diff --git a/scripts/filter_dataset.py b/scripts/filter_dataset.py
new file mode 100755
index 0000000..7e26d35
--- /dev/null
+++ b/scripts/filter_dataset.py
@@ -0,0 +1,280 @@
+#!/usr/bin/env python3
+"""Drop rows whose assistant loss mask is all zero, then write sharded parquet.
+
+A row is kept only if ``tokenizer.apply_chat_template(..., return_assistant_tokens_mask=True)``
+marks at least one token for the loss.  Rows with an all-zero mask contribute no
+gradient during SFT, so they only waste tokens in a packed batch.
+
+The number of output shards is inferred from the estimated size of the kept rows
+and ``--max-shard-size``.
+
+Usage
+-----
+    python scripts/filter_dataset.py \\
+        --dataset allenai/Dolci-Instruct-SFT \\
+        --tokenizer Qwen/Qwen3-8B \\
+        --chat-template chatml \\
+        --output data/dolci-instruct-sft-nonempty
+
+    # Report only, write nothing.
+    python scripts/filter_dataset.py --dataset ... --tokenizer ... --dry-run
+
+The output directory holds ``train-00000-of-000NN.parquet`` files that
+``load_dataset("<output>", split="train")`` reads directly.
+"""
+
+from __future__ import annotations
+
+import argparse
+import math
+import os
+import re
+import sys
+from pathlib import Path
+
+from datasets import load_dataset
+from transformers import AutoTokenizer
+
+_PROJECT_ROOT = Path(__file__).resolve().parent.parent
+if str(_PROJECT_ROOT / "src") not in sys.path:
+    sys.path.insert(0, str(_PROJECT_ROOT / "src"))
+
+from post_training.chat_templates.registry import (
+    CHAT_TEMPLATES,
+    get_chat_template,
+    has_generation_markers,
+)
+
+_SIZE_UNITS = {
+    "B": 1,
+    "KB": 10**3,
+    "MB": 10**6,
+    "GB": 10**9,
+    "TB": 10**12,
+    "KIB": 2**10,
+    "MIB": 2**20,
+    "GIB": 2**30,
+    "TIB": 2**40,
+}
+_SIZE_RE = re.compile(r"^\s*([0-9]*\.?[0-9]+)\s*([A-Za-z]*)\s*$")
+
+
+def parse_size(value: str) -> int:
+    """Return *value* (for example ``"500MB"``) as a number of bytes."""
+    match = _SIZE_RE.match(value)
+    if match is None:
+        raise argparse.ArgumentTypeError(f"Cannot parse size '{value}'.")
+    number, unit = match.groups()
+    unit = (unit or "B").upper()
+    if unit not in _SIZE_UNITS:
+        raise argparse.ArgumentTypeError(
+            f"Unknown size unit '{unit}'. Use one of: {', '.join(_SIZE_UNITS)}."
+        )
+    size = int(float(number) * _SIZE_UNITS[unit])
+    if size <= 0:
+        raise argparse.ArgumentTypeError(f"Size must be positive, got '{value}'.")
+    return size
+
+
+def resolve_chat_template(spec: str | None) -> str | None:
+    """Return the Jinja source for *spec*.
+
+    *spec* is either a registry name (see ``CHAT_TEMPLATES``) or a path to a
+    ``.jinja`` file.  ``None`` keeps the template that comes with the tokenizer.
+    """
+    if spec is None:
+        return None
+    path = Path(spec)
+    if path.suffix == ".jinja" or path.exists():
+        if not path.exists():
+            raise FileNotFoundError(f"Chat template file not found: {path}")
+        return path.read_text()
+    return get_chat_template(spec)
+
+
+# ``datasets`` >= 4 loads a directory or a repo id, but not a single data file.
+# For a file the builder name comes from the suffix and the file goes to data_files.
+_FILE_BUILDERS = {
+    ".parquet": "parquet",
+    ".json": "json",
+    ".jsonl": "json",
+    ".csv": "csv",
+    ".arrow": "arrow",
+    ".txt": "text",
+}
+
+
+def load_source(dataset: str, dataset_config: str | None, split: str, num_proc: int):
+    """Load *dataset* from the Hub, a local directory, or a single data file."""
+    path = Path(dataset)
+    if path.is_file():
+        builder = _FILE_BUILDERS.get(path.suffix)
+        if builder is None:
+            raise SystemExit(
+                f"Cannot load the file '{path}'. Known suffixes: "
+                f"{', '.join(sorted(_FILE_BUILDERS))}."
+            )
+        return load_dataset(
+            builder,
+            data_files={split: str(path)},
+            split=split,
+            num_proc=num_proc,
+        )
+    return load_dataset(dataset, dataset_config, split=split, num_proc=num_proc)
+
+
+def parse_args() -> argparse.Namespace:
+    parser = argparse.ArgumentParser(
+        description="Filter out dataset rows with an all-zero assistant loss mask.",
+        formatter_class=argparse.ArgumentDefaultsHelpFormatter,
+    )
+    parser.add_argument(
+        "--dataset",
+        required=True,
+        help="HF repo id, local directory, or single data file (parquet/json/csv/arrow).",
+    )
+    parser.add_argument(
+        "--dataset-config",
+        default=None,
+        help="Dataset config/subset name.",
+    )
+    parser.add_argument("--split", default="train", help="Split to load.")
+    parser.add_argument(
+        "--messages-column",
+        default="messages",
+        help="Column that holds the list of chat messages.",
+    )
+    parser.add_argument(
+        "--tokenizer",
+        required=True,
+        help="HF repo id or local path of the tokenizer.",
+    )
+    parser.add_argument(
+        "--chat-template",
+        default=None,
+        help=(
+            "Chat template: a registry name "
+            f"({', '.join(sorted(CHAT_TEMPLATES))}) or a path to a .jinja file. "
+            "Omit it to keep the template of the tokenizer."
+        ),
+    )
+    parser.add_argument(
+        "--output",
+        type=Path,
+        default=None,
+        help="Directory for the parquet shards. Required unless --dry-run is set.",
+    )
+    parser.add_argument(
+        "--max-shard-size",
+        type=parse_size,
+        default="500MB",
+        help="Target size of one parquet shard. The shard count follows from it.",
+    )
+    parser.add_argument(
+        "--num-shards",
+        type=int,
+        default=None,
+        help="Fixed shard count. It overrides --max-shard-size.",
+    )
+    parser.add_argument(
+        "--num-proc",
+        type=int,
+        default=int(os.environ.get("SLURM_CPUS_PER_TASK", os.cpu_count() or 1)),
+        help="Worker processes for load_dataset() and map().",
+    )
+    parser.add_argument(
+        "--num-examples",
+        type=int,
+        default=10,
+        help="Number of dropped rows to print.",
+    )
+    parser.add_argument(
+        "--dry-run",
+        action="store_true",
+        help="Print the statistics only and write no files.",
+    )
+    args = parser.parse_args()
+    if args.output is None and not args.dry_run:
+        parser.error("--output is required unless --dry-run is set.")
+    if args.num_shards is not None and args.num_shards < 1:
+        parser.error("--num-shards must be at least 1.")
+    return args
+
+
+def infer_num_shards(dataset, max_shard_size: int) -> int:
+    """Return the shard count that keeps each shard near *max_shard_size* bytes."""
+    num_rows = len(dataset)
+    if num_rows == 0:
+        return 1
+    nbytes = dataset.data.nbytes
+    num_shards = math.ceil(nbytes / max_shard_size)
+    return max(1, min(num_shards, num_rows))
+
+
+def main() -> None:
+    args = parse_args()
+
+    tokenizer = AutoTokenizer.from_pretrained(args.tokenizer)
+    template = resolve_chat_template(args.chat_template)
+    if template is not None:
+        tokenizer.chat_template = template
+    if not has_generation_markers(tokenizer.chat_template):
+        raise SystemExit(
+            "The chat template has no {% generation %} markers, so every mask is "
+            "all zero and the filter drops every row. Pass a template that has them."
+        )
+
+    dataset = load_source(args.dataset, args.dataset_config, args.split, args.num_proc)
+    if args.messages_column not in dataset.column_names:
+        raise SystemExit(
+            f"Column '{args.messages_column}' is not in the dataset. "
+            f"Available columns: {', '.join(dataset.column_names)}."
+        )
+    print(f"Loaded {len(dataset):,} rows from '{args.dataset}' (split={args.split}).")
+
+    # remove_columns keeps the map cache small: it holds the flag alone, not a
+    # copy of the data.
+    in_loss = dataset.map(
+        lambda row: {
+            "in_loss": any(
+                tokenizer.apply_chat_template(
+                    row[args.messages_column],
+                    return_dict=True,
+                    return_assistant_tokens_mask=True,
+                )["assistant_masks"]
+            )
+        },
+        num_proc=args.num_proc,
+        remove_columns=dataset.column_names,
+        desc="masking",
+    )["in_loss"]
+    in_loss = list(in_loss)
+
+    empty = [i for i, keep in enumerate(in_loss) if not keep]
+    share = 100 * len(empty) / len(dataset) if len(dataset) else 0.0
+    print(f"{len(dataset):,} rows, {len(empty):,} with an all-zero mask ({share:.3f}%)")
+    for i in empty[: args.num_examples]:
+        roles = " -> ".join(m["role"] for m in dataset[i][args.messages_column])
+        print(f"  row {i}: {roles}")
+
+    kept = dataset.select([i for i, keep in enumerate(in_loss) if keep])
+    if args.dry_run:
+        print(f"Dry run: {len(kept):,} of {len(dataset):,} rows pass the filter.")
+        return
+
+    num_shards = (
+        args.num_shards
+        if args.num_shards is not None
+        else infer_num_shards(kept, args.max_shard_size)
+    )
+    args.output.mkdir(parents=True, exist_ok=True)
+    for i in range(num_shards):
+        path = args.output / f"{args.split}-{i:05d}-of-{num_shards:05d}.parquet"
+        kept.shard(num_shards=num_shards, index=i, contiguous=True).to_parquet(path)
+        print(f"wrote {path}")
+
+    print(f"kept {len(kept):,} of {len(dataset):,} rows in {args.output} ({num_shards} shards)")
+
+
+if __name__ == "__main__":
+    main()