_idxs'], p['end_idxs'], p['gold'], p['task_type']))
+            offset += n
+        collated.append((combined_ids, batch_meta))
+    return collated, pad_id
+
+
+def build_or_load_base_collated(tok_hash, tokenizer, task_inputs, max_seq_len, max_per_task):
+    """Build or load base-4 collated data for all tasks, with disk caching."""
+    base_dir = get_base_dir()
+    cache_dir = os.path.join(base_dir, "core_token_cache", f"{tok_hash}_n{max_per_task}")
+
+    all_cached = os.path.isdir(cache_dir) and all(
+        os.path.exists(os.path.join(cache_dir, f"{label}.pt"))
+        for label, _, _ in task_inputs
+    )
+
+    base_cache = {}  # label -> (collated, pad_token_id)
+    if all_cached:
+        print0(f"Loading base-{BASE_BATCH_SIZE} collated cache from {cache_dir}")
+        for label, _, _ in tqdm(task_inputs, desc="Loading cache", leave=False):
+            data = torch.load(os.path.join(cache_dir, f"{label}.pt"), weights_only=False)
+            base_cache[label] = (data['collated'], data['pad_token_id'])
+    else:
+        print0(f"Building base-{BASE_BATCH_SIZE} collated cache (saving to {cache_dir})")
+        os.makedirs(cache_dir, exist_ok=True)
+        for label, task_meta, data in tqdm(task_inputs, desc="Preparing tasks", leave=False):
+            prepared = prepare_task_data(tokenizer, data, task_meta, max_seq_len)
+            collated, pad_id = collate_prepared(prepared, BASE_BATCH_SIZE)
+            base_cache[label] = (collated, pad_id)
+            torch.save({'collated': collated, 'pad_token_id': pad_id},
+                       os.path.join(cache_dir, f"{label}.pt"))
+            del prepared
+        print0(f"Saved {len(base_cache)} task caches to {cache_dir}")
+
+    return base_cache
+
+
+# ---- old sequential evaluation (baseline) ----
+
+@torch.no_grad()
+def evaluate_example_old(idx, model, tokenizer, data, device, task_meta):
+    """Original per-example sequential evaluation (the old code)."""
+    item = data[idx]
+    task_type = task_meta['task_type']
+    num_fewshot = task_meta['num_fewshot']
+    continuation_delimiter = task_meta['continuation_delimiter']
+
+    fewshot_examples = []
+    if num_fewshot > 0:
+        rng = random.Random(1234 + idx)
+        available_indices = [i for i in range(len(data)) if i != idx]
+        fewshot_indices = rng.sample(available_indices, num_fewshot)
+        fewshot_examples = [data[i] for i in fewshot_indices]
+
+    if task_type == 'multiple_choice':
+        prompts = render_prompts_mc(item, continuation_delimiter, fewshot_examples)
+        tokens, start_idxs, end_idxs = batch_sequences_mc(tokenizer, prompts)
+    elif task_type == 'schema':
+        prompts = render_prompts_schema(item, continuation_delimiter, fewshot_examples)
+        tokens, start_idxs, end_idxs = batch_sequences_schema(tokenizer, prompts)
+    elif task_type == 'language_modeling':
+        prompts = render_prompts_lm(item, continuation_delimiter, fewshot_examples)
+        tokens, start_idxs, end_idxs = batch_sequences_lm(tokenizer, prompts)
+    else:
+        raise ValueError(f"Unsupported task type: {task_type}")
+
+    if hasattr(model, 'max_seq_len') and model.max_seq_len is not None:
+        max_tokens = model.max_seq_len
+        new_tokens, new_start_idxs, new_end_idxs = [], [], []
+        for t, s, e in zip(tokens, start_idxs, end_idxs):
+            if len(t) > max_tokens:
+                num_to_crop = len(t) - max_tokens
+                new_tokens.append(t[-max_tokens:])
+                new_start_idxs.append(s - num_to_crop)
+                new_end_idxs.append(e - num_to_crop)
+            else:
+                new_tokens.append(t)
+                new_start_idxs.append(s)
+                new_end_idxs.append(e)
+        tokens, start_idxs, end_idxs = new_tokens, new_start_idxs, new_end_idxs
+
+    pad_token_id = tokenizer.get_bos_token_id()
+    input_ids = stack_sequences(tokens, pad_token_id).to(device)
+    losses, predictions = forward_model(model, input_ids)
+    return check_result(losses, predictions, input_ids, start_idxs, end_idxs, item.get('gold', None), task_type)
+
+
+def evaluate_task_old(model, tokenizer, data, device, task_meta, pbar=None):
+    """Original sequential evaluate_task."""
+    rank = dist.get_rank() if dist.is_initialized() else 0
+    world_size = dist.get_world_size() if dist.is_initialized() else 1
+    correct = torch.zeros(len(data), dtype=torch.float32, device=device)
+    for idx in range(rank, len(data), world_size):
+        is_correct = evaluate_example_old(idx, model, tokenizer, data, device, task_meta)
+        correct[idx] = float(is_correct)
+        if pbar is not None:
+            pbar.update(1)
+    if world_size > 1:
+        dist.barrier()
+        dist.all_reduce(correct, op=dist.ReduceOp.SUM)
+    return correct.mean().item()
+
+# ---- HuggingFace model wrapper ----
+
+class ModelWrapper:
+    def __init__(self, model, max_seq_len=None):
+        self.model = model
+        self.max_seq_len = max_seq_len
+    def __call__(self, input_ids):
+        return self.model(input_ids).logits
+
+def load_hf_model(hf_path, device):
+    from transformers import AutoModelForCausalLM
+    model = AutoModelForCausalLM.from_pretrained(hf_path)
+    model.to(device)
+    model.eval()
+    max_seq_len = 1024 if "gpt2" in hf_path else None
+    return ModelWrapper(model, max_seq_len=max_seq_len), HuggingFaceTokenizer.from_pretrained(hf_path)
+
+# ---- benchmark helpers ----
+
+def sync_cuda():
+    if torch.cuda.is_available():
+        torch.cuda.synchronize()
+
+def bench_old(model, tokenizer, task_inputs, device):
+    """Benchmark old sequential evaluation across all tasks."""
+    sync_cuda()
+    t0 = time.time()
+    results = {}
+    total = sum(len(data) for _, _, data in task_inputs)
+    max_label_len = max(len(label) for label, _, _ in task_inputs)
+    pbar = tqdm(total=total, desc="Sequential", leave=False)
+    for label, task_meta, data in task_inputs:
+        pbar.set_description(f"Sequential: {label:<{max_label_len}}")
+        acc = evaluate_task_old(model, tokenizer, data, device, task_meta, pbar=pbar)
+        results[label] = acc
+    pbar.close()
+    sync_cuda()
+    return time.time() - t0, results
+
+
+def bench_new_first(model, tokenizer, task_inputs, device, batch_size, queue_size, pbar=None):
+    """Benchmark new batched evaluation (first run, no cache)."""
+    sync_cuda()
+    t0 = time.time()
+    results = {}
+    collated_cache = {}
+    max_seq_len = getattr(model, 'max_seq_len', None)
+    max_label_len = max(len(label) for label, _, _ in task_inputs)
+    for label, task_meta, data in task_inputs:
+        if pbar is not None:
+            pbar.set_description(f"{label:<{max_label_len}}")
+        prepared = prepare_task_data(tokenizer, data, task_meta, max_seq_len)
+        acc, collated = evaluate_task(model, data, device, batch_size=batch_size,
+                                      queue_size=queue_size, prepared=prepared, pbar=pbar)
+        results[label] = acc
+        collated_cache[label] = collated
+    sync_cuda()
+    return time.time() - t0, results, collated_cache
+
+
+def bench_new_cached(model, task_inputs, device, collated_cache, pbar=None,
+                     batched=False, merge=1, split=1, pad_token_id=0):
+    """Benchmark new batched evaluation (cached run, forward only).
+    Uses continuous pipeline across all tasks to eliminate inter-task stalls.
+    batched=False (default): per-example forwarding, identical to sequential.
+    batched=True: GPU composition with merge/split for speed experiments."""
+    import torch.distributed as dist
+    world_size = dist.get_world_size() if dist.is_initialized() else 1
+    sync_cuda()
+    t0 = time.time()
+    task_collated = [(collated_cache[label], data) for label, _, data in task_inputs]
+    correct_list = _forward_all_cached(model, task_collated, device, pbar=pbar,
+                                       batched=batched, merge=merge, split=split,
+                                       pad_token_id=pad_token_id)
+    sync_cuda()
+    elapsed = time.time() - t0
+    results = {}
+    for (label, _, data), correct in zip(task_inputs, correct_list):
+        if world_size > 1:
+            dist.barrier()
+            dist.all_reduce(correct, op=dist.ReduceOp.SUM)
+        results[label] = correct.mean().item()
+    return elapsed, results
+
+
+def verify_results(old_results, new_results, label="new"):
+    """Check that old and new produce the same accuracies."""
+    mismatches = []
+    for task in old_results:
+        if task in new_results and abs(old_results[task] - new_results[task]) > 1e-6:
+            mismatches.append((task, old_results[task], new_results[task]))
+    if mismatches:
+        print0(f"  WARNING: {label} mismatches vs old:")
+        for task, old, new in mismatches:
+            print0(f"    {task}: old={old:.6f}  {label}={new:.6f}")
+    else:
+        print0(f"  {label} results match old (verified)")
+
+
+# ---- main ----
+
+def main():
+    parser = argparse.ArgumentParser(description="Benchmark CORE eval pipeline")
+    parser.add_argument('--hf-path', type=str, default=None, help='HuggingFace model path')
+    parser.add_argument('--model-tag', type=str, default=None, help='nanochat model tag')
+    parser.add_argument('--step', type=int, default=None, help='Model step to load')
+    parser.add_argument('--max-per-task', type=int, default=500, help='Max examples per task')
+    parser.add_argument('--device-type', type=str, default='', help='cuda|cpu|mps')
+    parser.add_argument('--batch-sizes', type=str, default='1,2,4,8,16,32,64', help='Comma-separated batch sizes to sweep')
+    parser.add_argument('--queue-sizes', type=str, default='2,4,8,16', help='Comma-separated queue sizes to sweep')
+    parser.add_argument('--skip-old', action='store_true', help='Skip old sequential baseline (slow)')
+    parser.add_argument('--skip-first', action='store_true', help='Skip first-run sweep (requires cached collated data)')
+    args = parser.parse_args()
+
+    batch_sizes = [int(x) for x in args.batch_sizes.split(',')]
+    queue_sizes = [int(x) for x in args.queue_sizes.split(',')]
+
+    device_type = autodetect_device_type() if args.device_type == '' else args.device_type
+    ddp, ddp_rank, ddp_local_rank, ddp_world_size, device = compute_init(device_type)
+    autocast_ctx = torch.amp.autocast(device_type=device_type, dtype=torch.bfloat16) if device_type == "cuda" else nullcontext()
+
+    # Load model
+    if args.hf_path is not None:
+        model, tokenizer = load_hf_model(args.hf_path, device)
+        model_name = args.hf_path
+    else:
+        model, tokenizer, meta = load_model("base", device, phase="eval", model_tag=args.model_tag, step=args.step)
+        model_name = f"base_model (step {meta['step']})"
+
+    print0(f"Model: {model_name}")
+    print0(f"Max per task: {args.max_per_task}")
+    print0(f"Device: {device}")
+    print0("")
+
+    # Load tasks
+    task_inputs = load_tasks(max_per_task=args.max_per_task)
+    total_examples = sum(len(data) for _, _, data in task_inputs)
+    print0(f"Loaded {len(task_inputs)} tasks, {total_examples} total examples")
+    print0("")
+
+    # ---- 1. Old sequential baseline ----
+    old_results = None
+    if not args.skip_old:
+        print0("=" * 80)
+        print0("OLD: Sequential per-example evaluation")
+        print0("=" * 80)
+        with autocast_ctx:
+            old_time, old_results = bench_old(model, tokenizer, task_inputs, device)
+        print0(f"  Time: {old_time:.2f}s  ({total_examples / old_time:.1f} examples/s)")
+        print0("")
+
+    # ---- 2. Sweep batch_size x queue_size for first run ----
+    # Compute tok_hash early — needed for both skip-first check and cache loading
+    max_seq_len = getattr(model, 'max_seq_len', None)
+    if args.hf_path is not None:
+        tok_hash = hashlib.sha256(args.hf_path.encode()).hexdigest()[:16]
+    else:
+        base_dir = get_base_dir()
+        for fname in ("tokenizer.pkl", "tokenizer.json"):
+            tok_path = os.path.join(base_dir, "tokenizer", fname)
+            if os.path.exists(tok_path):
+                tok_hash = file_checksum(tok_path)
+                break
+
+    # Check if we can skip the first-run sweep
+    best_time = None
+    best_params = None
+    if args.skip_first:
+        cache_dir = os.path.join(get_base_dir(), "core_token_cache", f"{tok_hash}_n{args.max_per_task}")
+        has_cache = os.path.isdir(cache_dir) and all(
+            os.path.exists(os.path.join(cache_dir, f"{label}.pt"))
+            for label, _, _ in task_inputs
+        )
+        if has_cache:
+            print0("Skipping first-run sweep (--skip-first, cache exists)")
+            print0("")
+        else:
+            print0(f"--skip-first: cache missing, running single bs={BASE_BATCH_SIZE} eval to create it...")
+            pbar = tqdm(total=total_examples, desc="Creating cache", leave=False)
+            with autocast_ctx:
+                _, _, collated_cache = bench_new_first(model, tokenizer, task_inputs, device,
+                                                       batch_size=BASE_BATCH_SIZE, queue_size=2, pbar=pbar)
+            pbar.close()
+            pad_id = tokenizer.get_bos_token_id()
+            os.makedirs(cache_dir, exist_ok=True)
+            for label in collated_cache:
+                torch.save({'collated': collated_cache[label], 'pad_token_id': pad_id},
+                           os.path.join(cache_dir, f"{label}.pt"))
+            print0(f"Cache created ({len(collated_cache)} tasks)")
+            print0("")
+
+    if not args.skip_first:
+        print0("=" * 80)
+        print0("NEW: Batched evaluation — hyperparameter sweep (first run)")
+        print0("=" * 80)
+        print0("")
+
+        # Header
+        qs_header = "".join(f"{'q=' + str(q):>10}" for q in queue_sizes)
+        print0(f"  {'batch_size':>10}{qs_header}")
+        print0(f"  {'':>10}" + "-" * (10 * len(queue_sizes)))
+
+        best_time = float('inf')
+        best_params = None
+        best_collated = None
+        sweep_results = {}
+        total_combos = len(batch_sizes) * len(queue_sizes)
+        outer_pbar = tqdm(total=total_combos, desc="First-run sweep", leave=False, position=0)
+        inner_pbar = tqdm(total=total_examples, desc="", leave=False, position=1)
+
+        for bs in batch_sizes:
+            row = f"  {bs:>10}"
+            for qs in queue_sizes:
+                outer_pbar.set_description(f"First-run: bs={bs} qs={qs}")
+                inner_pbar.reset()
+                with autocast_ctx:
+                    t, results, collated_cache = bench_new_first(model, tokenizer, task_inputs, device, bs, qs, pbar=inner_pbar)
+                sweep_results[(bs, qs)] = t
+                row += f"{t:>9.2f}s"
+                if t < best_time:
+                    best_time = t
+                    best_params = (bs, qs)
+                    best_collated = collated_cache
+                    best_first_results = results
+                outer_pbar.update(1)
+            outer_pbar.write(row)
+        inner_pbar.close()
+        outer_pbar.close()
+
+        print0("")
+        print0(f"  Best: batch_size={best_params[0]}, queue_size={best_params[1]} -> {best_time:.2f}s  ({total_examples / best_time:.1f} examples/s)")
+
+        # Verify correctness
+        if old_results is not None:
+            verify_results(old_results, best_first_results, label="new-first")
+        print0("")
+
+    # ---- 3. Build/load base-4 collated cache ----
+    base_cache = build_or_load_base_collated(tok_hash, tokenizer, task_inputs, max_seq_len, args.max_per_task)
+
+    # ---- 4. Cached run sweep (forward only, composed from base-4) ----
+    print0("=" * 80)
+    print0(f"NEW: Cached run (forward only, composed from base-{BASE_BATCH_SIZE})")
+    print0("=" * 80)
+    print0("")
+
+    best_cached_time = float('inf')
+    best_cached_params = None
+    pad_id = next(iter(base_cache.values()))[1]
+    # Preload ALL base-4 batches to GPU once (~144MB for full CORE eval).
+    # All batch-size sweeps compose from these GPU-resident tensors — zero CPU→GPU transfers.
+    gpu_collated = {}
+    for label, (collated, _) in base_cache.items():
+        gpu_collated[label] = [(ids.to(device), meta) for ids, meta in collated]
+    outer_pbar = tqdm(total=len(batch_sizes), desc="Cached sweep", leave=False, position=0)
+    inner_pbar = tqdm(total=total_examples, desc="", leave=False, position=1)
+
+    for bs in batch_sizes:
+        outer_pbar.set_description(f"Cached: bs={bs}")
+        inner_pbar.reset()
+
+        # All composition happens on GPU: merge for bs >= base, split for bs < base
+        if bs >= BASE_BATCH_SIZE:
+            merge, split = bs // BASE_BATCH_SIZE, 1
+        else:
+            merge, split = 1, BASE_BATCH_SIZE // bs
+
+        with autocast_ctx:
+            t, cached_results = bench_new_cached(model, task_inputs, device, gpu_collated,
+                                                 pbar=inner_pbar, batched=True,
+                                                 merge=merge, split=split, pad_token_id=pad_id)
+
+        outer_pbar.write(f"  batch_size={bs:>3}:  {t:.2f}s  ({total_examples / t:.1f} examples/s)")
+        outer_pbar.update(1)
+
+        if t < best_cached_time:
+            best_cached_time = t
+            best_cached_params = bs
+            best_cached_results = cached_results
+    inner_pbar.close()
+    outer_pbar.close()
+
+    print0("")
+    print0(f"  Best: batch_size={best_cached_params} -> {best_cached_time:.2f}s  ({total_examples / best_cached_time:.1f} examples/s)")
+
+    # Verify with per-example forwarding (identical to sequential — must match old)
+    inner_pbar = tqdm(total=total_examples, desc="Verifying", leave=False)
+    with autocast_ctx:
+        _, exact_results = bench_new_cached(model, task_inputs, device, gpu_collated, pbar=inner_pbar)
+    inner_pbar.close()
+    ref_results = old_results or (best_first_results if best_time is not None else None)
+    if ref_results is not None:
+        verify_results(ref_results, exact_results, label="new-cached(per-example)")
+    print0("")
+
+    # ---- Summary ----
+    print0("=" * 80)
+    print0("SUMMARY")
+    print0("=" * 80)
+    if old_results is not None:
+        print0(f"  Old (sequential):    {old_time:>8.2f}s")
+    if best_time is not None:
+        print0(f"  New (first run):     {best_time:>8.2f}s   batch_size={best_params[0]}, queue_size={best_params[1]}")
+    print0(f"  New (cached):        {best_cached_time:>8.2f}s   batch_size={best_cached_params}")
+    if old_results is not None:
+        if best_time is not None:
+            print0(f"  Speedup (first):     {old_time / best_time:>8.2f}x")
+        print0(f"  Speedup (cached):    {old_time / best_cached_time:>8.2f}x")
+
+    compute_cleanup()
+
+if __name__ == "__main__":
+    main()