diff --git a/pyproject.toml b/pyproject.toml
index 8b6fd95..fb4fc97 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -29,6 +29,8 @@ dependencies = [
 [dependency-groups]
 dev = [
     "pytest>=8.0.0",
+    "pytest-cov>=7.0.0",
+    "pytest-mock>=3.15.1",
 ]
 
 [tool.pytest.ini_options]
diff --git a/runs/miniseries.sh b/runs/miniseries.sh
index 01c4459..96f14fb 100644
--- a/runs/miniseries.sh
+++ b/runs/miniseries.sh
@@ -28,7 +28,7 @@ fi
 # Series name: from arg, env var, or default to today's date (e.g., jan11)
 SERIES_NAME="${1:-${SERIES_NAME:-$(date +%b%d | tr '[:upper:]' '[:lower:]')}}"
 # Depths to train (the "miniseries")
-DEPTHS=(12 14 16 18 20 22 24 26)
+DEPTHS=(12 14)
 # Hardware
 NPROC_PER_NODE="${NPROC_PER_NODE:-8}"
 # Logging
@@ -63,7 +63,7 @@ for d in "${DEPTHS[@]}"; do
     elif [ $d -ge 20 ]; then
         DEVICE_BATCH_SIZE_ARG="--device-batch-size=16"
     else
-        DEVICE_BATCH_SIZE_ARG="--device-batch-size=32"
+        DEVICE_BATCH_SIZE_ARG="--device-batch-size=16"
     fi
 
     torchrun --standalone --nproc_per_node=$NPROC_PER_NODE -m scripts.base_train -- \
diff --git a/tests/test_checkpoint_manager_new.py b/tests/test_checkpoint_manager_new.py
new file mode 100644
index 0000000..ffac1f2
--- /dev/null
+++ b/tests/test_checkpoint_manager_new.py
@@ -0,0 +1,175 @@
+import json
+import os
+from types import SimpleNamespace
+
+import pytest
+import torch
+
+import nanochat.checkpoint_manager as ckpt
+
+
+def test_log0_respects_rank(monkeypatch):
+    seen = {"n": 0}
+    monkeypatch.setattr(ckpt.logger, "info", lambda _m: seen.__setitem__("n", seen["n"] + 1))
+    monkeypatch.setenv("RANK", "0")
+    ckpt.log0("hi")
+    monkeypatch.setenv("RANK", "1")
+    ckpt.log0("skip")
+    assert seen["n"] == 1
+
+
+def test_patch_missing_helpers():
+    cfg = {"vocab_size": 32}
+    ckpt._patch_missing_config_keys(cfg)
+    assert cfg["window_pattern"] == "L"
+    ckpt._patch_missing_config_keys(cfg)
+    assert cfg["window_pattern"] == "L"
+
+    model_data = {}
+    model_cfg = SimpleNamespace(n_layer=3)
+    ckpt._patch_missing_keys(model_data, model_cfg)
+    assert torch.all(model_data["resid_lambdas"] == 1)
+    assert torch.all(model_data["x0_lambdas"] == 0)
+
+    ckpt._patch_missing_keys(model_data, model_cfg)
+    assert model_data["resid_lambdas"].numel() == 3
+
+
+def test_save_and_load_checkpoint(tmp_path):
+    model_data = {"w": torch.tensor([1, 2])}
+    optim_data = {"g": torch.tensor([3, 4])}
+    meta_data = {"model_config": {"vocab_size": 8, "n_layer": 1, "n_head": 1, "n_kv_head": 1, "n_embd": 4, "sequence_len": 4}}
+    ckpt.save_checkpoint(str(tmp_path), 7, model_data, optim_data, meta_data, rank=0)
+    ckpt.save_checkpoint(str(tmp_path), 8, model_data, None, meta_data, rank=0)
+    got_model, got_optim, got_meta = ckpt.load_checkpoint(str(tmp_path), 7, "cpu", load_optimizer=True, rank=0)
+    assert torch.equal(got_model["w"], model_data["w"])
+    assert torch.equal(got_optim["g"], optim_data["g"])
+    assert got_meta["model_config"]["vocab_size"] == 8
+
+    got_model2, got_optim2, got_meta2 = ckpt.load_checkpoint(str(tmp_path), 8, "cpu", load_optimizer=False, rank=0)
+    assert got_optim2 is None
+    assert got_meta2 == meta_data
+    assert torch.equal(got_model2["w"], model_data["w"])
+
+
+def test_build_model_cpu_and_train_eval(monkeypatch):
+    model_data = {"_orig_mod.w": torch.ones(1, dtype=torch.bfloat16)}
+    meta_data = {"model_config": {"vocab_size": 16, "n_layer": 2, "n_head": 1, "n_kv_head": 1, "n_embd": 4, "sequence_len": 8}}
+
+    class FakeTokenizer:
+        def get_vocab_size(self):
+            return 16
+
+    class FakeModel:
+        def __init__(self, cfg):
+            self.cfg = cfg
+            self.mode = None
+            self.loaded = None
+
+        def to_empty(self, device):
+            self.device = device
+
+        def init_weights(self):
+            self.init_called = True
+
+        def load_state_dict(self, data, strict, assign):
+            self.loaded = (data, strict, assign)
+
+        def eval(self):
+            self.mode = "eval"
+
+        def train(self):
+            self.mode = "train"
+
+    monkeypatch.setattr(ckpt, "load_checkpoint", lambda *a, **k: (model_data.copy(), None, meta_data))
+    monkeypatch.setattr(ckpt, "GPTConfig", lambda **kw: SimpleNamespace(**kw))
+    monkeypatch.setattr(ckpt, "GPT", lambda cfg: FakeModel(cfg))
+    monkeypatch.setattr(ckpt, "get_tokenizer", lambda: FakeTokenizer())
+
+    m, tok, meta = ckpt.build_model("/x", 1, torch.device("cpu"), phase="eval")
+    assert m.mode == "eval"
+    assert tok.get_vocab_size() == 16
+    assert meta == meta_data
+    assert "w" in m.loaded[0]
+    assert m.loaded[0]["w"].dtype == torch.float32
+
+    m2, _, _ = ckpt.build_model("/x", 1, torch.device("cpu"), phase="train")
+    assert m2.mode == "train"
+
+    with pytest.raises(AssertionError):
+        ckpt.build_model("/x", 1, torch.device("cpu"), phase="bad")
+
+    class BadTok(FakeTokenizer):
+        def get_vocab_size(self):
+            return 99
+
+    monkeypatch.setattr(ckpt, "get_tokenizer", lambda: BadTok())
+    with pytest.raises(AssertionError):
+        ckpt.build_model("/x", 1, torch.device("cpu"), phase="eval")
+
+
+def test_find_largest_model_and_last_step(tmp_path, monkeypatch):
+    with pytest.raises(FileNotFoundError):
+        ckpt.find_largest_model(str(tmp_path))
+
+    (tmp_path / "d3").mkdir()
+    (tmp_path / "d10").mkdir()
+    assert ckpt.find_largest_model(str(tmp_path)) == "d10"
+
+    # Fallback path: no d<number> tags, choose newest by mtime.
+    other = tmp_path / "abc"
+    newest = tmp_path / "zzz"
+    other.mkdir(exist_ok=True)
+    newest.mkdir(exist_ok=True)
+    monkeypatch.setattr(ckpt.re, "match", lambda _pat, _s: None)
+    assert ckpt.find_largest_model(str(tmp_path)) == "zzz"
+
+    step_dir = tmp_path / "steps"
+    step_dir.mkdir()
+    with pytest.raises(FileNotFoundError):
+        ckpt.find_last_step(str(step_dir))
+    (step_dir / "model_000012.pt").write_bytes(b"")
+    (step_dir / "model_000007.pt").write_bytes(b"")
+    assert ckpt.find_last_step(str(step_dir)) == 12
+
+
+def test_load_model_from_dir_and_load_model(monkeypatch):
+    calls = {}
+
+    monkeypatch.setattr(ckpt, "find_largest_model", lambda d: "d12")
+    monkeypatch.setattr(ckpt, "find_last_step", lambda d: 42)
+
+    def fake_build_model(checkpoint_dir, step, device, phase):
+        calls["args"] = (checkpoint_dir, step, device, phase)
+        return "m", "t", {"k": 1}
+
+    monkeypatch.setattr(ckpt, "build_model", fake_build_model)
+    out = ckpt.load_model_from_dir("/root/checkpoints", device="cpu", phase="eval")
+    assert out == ("m", "t", {"k": 1})
+    assert calls["args"][0].endswith("/root/checkpoints/d12")
+    assert calls["args"][1] == 42
+
+    monkeypatch.setattr(ckpt, "get_base_dir", lambda: "/base")
+    monkeypatch.setattr(ckpt, "load_model_from_dir", lambda checkpoints_dir, *a, **k: ("ok", checkpoints_dir, a, k))
+    got = ckpt.load_model("base", "cpu", phase="eval")
+    assert got[0] == "ok"
+    assert got[1].endswith("/base/base_checkpoints")
+    assert ckpt.load_model("sft", "cpu", phase="eval")[1].endswith("/base/chatsft_checkpoints")
+    assert ckpt.load_model("rl", "cpu", phase="eval")[1].endswith("/base/chatrl_checkpoints")
+
+
+def test_load_optimizer_state(monkeypatch, tmp_path):
+    model_root = tmp_path / "base_checkpoints" / "d7"
+    model_root.mkdir(parents=True)
+    opt_path = model_root / "optim_000003_rank2.pt"
+    torch.save({"x": torch.tensor([1])}, opt_path)
+
+    monkeypatch.setattr(ckpt, "get_base_dir", lambda: str(tmp_path))
+    monkeypatch.setattr(ckpt, "find_largest_model", lambda _d: "d7")
+    monkeypatch.setattr(ckpt, "find_last_step", lambda _d: 3)
+
+    got = ckpt.load_optimizer_state("base", "cpu", rank=2, model_tag=None, step=None)
+    assert torch.equal(got["x"], torch.tensor([1]))
+
+    missing = ckpt.load_optimizer_state("base", "cpu", rank=99, model_tag="d7", step=3)
+    assert missing is None
diff --git a/tests/test_common_new.py b/tests/test_common_new.py
new file mode 100644
index 0000000..7a11954
--- /dev/null
+++ b/tests/test_common_new.py
@@ -0,0 +1,234 @@
+import io
+import logging
+import os
+import types
+
+import pytest
+import torch
+
+import nanochat.common as common
+
+
+def test_colored_formatter_info_and_non_info():
+    fmt = common.ColoredFormatter("%(levelname)s %(message)s")
+    rec = logging.LogRecord("x", logging.INFO, __file__, 1, "Shard 2: 10 MB 5%", (), None)
+    out = fmt.format(rec)
+    assert "Shard 2" in out
+    assert "10 MB" in out
+    assert "5%" in out
+
+    rec2 = logging.LogRecord("x", logging.WARNING, __file__, 1, "warn", (), None)
+    out2 = fmt.format(rec2)
+    assert "warn" in out2
+
+
+def test_setup_default_logging(monkeypatch):
+    calls = {}
+
+    def fake_basic_config(**kwargs):
+        calls["kwargs"] = kwargs
+
+    monkeypatch.setattr(common.logging, "basicConfig", fake_basic_config)
+    common.setup_default_logging()
+    assert calls["kwargs"]["level"] == logging.INFO
+    assert len(calls["kwargs"]["handlers"]) == 1
+
+
+def test_get_base_dir_env_and_default(monkeypatch, tmp_path):
+    monkeypatch.setenv("NANOCHAT_BASE_DIR", str(tmp_path / "from_env"))
+    got = common.get_base_dir()
+    assert got.endswith("from_env")
+    assert os.path.isdir(got)
+
+    monkeypatch.delenv("NANOCHAT_BASE_DIR", raising=False)
+    monkeypatch.setattr(common.os.path, "expanduser", lambda _: str(tmp_path))
+    got2 = common.get_base_dir()
+    assert got2.endswith(".cache/nanochat")
+    assert os.path.isdir(got2)
+
+
+def test_download_file_with_lock_paths(monkeypatch, tmp_path):
+    monkeypatch.setattr(common, "get_base_dir", lambda: str(tmp_path))
+    out = tmp_path / "f.bin"
+    out.write_bytes(b"x")
+    assert common.download_file_with_lock("http://x", "f.bin") == str(out)
+
+    out.unlink()
+    payload = b"hello"
+    marker = {"post": None}
+
+    class Resp:
+        def __enter__(self):
+            return self
+
+        def __exit__(self, exc_type, exc, tb):
+            return False
+
+        def read(self):
+            return payload
+
+    monkeypatch.setattr(common.urllib.request, "urlopen", lambda _url: Resp())
+    got = common.download_file_with_lock(
+        "http://example.test",
+        "f.bin",
+        postprocess_fn=lambda p: marker.__setitem__("post", p),
+    )
+    assert got == str(out)
+    assert out.read_bytes() == payload
+    assert marker["post"] == str(out)
+
+
+def test_download_file_with_lock_recheck_after_lock(monkeypatch, tmp_path):
+    monkeypatch.setattr(common, "get_base_dir", lambda: str(tmp_path))
+    file_path = str(tmp_path / "race.bin")
+
+    calls = {"n": 0}
+    real_exists = common.os.path.exists
+
+    def fake_exists(path):
+        if path == file_path:
+            calls["n"] += 1
+            # first check (before lock): missing; second check (inside lock): present
+            return calls["n"] >= 2
+        return real_exists(path)
+
+    class Lock:
+        def __init__(self, _p):
+            pass
+
+        def __enter__(self):
+            return self
+
+        def __exit__(self, exc_type, exc, tb):
+            return False
+
+    monkeypatch.setattr(common.os.path, "exists", fake_exists)
+    monkeypatch.setattr(common, "FileLock", Lock)
+    got = common.download_file_with_lock("http://unused", "race.bin")
+    assert got == file_path
+
+
+def test_print0_and_banner(monkeypatch, capsys):
+    monkeypatch.setenv("RANK", "1")
+    common.print0("hidden")
+    assert capsys.readouterr().out == ""
+
+    monkeypatch.setenv("RANK", "0")
+    common.print0("shown")
+    assert "shown" in capsys.readouterr().out
+
+    common.print_banner()
+    assert "█████" in capsys.readouterr().out
+
+
+def test_ddp_helpers(monkeypatch):
+    for k in ("RANK", "LOCAL_RANK", "WORLD_SIZE"):
+        monkeypatch.delenv(k, raising=False)
+    assert common.is_ddp_requested() is False
+    assert common.get_dist_info() == (False, 0, 0, 1)
+
+    monkeypatch.setenv("RANK", "3")
+    monkeypatch.setenv("LOCAL_RANK", "2")
+    monkeypatch.setenv("WORLD_SIZE", "8")
+    assert common.is_ddp_requested() is True
+    assert common.get_dist_info() == (True, 3, 2, 8)
+
+    monkeypatch.setattr(common.dist, "is_available", lambda: True)
+    monkeypatch.setattr(common.dist, "is_initialized", lambda: True)
+    assert common.is_ddp_initialized() is True
+
+
+def test_autodetect_device_type(monkeypatch):
+    monkeypatch.setattr(common.torch.cuda, "is_available", lambda: True)
+    assert common.autodetect_device_type() == "cuda"
+
+    monkeypatch.setattr(common.torch.cuda, "is_available", lambda: False)
+    monkeypatch.setattr(common.torch.backends.mps, "is_available", lambda: True)
+    assert common.autodetect_device_type() == "mps"
+
+    monkeypatch.setattr(common.torch.backends.mps, "is_available", lambda: False)
+    assert common.autodetect_device_type() == "cpu"
+
+
+def test_compute_init_cpu_and_cleanup(monkeypatch):
+    monkeypatch.setattr(common, "get_dist_info", lambda: (False, 0, 0, 1))
+    out = common.compute_init("cpu")
+    assert out[0] is False
+    assert out[-1].type == "cpu"
+
+    called = {"destroy": 0}
+    monkeypatch.setattr(common, "is_ddp_initialized", lambda: True)
+    monkeypatch.setattr(common.dist, "destroy_process_group", lambda: called.__setitem__("destroy", 1))
+    common.compute_cleanup()
+    assert called["destroy"] == 1
+
+
+def test_compute_init_mps_and_cuda_paths(monkeypatch):
+    monkeypatch.setattr(common, "get_dist_info", lambda: (True, 1, 0, 2))
+    monkeypatch.setattr(common.torch.backends.mps, "is_available", lambda: True)
+    out = common.compute_init("mps")
+    assert out[0] is True
+    assert out[-1].type == "mps"
+
+    monkeypatch.setattr(common.torch.cuda, "is_available", lambda: True)
+    set_device_calls = {}
+    init_calls = {}
+    barrier_calls = {}
+    matmul_calls = {}
+    seed_calls = {}
+    cuda_seed_calls = {}
+
+    monkeypatch.setattr(common.torch.cuda, "set_device", lambda d: set_device_calls.__setitem__("device", d))
+    monkeypatch.setattr(common.dist, "init_process_group", lambda **kw: init_calls.__setitem__("kw", kw))
+    monkeypatch.setattr(common.dist, "barrier", lambda: barrier_calls.__setitem__("n", 1))
+    monkeypatch.setattr(common.torch, "set_float32_matmul_precision", lambda x: matmul_calls.__setitem__("x", x))
+    monkeypatch.setattr(common.torch, "manual_seed", lambda x: seed_calls.__setitem__("x", x))
+    monkeypatch.setattr(common.torch.cuda, "manual_seed", lambda x: cuda_seed_calls.__setitem__("x", x))
+
+    out2 = common.compute_init("cuda")
+    assert out2[0] is True
+    assert out2[-1].type == "cuda"
+    assert matmul_calls["x"] == "high"
+    assert seed_calls["x"] == 42
+    assert cuda_seed_calls["x"] == 42
+    assert "device_id" in init_calls["kw"]
+    assert barrier_calls["n"] == 1
+    assert set_device_calls["device"].type == "cuda"
+
+
+def test_compute_init_assertions(monkeypatch):
+    monkeypatch.setattr(common.torch.cuda, "is_available", lambda: False)
+    with pytest.raises(AssertionError):
+        common.compute_init("cuda")
+    monkeypatch.setattr(common.torch.backends.mps, "is_available", lambda: False)
+    with pytest.raises(AssertionError):
+        common.compute_init("mps")
+    with pytest.raises(AssertionError):
+        common.compute_init("bad")
+
+
+def test_dummy_wandb_and_peak_flops(monkeypatch):
+    w = common.DummyWandb()
+    assert w.log() is None
+    assert w.finish() is None
+
+    assert common.get_peak_flops("NVIDIA H100 NVL") == 835e12
+    assert common.get_peak_flops("A100-SXM") == 312e12
+
+    class Props:
+        max_compute_units = 8
+
+    class XPU:
+        @staticmethod
+        def get_device_properties(_name):
+            return Props()
+
+    monkeypatch.setattr(common.torch, "xpu", XPU())
+    pvc = common.get_peak_flops("Data Center GPU Max 1550")
+    assert pvc == 512 * 8 * 1300 * 10**6
+
+    warned = {}
+    monkeypatch.setattr(common.logger, "warning", lambda msg: warned.__setitem__("msg", msg))
+    unknown = common.get_peak_flops("mystery gpu")
+    assert unknown == float("inf")
+    assert "Peak flops undefined" in warned["msg"]
diff --git a/tests/test_core_eval_new.py b/tests/test_core_eval_new.py
new file mode 100644
index 0000000..e2f42b9
--- /dev/null
+++ b/tests/test_core_eval_new.py
@@ -0,0 +1,231 @@
+import random
+from dataclasses import dataclass
+
+import pytest
+import torch
+
+import nanochat.core_eval as core_eval
+
+
+@dataclass
+class ItemMC:
+    query: str
+    choices: list
+    gold: int
+
+
+@dataclass
+class ItemSchema:
+    context_options: list
+    continuation: str
+    gold: int
+
+
+@dataclass
+class ItemLM:
+    context: str
+    continuation: str
+    gold: int = 0
+
+
+class TinyTokenizer:
+    def __init__(self, bos=99):
+        self._bos = bos
+
+    def get_bos_token_id(self):
+        return self._bos
+
+    def __call__(self, prompts, prepend=None):
+        return self.encode(prompts, prepend=prepend)
+
+    def encode(self, prompts, prepend=None):
+        if isinstance(prompts, str):
+            prompts = [prompts]
+        out = []
+        for p in prompts:
+            ids = [len(x) % 11 + 1 for x in p.split()]
+            if prepend is not None:
+                ids = [prepend] + ids
+            out.append(ids)
+        return out
+
+
+def test_prompt_renderers():
+    mc_item = {"query": "Q?", "choices": ["A", "B"], "gold": 1}
+    mc_shots = [ItemMC(query="Q0", choices=["x", "y"], gold=0)]
+    prompts_mc = core_eval.render_prompts_mc(mc_item, " -> ", mc_shots)
+    assert len(prompts_mc) == 2
+    assert "Q0 -> x" in prompts_mc[0]
+
+    schema_item = {"context_options": ["ctx1", "ctx2"], "continuation": "cont", "gold": 0}
+    schema_shots = [ItemSchema(context_options=["ca", "cb"], continuation="x", gold=1)]
+    prompts_schema = core_eval.render_prompts_schema(schema_item, " || ", schema_shots)
+    assert len(prompts_schema) == 2
+    assert "cb || x" in prompts_schema[0]
+
+    lm_item = {"context": "c1  ", "continuation": "end", "gold": 0}
+    lm_shots = [ItemLM(context="ctx ", continuation="tail")]
+    prompts_lm = core_eval.render_prompts_lm(lm_item, " ## ", lm_shots)
+    assert len(prompts_lm) == 2
+    assert prompts_lm[0].endswith("##")
+    assert prompts_lm[1].endswith("## end")
+
+
+def test_sequence_helpers():
+    assert core_eval.find_common_length([[1, 2, 3], [1, 2, 4]], direction="left") == 2
+    assert core_eval.find_common_length([[1, 2, 3], [0, 2, 3]], direction="right") == 2
+    assert core_eval.find_common_length([[7, 8], [7, 8]], direction="left") == 2
+
+    stacked = core_eval.stack_sequences([[1, 2], [3]], pad_token_id=0)
+    assert stacked.tolist() == [[1, 2], [3, 0]]
+
+
+def test_batch_sequence_helpers():
+    tok = TinyTokenizer(bos=5)
+    prompts = ["aa bb", "aa bb cc"]
+    tokens_mc, s_mc, e_mc = core_eval.batch_sequences_mc(tok, prompts)
+    assert len(tokens_mc) == 2
+    assert len(s_mc) == 2 and len(e_mc) == 2
+
+    tokens_schema, s_schema, e_schema = core_eval.batch_sequences_schema(tok, prompts)
+    assert len(tokens_schema) == 2
+    assert len(s_schema) == 2 and len(e_schema) == 2
+
+    lm_prompts = ["a b", "a b c d"]
+    tokens_lm, s_lm, e_lm = core_eval.batch_sequences_lm(tok, lm_prompts)
+    assert len(tokens_lm) == 1
+    assert s_lm[0] < e_lm[0]
+
+    bad = [[1, 2], [8, 9]]
+    class BadTok:
+        def get_bos_token_id(self):
+            return 1
+
+        def __call__(self, *args, **kwargs):
+            del args, kwargs
+            return bad
+
+    with pytest.raises(AssertionError):
+        core_eval.batch_sequences_lm(BadTok(), ["x", "y"])
+
+
+def test_forward_model():
+    class M:
+        def __call__(self, input_ids):
+            b, t = input_ids.shape
+            v = 6
+            logits = torch.zeros((b, t, v), dtype=torch.float32)
+            for i in range(b):
+                for j in range(t):
+                    logits[i, j, (j + 1) % v] = 10.0
+            return logits
+
+    input_ids = torch.tensor([[1, 2, 3], [2, 1, 0]], dtype=torch.long)
+    losses, preds = core_eval.forward_model(M(), input_ids)
+    assert losses.shape == input_ids.shape
+    assert torch.isnan(losses[:, -1]).all()
+    assert preds.shape == input_ids.shape
+
+
+def _simple_model_for_eval():
+    class M:
+        max_seq_len = 3
+
+        def __call__(self, input_ids):
+            b, t = input_ids.shape
+            # Prefer token 3 always to create deterministic MC scores.
+            logits = torch.zeros((b, t, 8), dtype=torch.float32)
+            logits[..., 3] = 3.0
+            return logits
+
+    return M()
+
+
+def _simple_model_no_crop():
+    class M:
+        max_seq_len = None
+
+        def __call__(self, input_ids):
+            b, t = input_ids.shape
+            logits = torch.zeros((b, t, 8), dtype=torch.float32)
+            logits[..., 3] = 3.0
+            return logits
+
+    return M()
+
+
+def test_evaluate_example_main_paths(monkeypatch):
+    tok = TinyTokenizer(bos=1)
+    data_mc = [
+        {"query": "q0", "choices": ["a", "b"], "gold": 0},
+        {"query": "q1", "choices": ["c", "d"], "gold": 1},
+        {"query": "q2", "choices": ["e", "f"], "gold": 0},
+    ]
+    meta_mc = {"task_type": "multiple_choice", "num_fewshot": 1, "continuation_delimiter": " => "}
+    out_mc = core_eval.evaluate_example(0, _simple_model_for_eval(), tok, data_mc, "cpu", meta_mc)
+    assert isinstance(out_mc, bool)
+
+    data_schema = [
+        {"context_options": ["ctx0", "ctx1"], "continuation": "next", "gold": 0},
+        {"context_options": ["ctx2", "ctx3"], "continuation": "next", "gold": 1},
+    ]
+    meta_schema = {"task_type": "schema", "num_fewshot": 0, "continuation_delimiter": " -> "}
+    out_schema = core_eval.evaluate_example(0, _simple_model_no_crop(), tok, data_schema, "cpu", meta_schema)
+    assert isinstance(out_schema, bool)
+
+    # LM path with explicit prefix check and correctness.
+    class LMTokenizer(TinyTokenizer):
+        def encode(self, prompts, prepend=None):
+            if prompts == ["p0", "p1"]:
+                return [[1, 2], [1, 2, 3]]
+            return super().encode(prompts, prepend=prepend)
+
+    lm_data = [{"context": "x", "continuation": "y", "gold": 0}]
+    meta_lm = {"task_type": "language_modeling", "num_fewshot": 0, "continuation_delimiter": ""}
+    monkeypatch.setattr(core_eval, "render_prompts_lm", lambda *_a, **_k: ["p0", "p1"])
+    out_lm = core_eval.evaluate_example(0, _simple_model_for_eval(), LMTokenizer(), lm_data, "cpu", meta_lm)
+    assert isinstance(out_lm, bool)
+
+    with pytest.raises(ValueError):
+        core_eval.evaluate_example(0, _simple_model_for_eval(), tok, data_mc, "cpu", {"task_type": "bad", "num_fewshot": 0, "continuation_delimiter": ""})
+
+
+def test_evaluate_example_reaches_final_else(monkeypatch):
+    # Make first task-type comparison pass once, then fail later so final else is executed.
+    class FlakyType:
+        def __init__(self):
+            self.first = True
+
+        def __eq__(self, other):
+            if self.first and other == "multiple_choice":
+                self.first = False
+                return True
+            return False
+
+    tok = TinyTokenizer(bos=1)
+    data_mc = [{"query": "q", "choices": ["a", "b"], "gold": 0}]
+    meta = {"task_type": FlakyType(), "num_fewshot": 0, "continuation_delimiter": " :: "}
+    with pytest.raises(ValueError):
+        core_eval.evaluate_example(0, _simple_model_for_eval(), tok, data_mc, "cpu", meta)
+
+
+def test_evaluate_task(monkeypatch):
+    data = [{"x": 1}, {"x": 2}, {"x": 3}, {"x": 4}]
+    task_meta = {"task_type": "multiple_choice", "num_fewshot": 0, "continuation_delimiter": ""}
+
+    monkeypatch.setattr(core_eval, "evaluate_example", lambda idx, *_a, **_k: (idx % 2) == 0)
+    monkeypatch.setattr(core_eval.dist, "is_initialized", lambda: False)
+    out = core_eval.evaluate_task(model=None, tokenizer=None, data=data, device="cpu", task_meta=task_meta)
+    assert 0.0 <= out <= 1.0
+
+    # Distributed branch.
+    calls = {"barrier": 0, "reduce": 0}
+    monkeypatch.setattr(core_eval.dist, "is_initialized", lambda: True)
+    monkeypatch.setattr(core_eval.dist, "get_rank", lambda: 0)
+    monkeypatch.setattr(core_eval.dist, "get_world_size", lambda: 2)
+    monkeypatch.setattr(core_eval.dist, "barrier", lambda: calls.__setitem__("barrier", calls["barrier"] + 1))
+    monkeypatch.setattr(core_eval.dist, "all_reduce", lambda *_a, **_k: calls.__setitem__("reduce", calls["reduce"] + 1))
+    out2 = core_eval.evaluate_task(model=None, tokenizer=None, data=data, device="cpu", task_meta=task_meta)
+    assert 0.0 <= out2 <= 1.0
+    assert calls["barrier"] == 1
+    assert calls["reduce"] == 1
diff --git a/tests/test_dataset_dataloader_new.py b/tests/test_dataset_dataloader_new.py
new file mode 100644
index 0000000..101d393
--- /dev/null
+++ b/tests/test_dataset_dataloader_new.py
@@ -0,0 +1,282 @@
+import os
+import runpy
+import types
+
+import pytest
+import torch
+
+import nanochat.dataset as dataset
+import nanochat.dataloader as dataloader
+
+
+def test_list_parquet_files(tmp_path):
+    (tmp_path / "a.parquet").write_bytes(b"")
+    (tmp_path / "b.tmp").write_bytes(b"")
+    (tmp_path / "c.parquet.tmp").write_bytes(b"")
+    (tmp_path / "d.parquet").write_bytes(b"")
+    got = dataset.list_parquet_files(str(tmp_path))
+    assert got == [str(tmp_path / "a.parquet"), str(tmp_path / "d.parquet")]
+
+
+def test_parquets_iter_batched(monkeypatch):
+    files = ["/p0.parquet", "/p1.parquet"]
+    monkeypatch.setattr(dataset, "list_parquet_files", lambda: files)
+
+    class RG:
+        def __init__(self, vals):
+            self._vals = vals
+
+        def column(self, _name):
+            return types.SimpleNamespace(to_pylist=lambda: self._vals)
+
+    class PF:
+        def __init__(self, _path):
+            self.num_row_groups = 3
+
+        def read_row_group(self, idx):
+            return RG([f"row-{idx}"])
+
+    monkeypatch.setattr(dataset.pq, "ParquetFile", PF)
+
+    train = list(dataset.parquets_iter_batched("train", start=0, step=2))
+    val = list(dataset.parquets_iter_batched("val", start=1, step=2))
+    assert train == [["row-0"], ["row-2"]]
+    assert val == [["row-1"]]
+
+    with pytest.raises(AssertionError):
+        list(dataset.parquets_iter_batched("bad"))
+
+
+def test_download_single_file(monkeypatch, tmp_path):
+    monkeypatch.setattr(dataset, "DATA_DIR", str(tmp_path))
+    monkeypatch.setattr(dataset, "BASE_URL", "http://example.test")
+
+    # Existing file -> skip.
+    pre = tmp_path / dataset.index_to_filename(1)
+    pre.write_bytes(b"x")
+    assert dataset.download_single_file(1) is True
+
+    # Success path.
+    class Resp:
+        def __init__(self):
+            self.ok = True
+
+        def raise_for_status(self):
+            return None
+
+        def iter_content(self, chunk_size):
+            del chunk_size
+            yield b"abc"
+            yield b""
+            yield b"def"
+
+    monkeypatch.setattr(dataset.requests, "get", lambda *a, **k: Resp())
+    assert dataset.download_single_file(2) is True
+    out = tmp_path / dataset.index_to_filename(2)
+    assert out.read_bytes() == b"abcdef"
+
+    # Failure with retries and eventual False.
+    calls = {"n": 0}
+
+    def bad_get(*_a, **_k):
+        calls["n"] += 1
+        raise dataset.requests.RequestException("boom")
+
+    slept = {"n": 0}
+    monkeypatch.setattr(dataset.requests, "get", bad_get)
+    monkeypatch.setattr(dataset.time, "sleep", lambda _s: slept.__setitem__("n", slept["n"] + 1))
+    assert dataset.download_single_file(3) is False
+    assert calls["n"] == 5
+    assert slept["n"] == 4
+
+    # Execute final return False line by skipping attempts loop.
+    monkeypatch.setattr(dataset, "range", lambda *_a, **_k: [], raising=False)
+    assert dataset.download_single_file(4) is False
+
+
+def test_document_batches(monkeypatch):
+    monkeypatch.setattr(dataloader, "list_parquet_files", lambda: ["/a.parquet", "/b.parquet", "/c.parquet"])
+    monkeypatch.setattr(dataloader, "get_dist_info", lambda: (True, 1, 0, 2))
+
+    class RG:
+        def __init__(self, vals):
+            self._vals = vals
+
+        def column(self, _name):
+            return types.SimpleNamespace(to_pylist=lambda: self._vals)
+
+    class PF:
+        def __init__(self, path):
+            self.path = path
+            # Make first file tiny to trigger continue in resume branch.
+            self.num_row_groups = 1 if "a.parquet" in path else 4
+
+        def read_row_group(self, idx):
+            return RG([f"{self.path}-r{idx}-d0", f"{self.path}-r{idx}-d1"])
+
+    monkeypatch.setattr(dataloader.pq, "ParquetFile", PF)
+
+    # Resume in first file should continue to next file due rg_idx>=num_row_groups.
+    gen = dataloader._document_batches("train", {"pq_idx": 0, "rg_idx": 3, "epoch": 5}, tokenizer_batch_size=1)
+    b1, state1 = next(gen)
+    assert len(b1) == 1
+    assert state1[0] == 1
+    assert state1[2] >= 5
+
+    # Non-resume and val split.
+    gen2 = dataloader._document_batches("val", None, tokenizer_batch_size=2)
+    b2, state2 = next(gen2)
+    assert len(b2) == 2
+    assert state2[0] == 0
+
+    monkeypatch.setattr(dataloader, "list_parquet_files", lambda: [])
+    with pytest.raises(AssertionError):
+        next(dataloader._document_batches("train", None, tokenizer_batch_size=1))
+
+
+def test_document_batches_resume_cleared_and_epoch_increment(monkeypatch):
+    monkeypatch.setattr(dataloader, "list_parquet_files", lambda: ["/only.parquet", "/val.parquet"])
+    monkeypatch.setattr(dataloader, "get_dist_info", lambda: (True, 0, 0, 2))
+
+    class RG:
+        def __init__(self, vals):
+            self._vals = vals
+
+        def column(self, _name):
+            return types.SimpleNamespace(to_pylist=lambda: self._vals)
+
+    class PF:
+        def __init__(self, _path):
+            self.num_row_groups = 3
+
+        def read_row_group(self, idx):
+            return RG([f"r{idx}"])
+
+    monkeypatch.setattr(dataloader.pq, "ParquetFile", PF)
+    gen = dataloader._document_batches("train", {"pq_idx": 0, "rg_idx": 0, "epoch": 1}, tokenizer_batch_size=1)
+    b1, s1 = next(gen)
+    b2, s2 = next(gen)
+    assert b1 == ["r2"] and s1 == (0, 2, 1)
+    # next epoch yield proves first_pass->False and epoch increment happened
+    assert b2 == ["r0"] and s2[2] >= 2
+
+
+def test_tokenizing_loader_and_wrapper(monkeypatch):
+    class Tok:
+        def get_bos_token_id(self):
+            return 7
+
+        def encode(self, doc_batch, prepend=None, num_threads=None):
+            del num_threads
+            out = []
+            for t in doc_batch:
+                row = [prepend] if prepend is not None else []
+                row.extend([len(t), len(t) + 1, len(t) + 2])
+                out.append(row)
+            return out
+
+    batches = iter([
+        (["a", "bb", "ccc"], (1, 2, 3)),
+        (["dddd", "eeeee", "ffffff"], (4, 5, 6)),
+    ])
+    monkeypatch.setattr(dataloader, "_document_batches", lambda *a, **k: batches)
+
+    gen = dataloader.tokenizing_distributed_data_loader_with_state_bos_bestfit(
+        tokenizer=Tok(),
+        B=2,
+        T=4,
+        split="train",
+        tokenizer_threads=1,
+        tokenizer_batch_size=2,
+        device="cpu",
+        resume_state_dict=None,
+        buffer_size=2,
+    )
+    x, y, st = next(gen)
+    assert x.shape == (2, 4)
+    assert y.shape == (2, 4)
+    assert st == {"pq_idx": 4, "rg_idx": 5, "epoch": 6}
+
+    with pytest.raises(AssertionError):
+        next(
+            dataloader.tokenizing_distributed_data_loader_with_state_bos_bestfit(
+                tokenizer=Tok(),
+                B=1,
+                T=2,
+                split="bad",
+                device="cpu",
+            )
+        )
+
+    batches2 = iter([(["z"], (0, 0, 1))])
+    monkeypatch.setattr(dataloader, "_document_batches", lambda *a, **k: batches2)
+    gen2 = dataloader.tokenizing_distributed_data_loader_bos_bestfit(
+        tokenizer=Tok(),
+        B=1,
+        T=2,
+        split="val",
+        device="cpu",
+        buffer_size=1,
+    )
+    x2, y2 = next(gen2)
+    assert x2.shape == (1, 2)
+    assert y2.shape == (1, 2)
+
+
+def test_download_single_file_cleanup_except_and_dataset_main(monkeypatch, tmp_path):
+    # Cleanup except path (os.remove raises).
+    monkeypatch.setattr(dataset, "DATA_DIR", str(tmp_path))
+    monkeypatch.setattr(dataset, "BASE_URL", "http://example.test")
+    monkeypatch.setattr(dataset, "range", lambda *_a, **_k: [5], raising=False)
+    monkeypatch.setattr(dataset.requests, "get", lambda *_a, **_k: (_ for _ in ()).throw(dataset.requests.RequestException("x")))
+
+    filename = dataset.index_to_filename(9)
+    fp = str(tmp_path / filename)
+    tp = str(tmp_path / f"{filename}.tmp")
+    with open(tp, "w", encoding="utf-8") as f:
+        f.write("y")
+
+    real_exists = dataset.os.path.exists
+    calls = {"fp": 0}
+
+    def fake_exists(path):
+        if path == fp:
+            calls["fp"] += 1
+            return calls["fp"] > 1  # first check in function should be False; cleanup checks True
+        if path == tp:
+            return True
+        return real_exists(path)
+
+    monkeypatch.setattr(dataset.os.path, "exists", fake_exists)
+    monkeypatch.setattr(dataset.os, "remove", lambda _p: (_ for _ in ()).throw(OSError("deny")))
+    assert dataset.download_single_file(9) is False
+
+    # __main__ block coverage.
+    class FakeParser:
+        def __init__(self, *a, **k):
+            pass
+
+        def add_argument(self, *a, **k):
+            return None
+
+        def parse_args(self):
+            return types.SimpleNamespace(num_files=2, num_workers=3)
+
+    class FakePool:
+        def __init__(self, processes):
+            self.processes = processes
+
+        def __enter__(self):
+            return self
+
+        def __exit__(self, exc_type, exc, tb):
+            return False
+
+        def map(self, fn, ids):
+            del fn
+            return [True for _ in ids]
+
+    monkeypatch.setattr("argparse.ArgumentParser", FakeParser)
+    monkeypatch.setattr("multiprocessing.Pool", FakePool)
+    monkeypatch.setenv("NANOCHAT_BASE_DIR", str(tmp_path / "base"))
+    runpy.run_module("nanochat.dataset", run_name="__main__")
diff --git a/tests/test_engine_extra_new.py b/tests/test_engine_extra_new.py
new file mode 100644
index 0000000..bfc7d1a
--- /dev/null
+++ b/tests/test_engine_extra_new.py
@@ -0,0 +1,202 @@
+import runpy
+from contextlib import contextmanager
+from types import SimpleNamespace
+
+import pytest
+import torch
+
+import nanochat.engine as engine
+
+
+def test_timeout_eval_and_calculator_paths(monkeypatch):
+    original_timeout = engine.timeout
+    with engine.timeout(1, "1+1"):
+        assert 1 + 1 == 2
+
+    assert engine.eval_with_timeout("1+2", max_time=1) == 3
+    assert engine.eval_with_timeout("bad +", max_time=1) is None
+
+    @contextmanager
+    def boom(_duration, _formula):
+        raise Exception("t")
+        yield  # pragma: no cover
+
+    monkeypatch.setattr(engine, "timeout", boom)
+    assert engine.eval_with_timeout("1+1", max_time=1) is None
+    monkeypatch.setattr(engine, "timeout", original_timeout)
+
+    assert engine.use_calculator("1,000 + 2") == 1002
+    assert engine.use_calculator("2 ** 8") is None
+    assert engine.use_calculator("x$y") is None
+    assert engine.use_calculator("__import__('os')") is None
+    assert engine.use_calculator("'abc'") is None
+    assert engine.use_calculator("'ababa'.count('a')") == 3
+
+
+def test_timeout_handler_line(monkeypatch):
+    captured = {}
+    monkeypatch.setattr(engine.signal, "signal", lambda _sig, fn: captured.__setitem__("fn", fn))
+    monkeypatch.setattr(engine.signal, "alarm", lambda _n: None)
+    with pytest.raises(Exception):
+        with engine.timeout(1, "slow_expr"):
+            captured["fn"](0, None)
+
+
+def test_sample_next_token_branches():
+    logits = torch.tensor([[0.1, 0.2, 0.9]], dtype=torch.float32)
+    rng = torch.Generator(device="cpu").manual_seed(123)
+
+    out0 = engine.sample_next_token(logits, rng, temperature=0.0, top_k=None)
+    assert out0.shape == (1, 1)
+    assert out0.item() == 2
+
+    out1 = engine.sample_next_token(logits, rng, temperature=1.0, top_k=2)
+    assert out1.shape == (1, 1)
+    assert out1.item() in {1, 2}
+
+    out2 = engine.sample_next_token(logits, rng, temperature=1.0, top_k=None)
+    assert out2.shape == (1, 1)
+
+    with pytest.raises(AssertionError):
+        engine.sample_next_token(logits, rng, temperature=-1.0)
+
+
+def test_kv_cache_prefill_assertions():
+    a = engine.KVCache(1, 1, 4, 2, 1, "cpu", torch.float32)
+    b = engine.KVCache(1, 1, 2, 2, 1, "cpu", torch.float32)
+    b.advance(1)
+    a.advance(1)
+    with pytest.raises(AssertionError):
+        a.prefill(b)  # non-empty destination
+
+    c = engine.KVCache(1, 2, 2, 2, 1, "cpu", torch.float32)
+    d = engine.KVCache(1, 1, 2, 2, 1, "cpu", torch.float32)
+    with pytest.raises(AssertionError):
+        c.prefill(d)  # head mismatch
+
+    e = engine.KVCache(1, 1, 1, 2, 1, "cpu", torch.float32)
+    f = engine.KVCache(1, 1, 2, 2, 1, "cpu", torch.float32)
+    with pytest.raises(AssertionError):
+        e.prefill(f)  # seq len too small
+
+
+class _TinyTok:
+    def __init__(self):
+        self.special = {
+            "<|python_start|>": 100,
+            "<|python_end|>": 101,
+            "<|output_start|>": 102,
+            "<|output_end|>": 103,
+            "<|assistant_end|>": 104,
+            "<|bos|>": 0,
+        }
+
+    def encode_special(self, s):
+        return self.special[s]
+
+    def get_bos_token_id(self):
+        return 0
+
+    def encode(self, s, prepend=None):
+        out = [9] if s == "2" else [55]
+        if prepend is not None:
+            return [prepend] + out
+        return out
+
+    def decode(self, ids):
+        return "1+1" if ids else ""
+
+
+class _TinyModel:
+    def __init__(self):
+        self.config = SimpleNamespace(n_kv_head=1, n_head=1, n_embd=4, n_layer=1, sequence_len=32)
+        self._device = torch.device("cpu")
+        self.vocab = 200
+
+    def get_device(self):
+        return self._device
+
+    def forward(self, ids, kv_cache=None):
+        b, t = ids.shape
+        if kv_cache is not None:
+            kv_cache.advance(t)
+        return torch.zeros((b, t, self.vocab), dtype=torch.float32)
+
+
+def test_engine_generate_tool_forcing(monkeypatch):
+    tok = _TinyTok()
+    model = _TinyModel()
+    eng = engine.Engine(model, tok)
+
+    sampled = [
+        [100],  # python_start
+        [55],   # expression token
+        [101],  # python_end -> enqueue forced output tokens
+        [104],  # ignored while forced queue drains
+        [104],
+        [104],
+        [104],  # finally consumed as assistant_end
+    ]
+    idx = {"i": 0}
+
+    def fake_sample(_logits, _rng, _temperature, _top_k):
+        row = sampled[min(idx["i"], len(sampled) - 1)]
+        idx["i"] += 1
+        return torch.tensor([[row[0]]], dtype=torch.long)
+
+    calls = {"expr": None}
+    monkeypatch.setattr(engine, "sample_next_token", fake_sample)
+    monkeypatch.setattr(engine, "use_calculator", lambda expr: calls.__setitem__("expr", expr) or 2)
+
+    rows = list(eng.generate([1, 2], num_samples=1, max_tokens=10, temperature=0.0))
+    flat_tokens = [r[0][0] for r in rows]
+    flat_masks = [r[1][0] for r in rows]
+    assert 102 in flat_tokens and 103 in flat_tokens  # output_start/output_end forced
+    assert 0 in flat_masks  # forced token mask
+    assert calls["expr"] == "1+1"
+
+
+def test_engine_generate_batch_and_input_validation(monkeypatch):
+    tok = _TinyTok()
+    model = _TinyModel()
+    eng = engine.Engine(model, tok)
+
+    # Stop immediately via BOS; also exercises max_tokens=None kv-length hint path.
+    monkeypatch.setattr(
+        engine,
+        "sample_next_token",
+        lambda *_a, **_k: torch.tensor([[tok.get_bos_token_id()]], dtype=torch.long),
+    )
+    rows = list(eng.generate([7], num_samples=1, max_tokens=None, temperature=0.0))
+    assert len(rows) == 1
+
+    results, masks = eng.generate_batch([7], num_samples=1, max_tokens=3, temperature=0.0)
+    assert results == [[7]]
+    assert masks == [[0]]
+
+    with pytest.raises(AssertionError):
+        list(eng.generate("bad", num_samples=1, max_tokens=1))  # type: ignore[arg-type]
+
+
+def test_engine_main_block_runs(monkeypatch):
+    # Patch external entrypoints used by the __main__ block.
+    monkeypatch.setattr("nanochat.common.autodetect_device_type", lambda: "cpu")
+    monkeypatch.setattr("nanochat.common.compute_init", lambda _d: (False, 0, 0, 1, torch.device("cpu")))
+
+    class MainTok(_TinyTok):
+        def encode(self, s, prepend=None):
+            out = [11, 12]
+            return ([prepend] + out) if prepend is not None else out
+
+        def decode(self, ids):
+            return "x"
+
+    class MainModel(_TinyModel):
+        def generate(self, tokens, max_tokens, temperature=1.0, top_k=None, seed=42):
+            del tokens, max_tokens, temperature, top_k, seed
+            yield 11
+            yield 12
+
+    monkeypatch.setattr("nanochat.checkpoint_manager.load_model", lambda *a, **k: (MainModel(), MainTok(), {}))
+    monkeypatch.setattr(engine.torch.cuda, "synchronize", lambda: None)
+    runpy.run_module("nanochat.engine", run_name="__main__")
diff --git a/tests/test_execution_new.py b/tests/test_execution_new.py
new file mode 100644
index 0000000..9f0db20
--- /dev/null
+++ b/tests/test_execution_new.py
@@ -0,0 +1,235 @@
+import builtins
+import io
+import os
+from contextlib import contextmanager
+
+import pytest
+
+import nanochat.execution as execution
+
+
+def test_execution_result_repr_variants():
+    r = execution.ExecutionResult(success=True, stdout="ok\n", stderr="")
+    s = repr(r)
+    assert "success=True" in s
+    assert "stdout='ok\\n'" in s
+
+    r2 = execution.ExecutionResult(success=False, stdout="", stderr="err", error="bad", timeout=True, memory_exceeded=True)
+    s2 = repr(r2)
+    assert "timeout=True" in s2
+    assert "memory_exceeded=True" in s2
+    assert "error='bad'" in s2
+    assert "stderr='err'" in s2
+
+
+def test_write_only_string_io_and_capture_io():
+    s = execution.WriteOnlyStringIO()
+    assert s.readable() is False
+    with pytest.raises(IOError):
+        s.read()
+    with pytest.raises(IOError):
+        s.readline()
+    with pytest.raises(IOError):
+        s.readlines()
+
+    with execution.capture_io() as (out, err):
+        print("hello")
+    assert "hello" in out.getvalue()
+    assert err.getvalue() == ""
+
+
+def test_time_limit_and_chdir_and_tempdir(tmp_path):
+    with execution.time_limit(0.5):
+        x = 1 + 1
+    assert x == 2
+
+    with pytest.raises(execution.TimeoutException):
+        with execution.time_limit(0.01):
+            import time
+            time.sleep(0.05)
+
+    cwd = os.getcwd()
+    with execution.chdir("."):
+        assert os.getcwd() == cwd
+    with execution.chdir(str(tmp_path)):
+        assert os.getcwd() == str(tmp_path)
+    assert os.getcwd() == cwd
+
+    with execution.create_tempdir() as d:
+        assert os.path.isdir(d)
+    assert not os.path.exists(d)
+
+
+def test_reliability_guard(monkeypatch):
+    # Snapshot a small subset we assert was modified/restored in test cleanup.
+    old_exit = builtins.exit
+    old_quit = builtins.quit
+
+    class FakePlatform:
+        @staticmethod
+        def uname():
+            class U:
+                system = "Linux"
+            return U()
+
+    calls = {"rlimit": 0, "disabled": 0}
+    monkeypatch.setattr(execution, "platform", FakePlatform())
+
+    class FakeResource:
+        RLIMIT_AS = 1
+        RLIMIT_DATA = 2
+        RLIMIT_STACK = 3
+
+        @staticmethod
+        def setrlimit(_k, _v):
+            calls["rlimit"] += 1
+
+    import sys
+    keys = ["ipdb", "joblib", "resource", "psutil", "tkinter"]
+    old_modules = {k: sys.modules.get(k, None) for k in keys}
+
+    class FakeOS:
+        def __init__(self):
+            self.environ = {}
+            self.kill = lambda *a, **k: None
+            self.system = lambda *a, **k: None
+            self.putenv = lambda *a, **k: None
+            self.remove = lambda *a, **k: None
+            self.removedirs = lambda *a, **k: None
+            self.rmdir = lambda *a, **k: None
+            self.fchdir = lambda *a, **k: None
+            self.setuid = lambda *a, **k: None
+            self.fork = lambda *a, **k: None
+            self.forkpty = lambda *a, **k: None
+            self.killpg = lambda *a, **k: None
+            self.rename = lambda *a, **k: None
+            self.renames = lambda *a, **k: None
+            self.truncate = lambda *a, **k: None
+            self.replace = lambda *a, **k: None
+            self.unlink = lambda *a, **k: None
+            self.fchmod = lambda *a, **k: None
+            self.fchown = lambda *a, **k: None
+            self.chmod = lambda *a, **k: None
+            self.chown = lambda *a, **k: None
+            self.chroot = lambda *a, **k: None
+            self.lchflags = lambda *a, **k: None
+            self.lchmod = lambda *a, **k: None
+            self.lchown = lambda *a, **k: None
+            self.getcwd = lambda: "."
+            self.chdir = lambda _p: None
+
+    class FakeShutil:
+        def __init__(self):
+            self.rmtree = lambda *a, **k: None
+            self.move = lambda *a, **k: None
+            self.chown = lambda *a, **k: None
+
+    class FakeSubprocess:
+        def __init__(self):
+            self.Popen = object
+
+    monkeypatch.setitem(sys.modules, "resource", FakeResource)
+    monkeypatch.setitem(sys.modules, "os", FakeOS())
+    monkeypatch.setitem(sys.modules, "shutil", FakeShutil())
+    monkeypatch.setitem(sys.modules, "subprocess", FakeSubprocess())
+    monkeypatch.setattr(execution.faulthandler, "disable", lambda: calls.__setitem__("disabled", 1))
+    execution.reliability_guard(1024)
+    assert calls["rlimit"] == 3
+    assert calls["disabled"] == 1
+    assert builtins.exit is None
+    assert builtins.quit is None
+
+    # Restore minimal globals so current test process remains healthy.
+    builtins.exit = old_exit
+    builtins.quit = old_quit
+    if isinstance(__builtins__, dict):
+        __builtins__["help"] = help
+    else:
+        setattr(__builtins__, "help", help)
+    for k, v in old_modules.items():
+        if v is None:
+            sys.modules.pop(k, None)
+        else:
+            sys.modules[k] = v
+
+
+def test_unsafe_execute_success_and_error_paths(monkeypatch):
+    # Avoid mutating process-wide dangerous globals during direct _unsafe_execute calls.
+    monkeypatch.setattr(execution, "reliability_guard", lambda maximum_memory_bytes=None: None)
+
+    out = {}
+    execution._unsafe_execute("print('hi')", timeout=1.0, maximum_memory_bytes=None, result_dict=out)
+    assert out["success"] is True
+    assert "hi" in out["stdout"]
+    assert out["stderr"] == ""
+
+    out2 = {}
+    execution._unsafe_execute("raise MemoryError('oom')", timeout=1.0, maximum_memory_bytes=None, result_dict=out2)
+    assert out2["memory_exceeded"] is True
+    assert "Memory limit exceeded" in out2["error"]
+
+    out3 = {}
+    execution._unsafe_execute("raise ValueError('bad')", timeout=1.0, maximum_memory_bytes=None, result_dict=out3)
+    assert out3["success"] is False
+    assert "ValueError: bad" in out3["error"]
+
+    @contextmanager
+    def boom(_seconds):
+        raise execution.TimeoutException("Timed out!")
+        yield  # pragma: no cover
+
+    monkeypatch.setattr(execution, "time_limit", boom)
+    out4 = {}
+    execution._unsafe_execute("print('x')", timeout=1.0, maximum_memory_bytes=None, result_dict=out4)
+    assert out4["timeout"] is True
+    assert out4["error"] == "Execution timed out"
+
+
+def test_execute_code_paths(monkeypatch):
+    # Normal path with real machinery.
+    r = execution.execute_code("print('ok')", timeout=1.0)
+    assert r.success is True
+    assert "ok" in r.stdout
+
+    # p.is_alive() path.
+    class FakeDict(dict):
+        pass
+
+    class FakeManager:
+        def dict(self):
+            return FakeDict()
+
+    class FakeProcessAlive:
+        def __init__(self, target, args):
+            self.target = target
+            self.args = args
+            self.killed = False
+
+        def start(self):
+            return None
+
+        def join(self, timeout):
+            del timeout
+            return None
+
+        def is_alive(self):
+            return True
+
+        def kill(self):
+            self.killed = True
+
+    monkeypatch.setattr(execution.multiprocessing, "Manager", lambda: FakeManager())
+    monkeypatch.setattr(execution.multiprocessing, "Process", FakeProcessAlive)
+    r2 = execution.execute_code("print(1)")
+    assert r2.timeout is True
+    assert "process killed" in r2.error
+
+    # Empty result_dict path.
+    class FakeProcessDone(FakeProcessAlive):
+        def is_alive(self):
+            return False
+
+    monkeypatch.setattr(execution.multiprocessing, "Process", FakeProcessDone)
+    r3 = execution.execute_code("print(2)")
+    assert r3.success is False
+    assert "no result returned" in r3.error
diff --git a/tests/test_flash_attention_new.py b/tests/test_flash_attention_new.py
new file mode 100644
index 0000000..028496f
--- /dev/null
+++ b/tests/test_flash_attention_new.py
@@ -0,0 +1,110 @@
+import sys
+import types
+
+import pytest
+import torch
+
+import nanochat.flash_attention as fa
+
+
+def test_load_flash_attention_3_paths(monkeypatch):
+    monkeypatch.setattr(fa.torch.cuda, "is_available", lambda: False)
+    assert fa._load_flash_attention_3() is None
+
+    monkeypatch.setattr(fa.torch.cuda, "is_available", lambda: True)
+    monkeypatch.setattr(fa.torch.cuda, "get_device_capability", lambda: (8, 0))
+    assert fa._load_flash_attention_3() is None
+
+    monkeypatch.setattr(fa.torch.cuda, "get_device_capability", lambda: (9, 0))
+    monkeypatch.setitem(sys.modules, "kernels", None)
+    assert fa._load_flash_attention_3() is None
+
+    class K:
+        @staticmethod
+        def get_kernel(_name):
+            return types.SimpleNamespace(flash_attn_interface="iface")
+
+    monkeypatch.setitem(sys.modules, "kernels", K)
+    assert fa._load_flash_attention_3() == "iface"
+
+
+def test_use_fa3_override(monkeypatch):
+    monkeypatch.setattr(fa, "HAS_FA3", False)
+    monkeypatch.setattr(fa, "_override_impl", "fa3")
+    with pytest.raises(AssertionError):
+        fa._use_fa3()
+
+    monkeypatch.setattr(fa, "_override_impl", "sdpa")
+    assert fa._use_fa3() is False
+
+    monkeypatch.setattr(fa, "_override_impl", None)
+    monkeypatch.setattr(fa, "HAS_FA3", True)
+    assert fa._use_fa3() is True
+
+
+def test_sdpa_attention_branches():
+    q = torch.randn(1, 2, 4, 3)
+    k = torch.randn(1, 2, 4, 3)
+    v = torch.randn(1, 2, 4, 3)
+    y1 = fa._sdpa_attention(q, k, v, window_size=(-1, -1), enable_gqa=False)
+    assert y1.shape == q.shape
+
+    q2 = torch.randn(1, 2, 1, 3)
+    k2 = torch.randn(1, 2, 6, 3)
+    v2 = torch.randn(1, 2, 6, 3)
+    y2 = fa._sdpa_attention(q2, k2, v2, window_size=(2, 0), enable_gqa=False)
+    assert y2.shape == q2.shape
+
+    q3 = torch.randn(1, 2, 3, 3)
+    k3 = torch.randn(1, 2, 6, 3)
+    v3 = torch.randn(1, 2, 6, 3)
+    y3 = fa._sdpa_attention(q3, k3, v3, window_size=(3, 0), enable_gqa=False)
+    assert y3.shape == q3.shape
+
+
+def test_public_flash_attn_paths(monkeypatch):
+    q = torch.randn(1, 3, 2, 4)
+    k = torch.randn(1, 3, 2, 4)
+    v = torch.randn(1, 3, 2, 4)
+
+    # FA3 path.
+    class FakeFA3:
+        @staticmethod
+        def flash_attn_func(q, k, v, causal, window_size):
+            del k, v, causal, window_size
+            return q + 1
+
+        @staticmethod
+        def flash_attn_with_kvcache(q, k_cache, v_cache, k=None, v=None, cache_seqlens=None, causal=False, window_size=(-1, -1)):
+            del k_cache, v_cache, k, v, cache_seqlens, causal, window_size
+            return q + 2
+
+    monkeypatch.setattr(fa, "_fa3", FakeFA3())
+    monkeypatch.setattr(fa, "HAS_FA3", True)
+    monkeypatch.setattr(fa, "_override_impl", "fa3")
+    out1 = fa.flash_attn_func(q, k, v, causal=True, window_size=(3, 0))
+    assert torch.allclose(out1, q + 1)
+
+    k_cache = torch.zeros(1, 8, 2, 4)
+    v_cache = torch.zeros(1, 8, 2, 4)
+    cache_seqlens = torch.zeros(1, dtype=torch.int32)
+    out2 = fa.flash_attn_with_kvcache(q[:, :1], k_cache, v_cache, k=k[:, :1], v=v[:, :1], cache_seqlens=cache_seqlens, causal=True, window_size=(3, 0))
+    assert torch.allclose(out2, q[:, :1] + 2)
+
+    # SDPA path with cache insert/update.
+    monkeypatch.setattr(fa, "_override_impl", "sdpa")
+    out3 = fa.flash_attn_func(q, k, v, causal=True, window_size=(3, 0))
+    assert out3.shape == q.shape
+
+    out4 = fa.flash_attn_with_kvcache(
+        q[:, :1],
+        k_cache,
+        v_cache,
+        k=k[:, :1],
+        v=v[:, :1],
+        cache_seqlens=cache_seqlens,
+        causal=True,
+        window_size=(2, 0),
+    )
+    assert out4.shape == q[:, :1].shape
+
diff --git a/tests/test_fp8_new.py b/tests/test_fp8_new.py
new file mode 100644
index 0000000..733531c
--- /dev/null
+++ b/tests/test_fp8_new.py
@@ -0,0 +1,83 @@
+import torch
+import torch.nn as nn
+import pytest
+
+import nanochat.fp8 as fp8
+
+
+def test_to_fp8_and_col_major():
+    x = torch.tensor([[0.0, 1.0], [2.0, -3.0]], dtype=torch.float32)
+    x_fp8, inv = fp8._to_fp8(x, torch.float8_e4m3fn)
+    assert x_fp8.dtype == torch.float8_e4m3fn
+    assert inv.ndim == 0
+
+    cm = fp8._to_col_major(torch.arange(6, dtype=torch.float32).view(2, 3))
+    assert cm.shape == (2, 3)
+    assert torch.equal(cm, torch.arange(6, dtype=torch.float32).view(2, 3))
+
+
+def test_float8_matmul_forward_backward(monkeypatch):
+    def fake_scaled_mm(a, b, scale_a=None, scale_b=None, out_dtype=None, use_fast_accum=None):
+        del scale_a, scale_b, use_fast_accum
+        out = a.float() @ b.float()
+        return out.to(out_dtype if out_dtype is not None else out.dtype)
+
+    monkeypatch.setattr(fp8.torch, "_scaled_mm", fake_scaled_mm)
+
+    x = torch.randn(3, 4, dtype=torch.float32, requires_grad=True)
+    w = torch.randn(5, 4, dtype=torch.float32, requires_grad=True)
+    y = fp8._Float8Matmul.apply(x, w)
+    assert y.shape == (3, 5)
+    y.sum().backward()
+    assert x.grad is not None
+    assert w.grad is not None
+
+
+def test_float8_linear_and_config(monkeypatch):
+    monkeypatch.setattr(fp8._Float8Matmul, "apply", lambda input_2d, weight: input_2d.float() @ weight.float().t())
+    layer = fp8.Float8Linear(4, 3, bias=True)
+    inp = torch.randn(2, 5, 4)
+    out = layer(inp)
+    assert out.shape == (2, 5, 3)
+
+    # Autocast branch.
+    monkeypatch.setattr(fp8.torch, "is_autocast_enabled", lambda: True)
+    monkeypatch.setattr(fp8.torch, "get_autocast_gpu_dtype", lambda: torch.float16)
+    out2 = layer(inp)
+    assert out2.shape == (2, 5, 3)
+
+    layer2 = fp8.Float8Linear(4, 3, bias=False)
+    out3 = layer2(inp)
+    assert out3.shape == (2, 5, 3)
+
+    src = nn.Linear(4, 3, bias=True)
+    converted = fp8.Float8Linear.from_float(src)
+    assert isinstance(converted, fp8.Float8Linear)
+    assert converted.weight is src.weight
+    assert converted.bias is src.bias
+
+    assert isinstance(fp8.Float8LinearConfig.from_recipe_name("tensorwise"), fp8.Float8LinearConfig)
+    with pytest.raises(ValueError):
+        fp8.Float8LinearConfig.from_recipe_name("rowwise")
+
+
+def test_convert_to_float8_training(monkeypatch):
+    monkeypatch.setattr(fp8.Float8Linear, "from_float", classmethod(lambda cls, mod: cls(mod.in_features, mod.out_features, bias=(mod.bias is not None))))
+
+    m = nn.Sequential(
+        nn.Linear(4, 4),
+        nn.ReLU(),
+        nn.Sequential(nn.Linear(4, 2), nn.Linear(2, 1)),
+    )
+    out = fp8.convert_to_float8_training(m)
+    assert isinstance(out[0], fp8.Float8Linear)
+    assert isinstance(out[2][0], fp8.Float8Linear)
+    assert isinstance(out[2][1], fp8.Float8Linear)
+
+    m2 = nn.Sequential(nn.Linear(4, 4), nn.Linear(4, 4))
+    out2 = fp8.convert_to_float8_training(
+        m2,
+        module_filter_fn=lambda _mod, fqn: fqn.endswith("0"),
+    )
+    assert isinstance(out2[0], fp8.Float8Linear)
+    assert isinstance(out2[1], nn.Linear)
diff --git a/tests/test_gpt_new.py b/tests/test_gpt_new.py
new file mode 100644
index 0000000..9cb3cad
--- /dev/null
+++ b/tests/test_gpt_new.py
@@ -0,0 +1,242 @@
+from types import SimpleNamespace
+
+import pytest
+import torch
+
+import nanochat.gpt as gpt
+
+
+def _cfg(**overrides):
+    base = dict(sequence_len=8, vocab_size=13, n_layer=2, n_head=2, n_kv_head=1, n_embd=4, window_pattern="SL")
+    base.update(overrides)
+    return gpt.GPTConfig(**base)
+
+
+def test_norm_has_ve_and_rotary():
+    x = torch.randn(2, 3, 4)
+    y = gpt.norm(x)
+    assert y.shape == x.shape
+    assert gpt.has_ve(1, 4) is True
+    assert gpt.has_ve(0, 4) is False
+
+    h = torch.randn(1, 2, 2, 4)
+    cos = torch.ones(1, 2, 1, 2)
+    sin = torch.zeros(1, 2, 1, 2)
+    out = gpt.apply_rotary_emb(h, cos, sin)
+    assert out.shape == h.shape
+
+
+def test_attention_forward_paths(monkeypatch):
+    cfg = _cfg(n_layer=1)
+    attn = gpt.CausalSelfAttention(cfg, layer_idx=0)
+
+    calls = {"func": 0, "kvcache": 0}
+
+    def fake_func(q, k, v, causal, window_size):
+        del k, v, causal, window_size
+        calls["func"] += 1
+        return q
+
+    def fake_kvcache(q, k_cache, v_cache, k=None, v=None, cache_seqlens=None, causal=False, window_size=(-1, -1)):
+        del k_cache, v_cache, k, v, cache_seqlens, causal, window_size
+        calls["kvcache"] += 1
+        return q
+
+    monkeypatch.setattr(gpt.flash_attn, "flash_attn_func", fake_func)
+    monkeypatch.setattr(gpt.flash_attn, "flash_attn_with_kvcache", fake_kvcache)
+
+    x = torch.randn(2, 3, cfg.n_embd)
+    cos = torch.ones(1, 3, 1, cfg.n_embd // cfg.n_head // 2)
+    sin = torch.zeros_like(cos)
+    y = attn(x, ve=None, cos_sin=(cos, sin), window_size=(cfg.sequence_len, 0), kv_cache=None)
+    assert y.shape == x.shape
+    assert calls["func"] == 1
+
+    class Cache:
+        def __init__(self):
+            self.n_layers = 1
+            self.cache_seqlens = torch.zeros(2, dtype=torch.int32)
+            self.k = torch.zeros(2, 10, cfg.n_kv_head, cfg.n_embd // cfg.n_head)
+            self.v = torch.zeros(2, 10, cfg.n_kv_head, cfg.n_embd // cfg.n_head)
+            self.advanced = 0
+
+        def get_layer_cache(self, _idx):
+            return self.k, self.v
+
+        def advance(self, t):
+            self.advanced += t
+
+    ve = torch.randn(2, 3, cfg.n_kv_head * (cfg.n_embd // cfg.n_head))
+    attn.ve_gate_channels = cfg.n_embd
+    attn.ve_gate = torch.nn.Linear(cfg.n_embd, cfg.n_kv_head, bias=False)
+    cache = Cache()
+    y2 = attn(x, ve=ve, cos_sin=(cos, sin), window_size=(cfg.sequence_len, 0), kv_cache=cache)
+    assert y2.shape == x.shape
+    assert calls["kvcache"] == 1
+    assert cache.advanced == 3
+
+    with pytest.raises(AssertionError):
+        gpt.CausalSelfAttention(_cfg(n_embd=5, n_head=2), layer_idx=0)
+    with pytest.raises(AssertionError):
+        gpt.CausalSelfAttention(_cfg(n_head=3, n_kv_head=2), layer_idx=0)
+
+
+def test_mlp_and_block(monkeypatch):
+    cfg = _cfg(n_layer=1)
+    mlp = gpt.MLP(cfg)
+    x = torch.randn(2, 3, cfg.n_embd)
+    y = mlp(x)
+    assert y.shape == x.shape
+
+    monkeypatch.setattr(gpt.flash_attn, "flash_attn_func", lambda q, k, v, causal, window_size: q)
+    block = gpt.Block(cfg, layer_idx=0)
+    cos = torch.ones(1, 3, 1, cfg.n_embd // cfg.n_head // 2)
+    sin = torch.zeros_like(cos)
+    out = block(x, ve=None, cos_sin=(cos, sin), window_size=(cfg.sequence_len, 0), kv_cache=None)
+    assert out.shape == x.shape
+
+
+def test_gpt_core_helpers_and_forward(monkeypatch):
+    monkeypatch.setattr(gpt, "print0", lambda *a, **k: None)
+    monkeypatch.setattr(gpt.flash_attn, "flash_attn_func", lambda q, k, v, causal, window_size: q)
+    monkeypatch.setattr(gpt.flash_attn, "flash_attn_with_kvcache", lambda q, k_cache, v_cache, **kw: q)
+
+    model = gpt.GPT(_cfg(), pad_vocab_size_to=8)
+    model.init_weights()
+    for block in model.transformer.h:
+        if block.attn.ve_gate is not None:
+            block.attn.ve_gate_channels = model.config.n_embd
+            block.attn.ve_gate = torch.nn.Linear(model.config.n_embd, model.config.n_kv_head, bias=False)
+
+    ws = model._compute_window_sizes(_cfg(window_pattern="SLS", n_layer=4))
+    assert len(ws) == 4
+    assert ws[-1][0] == 8
+    with pytest.raises(AssertionError):
+        model._compute_window_sizes(_cfg(window_pattern="Q"))
+
+    cos, sin = model._precompute_rotary_embeddings(4, 2, base=1000, device=torch.device("cpu"))
+    assert cos.shape == sin.shape
+
+    assert model.get_device().type == "cpu"
+    assert model.estimate_flops() > 0
+    counts = model.num_scaling_params()
+    assert counts["total"] == sum(p.numel() for p in model.parameters())
+
+    idx = torch.randint(0, model.config.vocab_size, (2, 4), dtype=torch.long)
+    logits = model.forward(idx)
+    assert logits.shape == (2, 4, model.config.vocab_size)
+
+    targets = torch.randint(0, model.config.vocab_size, (2, 4), dtype=torch.long)
+    loss = model.forward(idx, targets=targets, loss_reduction="mean")
+    assert loss.ndim == 0
+
+    with pytest.raises(AssertionError):
+        model.forward(torch.randint(0, model.config.vocab_size, (1, model.cos.size(1) + 1)))
+
+
+def test_setup_optimizer_paths(monkeypatch):
+    model = gpt.GPT(_cfg(), pad_vocab_size_to=8)
+
+    class FakeOpt:
+        def __init__(self, groups):
+            self.param_groups = groups
+
+    monkeypatch.setattr(gpt, "print0", lambda *a, **k: None)
+    monkeypatch.setattr(gpt, "MuonAdamW", FakeOpt)
+    monkeypatch.setattr(gpt, "DistMuonAdamW", FakeOpt)
+
+    monkeypatch.setattr(gpt, "get_dist_info", lambda: (False, 0, 0, 1))
+    opt1 = model.setup_optimizer()
+    assert isinstance(opt1, FakeOpt)
+    assert all("initial_lr" in g for g in opt1.param_groups)
+
+    monkeypatch.setattr(gpt, "get_dist_info", lambda: (True, 0, 0, 2))
+    opt2 = model.setup_optimizer()
+    assert isinstance(opt2, FakeOpt)
+
+
+def test_generate_paths(monkeypatch):
+    model = gpt.GPT(_cfg(), pad_vocab_size_to=8)
+
+    # Force predictable logits.
+    def fake_forward(ids, *args, **kwargs):
+        b, t = ids.shape
+        v = model.config.vocab_size
+        logits = torch.zeros((b, t, v), dtype=torch.float32)
+        logits[..., 2] = 2.0
+        logits[..., 3] = 1.0
+        return logits
+
+    monkeypatch.setattr(model, "forward", fake_forward)
+    tokens = [1, 2]
+    out_temp0 = list(model.generate(tokens, max_tokens=3, temperature=0.0, top_k=None, seed=42))
+    assert out_temp0 == [2, 2, 2]
+
+    out_topk = list(model.generate(tokens, max_tokens=2, temperature=1.0, top_k=1, seed=42))
+    assert out_topk == [2, 2]
+
+    with pytest.raises(AssertionError):
+        list(model.generate("bad", max_tokens=1))  # type: ignore[arg-type]
+
+
+def test_init_weights_cuda_cast_branch_with_fake_self(monkeypatch):
+    # This executes the CUDA-only cast lines in init_weights without requiring a real CUDA device.
+    class FakeTensor:
+        def __init__(self):
+            self.device = SimpleNamespace(type="cuda")
+
+    class FakeParam:
+        def __init__(self):
+            self.weight = FakeTensor()
+            self.to_calls = 0
+
+        def to(self, dtype):
+            del dtype
+            self.to_calls += 1
+
+    class FakeAttn:
+        def __init__(self):
+            self.c_q = SimpleNamespace(weight=object())
+            self.c_k = SimpleNamespace(weight=object())
+            self.c_v = SimpleNamespace(weight=object())
+            self.c_proj = SimpleNamespace(weight=object())
+            self.ve_gate = SimpleNamespace(weight=object())
+
+    class FakeMLP:
+        def __init__(self):
+            self.c_fc = SimpleNamespace(weight=object())
+            self.c_proj = SimpleNamespace(weight=object())
+
+    class FakeBlock:
+        def __init__(self):
+            self.attn = FakeAttn()
+            self.mlp = FakeMLP()
+
+    class Fillable:
+        def fill_(self, _v):
+            return self
+
+        def numel(self):
+            return 1
+
+    fake_self = SimpleNamespace()
+    fake_self.config = _cfg()
+    fake_self.rotary_seq_len = 8
+    fake_self.transformer = SimpleNamespace(
+        wte=FakeParam(),
+        h=[FakeBlock(), FakeBlock()],
+    )
+    fake_self.lm_head = SimpleNamespace(weight=object())
+    fake_self.resid_lambdas = Fillable()
+    fake_self.x0_lambdas = Fillable()
+    ve0 = FakeParam()
+    fake_self.value_embeds = {"0": ve0}
+    fake_self._precompute_rotary_embeddings = lambda *a, **k: (torch.zeros(1, 1, 1, 1, dtype=torch.bfloat16), torch.zeros(1, 1, 1, 1, dtype=torch.bfloat16))
+
+    monkeypatch.setattr(gpt.torch.nn.init, "normal_", lambda *a, **k: None)
+    monkeypatch.setattr(gpt.torch.nn.init, "uniform_", lambda *a, **k: None)
+    monkeypatch.setattr(gpt.torch.nn.init, "zeros_", lambda *a, **k: None)
+
+    gpt.GPT.init_weights(fake_self)
+    assert fake_self.transformer.wte.to_calls == 1
+    assert ve0.to_calls == 1
diff --git a/tests/test_loss_eval_new.py b/tests/test_loss_eval_new.py
new file mode 100644
index 0000000..005455e
--- /dev/null
+++ b/tests/test_loss_eval_new.py
@@ -0,0 +1,56 @@
+import math
+
+import torch
+
+import nanochat.loss_eval as loss_eval
+
+
+class FakeModel:
+    def get_device(self):
+        return torch.device("cpu")
+
+    def __call__(self, x, y, loss_reduction="none"):
+        del x, loss_reduction
+        # Return a deterministic per-token loss tensor matching y shape.
+        return torch.ones_like(y, dtype=torch.float32)
+
+
+def test_evaluate_bpb_paths(monkeypatch):
+    model = FakeModel()
+    token_bytes = torch.tensor([0, 1, 2, 3, 4], dtype=torch.int64)
+
+    # Path with ignored targets (<0).
+    batches1 = [
+        (
+            torch.tensor([[0, 1], [2, 3]], dtype=torch.long),
+            torch.tensor([[1, -1], [2, 0]], dtype=torch.long),
+        )
+    ]
+    monkeypatch.setattr(loss_eval.dist, "is_initialized", lambda: False)
+    out1 = loss_eval.evaluate_bpb(model, batches1, steps=1, token_bytes=token_bytes)
+    assert out1 > 0.0
+
+    # Fast path without ignored targets.
+    batches2 = [
+        (
+            torch.tensor([[0, 1]], dtype=torch.long),
+            torch.tensor([[3, 4]], dtype=torch.long),
+        )
+    ]
+    out2 = loss_eval.evaluate_bpb(model, batches2, steps=1, token_bytes=token_bytes)
+    assert out2 > 0.0
+
+    # Distributed reduction path.
+    calls = {"n": 0}
+    monkeypatch.setattr(loss_eval.dist, "is_initialized", lambda: True)
+    monkeypatch.setattr(loss_eval.dist, "get_world_size", lambda: 2)
+    monkeypatch.setattr(loss_eval.dist, "all_reduce", lambda *_a, **_k: calls.__setitem__("n", calls["n"] + 1))
+    out3 = loss_eval.evaluate_bpb(model, batches2, steps=1, token_bytes=token_bytes)
+    assert out3 > 0.0
+    assert calls["n"] == 2
+
+    # No counted bytes -> inf.
+    zero_bytes = torch.zeros_like(token_bytes)
+    out4 = loss_eval.evaluate_bpb(model, batches2, steps=1, token_bytes=zero_bytes)
+    assert out4 == float("inf")
+
diff --git a/tests/test_optim_new.py b/tests/test_optim_new.py
new file mode 100644
index 0000000..91ad56c
--- /dev/null
+++ b/tests/test_optim_new.py
@@ -0,0 +1,229 @@
+from types import SimpleNamespace
+
+import pytest
+import torch
+
+import nanochat.optim as optim
+
+
+def test_fused_step_functions_via_wrapped():
+    # AdamW fused kernel.
+    p = torch.ones(2, 2, dtype=torch.float32)
+    g = torch.full((2, 2), 0.1, dtype=torch.float32)
+    exp_avg = torch.zeros_like(p)
+    exp_avg_sq = torch.zeros_like(p)
+    optim.adamw_step_fused.__wrapped__(
+        p,
+        g,
+        exp_avg,
+        exp_avg_sq,
+        torch.tensor(1.0),
+        torch.tensor(0.01),
+        torch.tensor(0.9),
+        torch.tensor(0.99),
+        torch.tensor(1e-8),
+        torch.tensor(0.01),
+    )
+    assert not torch.equal(p, torch.ones_like(p))
+
+    # Muon fused kernel: tall matrix branch.
+    grads_tall = torch.randn(2, 4, 2, dtype=torch.float32)
+    params_tall = torch.randn(2, 4, 2, dtype=torch.float32)
+    m_tall = torch.zeros_like(grads_tall)
+    v_tall = torch.zeros(2, 4, 1, dtype=torch.float32)
+    optim.muon_step_fused.__wrapped__(
+        grads_tall,
+        params_tall,
+        m_tall,
+        v_tall,
+        torch.tensor(0.9),
+        torch.tensor(0.01),
+        torch.tensor(0.0),
+        torch.tensor(0.95),
+        2,
+        -1,
+    )
+
+    # Muon fused kernel: wide matrix branch.
+    grads_wide = torch.randn(2, 2, 4, dtype=torch.float32)
+    params_wide = torch.randn(2, 2, 4, dtype=torch.float32)
+    m_wide = torch.zeros_like(grads_wide)
+    v_wide = torch.zeros(2, 1, 4, dtype=torch.float32)
+    optim.muon_step_fused.__wrapped__(
+        grads_wide,
+        params_wide,
+        m_wide,
+        v_wide,
+        torch.tensor(0.9),
+        torch.tensor(0.01),
+        torch.tensor(0.0),
+        torch.tensor(0.95),
+        2,
+        -2,
+    )
+
+
+def test_muon_adamw_optimizer_paths(monkeypatch):
+    monkeypatch.setattr(
+        optim,
+        "adamw_step_fused",
+        lambda p, grad, exp_avg, exp_avg_sq, step_t, lr_t, beta1_t, beta2_t, eps_t, wd_t: p.data.add_(-grad * lr_t.item()),
+    )
+    monkeypatch.setattr(
+        optim,
+        "muon_step_fused",
+        lambda stacked_grads, stacked_params, momentum_buffer, second_momentum_buffer, momentum_t, lr_t, wd_t, beta2_t, ns_steps, red_dim: stacked_params.add_(-stacked_grads * lr_t.item()),
+    )
+
+    p1 = torch.nn.Parameter(torch.ones(4, 4))
+    p2 = torch.nn.Parameter(torch.ones(4, 4))
+    p1.grad = torch.full_like(p1, 0.1)
+    p2.grad = torch.full_like(p2, 0.2)
+
+    p1_nograd = torch.nn.Parameter(torch.ones(4, 4))
+    groups = [
+        dict(kind="adamw", params=[p1_nograd, p1], lr=0.01, betas=(0.9, 0.95), eps=1e-8, weight_decay=0.0),
+        dict(kind="muon", params=[p2], lr=0.02, momentum=0.9, ns_steps=2, beta2=None, weight_decay=0.0),
+    ]
+    opt = optim.MuonAdamW(groups)
+    opt.step()
+    assert p1.data.mean().item() < 1.0
+    assert p2.data.mean().item() < 1.0
+
+    # _step_muon early return on empty params.
+    opt._step_muon(dict(kind="muon", params=[], lr=0.01, momentum=0.9, ns_steps=1, beta2=0.95, weight_decay=0.0))
+
+    bad = optim.MuonAdamW([dict(kind="bad", params=[p1], lr=0.1)])
+    with pytest.raises(ValueError):
+        bad.step()
+
+
+class _Future:
+    def __init__(self):
+        self.waited = 0
+
+    def wait(self):
+        self.waited += 1
+        return None
+
+
+class _AsyncOp:
+    def __init__(self):
+        self.f = _Future()
+
+    def get_future(self):
+        return self.f
+
+
+def test_dist_muon_adamw_components(monkeypatch):
+    # Fake distributed ops.
+    def fake_all_reduce(t, op=None, async_op=False):
+        del op, async_op
+        return _AsyncOp()
+
+    def fake_reduce_scatter_tensor(out, inp, op=None, async_op=False):
+        del op, async_op
+        # take the leading chunk
+        flat = inp.reshape(inp.shape[0], -1)
+        out.copy_(inp[: out.shape[0]])
+        return _AsyncOp()
+
+    def fake_all_gather_into_tensor(out, inp, async_op=False):
+        del async_op
+        # repeat input chunks into output prefix as needed
+        n = inp.shape[0]
+        out.data[:n].copy_(inp.detach())
+        if out.shape[0] > n:
+            out.data[n:].zero_()
+        return _AsyncOp()
+
+    monkeypatch.setattr(optim.dist, "all_reduce", fake_all_reduce)
+    monkeypatch.setattr(optim.dist, "reduce_scatter_tensor", fake_reduce_scatter_tensor)
+    monkeypatch.setattr(optim.dist, "all_gather_into_tensor", fake_all_gather_into_tensor)
+    monkeypatch.setattr(optim.dist, "get_rank", lambda: 1)
+    monkeypatch.setattr(optim.dist, "get_world_size", lambda: 2)
+    monkeypatch.setattr(
+        optim,
+        "adamw_step_fused",
+        lambda p, grad, exp_avg, exp_avg_sq, step_t, lr_t, beta1_t, beta2_t, eps_t, wd_t: p.data.add_(-grad * lr_t.item()),
+    )
+    monkeypatch.setattr(
+        optim,
+        "muon_step_fused",
+        lambda stacked_grads, stacked_params, momentum_buffer, second_momentum_buffer, momentum_t, lr_t, wd_t, beta2_t, ns_steps, red_dim: stacked_params.add_(-stacked_grads * lr_t.item()),
+    )
+
+    p_small = torch.nn.Parameter(torch.ones(8), requires_grad=False)
+    p_small.grad = torch.full_like(p_small, 0.1)
+    p_large = torch.nn.Parameter(torch.ones(1024, 2), requires_grad=False)
+    p_large.grad = torch.full_like(p_large, 0.1)
+
+    p_mu0 = torch.nn.Parameter(torch.ones(2, 2), requires_grad=False)
+    p_mu1 = torch.nn.Parameter(torch.ones(2, 2), requires_grad=False)
+    p_mu2 = torch.nn.Parameter(torch.ones(2, 2), requires_grad=False)
+    for p in (p_mu0, p_mu1, p_mu2):
+        p.grad = torch.full_like(p, 0.2)
+
+    groups = [
+        dict(kind="adamw", params=[p_small, p_large], lr=0.01, betas=(0.9, 0.95), eps=1e-8, weight_decay=0.0),
+        dict(kind="muon", params=[p_mu0, p_mu1, p_mu2], lr=0.02, momentum=0.9, ns_steps=2, beta2=0.95, weight_decay=0.0),
+    ]
+    opt = optim.DistMuonAdamW(groups)
+
+    # _reduce_adamw small and large paths.
+    info_adam = opt._reduce_adamw(groups[0], world_size=2)
+    assert len(info_adam["param_infos"]) == 2
+    assert info_adam["param_infos"][p_small]["is_small"] is True
+    assert info_adam["param_infos"][p_large]["is_small"] is False
+
+    # Assertion on non-divisible shape[0] for large params.
+    bad_p = torch.nn.Parameter(torch.ones(1025, 2), requires_grad=False)
+    bad_p.grad = torch.ones_like(bad_p)
+    with pytest.raises(AssertionError):
+        opt._reduce_adamw(dict(kind="adamw", params=[bad_p], lr=0.01, betas=(0.9, 0.95), eps=1e-8, weight_decay=0.0), world_size=2)
+
+    info_mu = opt._reduce_muon(groups[1], world_size=2)
+    assert "chunk_size" in info_mu
+
+    gather_list = []
+    opt._compute_adamw(groups[0], info_adam, gather_list, rank=1, world_size=2)
+    assert len(gather_list) >= 1
+
+    opt._compute_muon(groups[1], info_mu, gather_list, rank=1)
+    assert len(gather_list) >= 2
+    opt._compute_muon(groups[1], info_mu, gather_list, rank=3)
+
+    # _finish_gathers handles both params=None and params=list branches.
+    gather_list.append(dict(future=_Future(), params=None))
+    opt._finish_gathers(gather_list)
+
+    # step() happy path.
+    opt.step()
+
+    # step() unknown kind.
+    bad = optim.DistMuonAdamW([dict(kind="bad", params=[p_small], lr=0.1)])
+    with pytest.raises(ValueError):
+        bad.step()
+
+
+def test_dist_optimizer_phase2_unknown_kind(monkeypatch):
+    p = torch.nn.Parameter(torch.ones(2, 2), requires_grad=False)
+    p.grad = torch.ones_like(p)
+    group = dict(kind="muon", params=[p], lr=0.01, momentum=0.9, ns_steps=1, beta2=0.95, weight_decay=0.0)
+    opt = optim.DistMuonAdamW([group])
+
+    monkeypatch.setattr(optim.dist, "get_rank", lambda: 0)
+    monkeypatch.setattr(optim.dist, "get_world_size", lambda: 1)
+
+    def mutate_kind(g, world_size):
+        del world_size
+        g["kind"] = "bad"
+        return {}
+
+    monkeypatch.setattr(opt, "_reduce_muon", mutate_kind)
+    monkeypatch.setattr(opt, "_compute_adamw", lambda *a, **k: None)
+    monkeypatch.setattr(opt, "_compute_muon", lambda *a, **k: None)
+    monkeypatch.setattr(opt, "_finish_gathers", lambda *a, **k: None)
+
+    with pytest.raises(ValueError):
+        opt.step()
diff --git a/tests/test_report_new.py b/tests/test_report_new.py
new file mode 100644
index 0000000..9a98d92
--- /dev/null
+++ b/tests/test_report_new.py
@@ -0,0 +1,206 @@
+import datetime
+import runpy
+import os
+import types
+
+import pytest
+
+import nanochat.report as report
+
+
+def test_run_command_paths(monkeypatch):
+    class R:
+        def __init__(self, stdout, returncode):
+            self.stdout = stdout
+            self.returncode = returncode
+
+    monkeypatch.setattr(report.subprocess, "run", lambda *a, **k: R("x\n", 1))
+    assert report.run_command("x") == "x"
+
+    monkeypatch.setattr(report.subprocess, "run", lambda *a, **k: R("  ", 0))
+    assert report.run_command("x") == ""
+
+    monkeypatch.setattr(report.subprocess, "run", lambda *a, **k: R("", 2))
+    assert report.run_command("x") is None
+
+    def boom(*a, **k):
+        raise RuntimeError("fail")
+
+    monkeypatch.setattr(report.subprocess, "run", boom)
+    assert report.run_command("x") is None
+
+
+def test_git_gpu_system_cost_helpers(monkeypatch):
+    seq = iter(["abc123", "main", " M x", "subject line\nbody"])
+    monkeypatch.setattr(report, "run_command", lambda _cmd: next(seq))
+    git = report.get_git_info()
+    assert git["commit"] == "abc123"
+    assert git["branch"] == "main"
+    assert git["dirty"] is True
+    assert git["message"] == "subject line"
+
+    monkeypatch.setattr(report.torch.cuda, "is_available", lambda: False)
+    assert report.get_gpu_info() == {"available": False}
+
+    class Props:
+        def __init__(self, name, total_memory):
+            self.name = name
+            self.total_memory = total_memory
+
+    monkeypatch.setattr(report.torch.cuda, "is_available", lambda: True)
+    monkeypatch.setattr(report.torch.cuda, "device_count", lambda: 2)
+    monkeypatch.setattr(report.torch.cuda, "get_device_properties", lambda i: Props(f"GPU{i}", 8 * 1024**3))
+    monkeypatch.setattr(report.torch, "version", types.SimpleNamespace(cuda="12.4"))
+    g = report.get_gpu_info()
+    assert g["available"] is True
+    assert g["count"] == 2
+    assert g["cuda_version"] == "12.4"
+
+    monkeypatch.setattr(report.socket, "gethostname", lambda: "host")
+    monkeypatch.setattr(report.platform, "system", lambda: "Linux")
+    monkeypatch.setattr(report.platform, "python_version", lambda: "3.10.0")
+    monkeypatch.setattr(report.torch, "__version__", "2.x")
+    monkeypatch.setattr(report.psutil, "cpu_count", lambda logical=False: 8 if not logical else 16)
+    monkeypatch.setattr(report.psutil, "virtual_memory", lambda: types.SimpleNamespace(total=32 * 1024**3))
+    monkeypatch.setenv("USER", "alice")
+    monkeypatch.setenv("NANOCHAT_BASE_DIR", "/n")
+    monkeypatch.setattr(report.os, "getcwd", lambda: "/cwd")
+    s = report.get_system_info()
+    assert s["hostname"] == "host"
+    assert s["cpu_count_logical"] == 16
+    assert s["nanochat_base_dir"] == "/n"
+
+    assert report.estimate_cost({"available": False}) is None
+    c1 = report.estimate_cost({"available": True, "count": 2, "names": ["H100"], "memory_gb": [80]}, runtime_hours=3)
+    assert c1["hourly_rate"] == 6.0
+    assert c1["estimated_total"] == 18.0
+    c2 = report.estimate_cost({"available": True, "count": 3, "names": ["Unknown"], "memory_gb": [1]})
+    assert c2["hourly_rate"] == 6.0
+
+
+def test_generate_header(monkeypatch, tmp_path):
+    monkeypatch.chdir(tmp_path)
+    (tmp_path / "uv.lock").write_text("a\nb\n", encoding="utf-8")
+    monkeypatch.setattr(report, "get_git_info", lambda: {"branch": "b", "commit": "c", "dirty": False, "message": "m"})
+    monkeypatch.setattr(report, "get_gpu_info", lambda: {"available": True, "count": 1, "names": ["A100"], "memory_gb": [40], "cuda_version": "12"})
+    monkeypatch.setattr(report, "get_system_info", lambda: {"platform": "Linux", "cpu_count": 8, "cpu_count_logical": 16, "memory_gb": 31.5, "python_version": "3.10", "torch_version": "2.9"})
+    monkeypatch.setattr(report, "estimate_cost", lambda _gpu: {"hourly_rate": 1.23, "gpu_type": "A100", "estimated_total": None})
+
+    def fake_run(cmd):
+        if "git ls-files" in cmd and "| xargs" not in cmd:
+            return "a.py\nb.md"
+        if "| xargs wc -lc" in cmd:
+            return "  10  100 total"
+        return ""
+
+    monkeypatch.setattr(report, "run_command", fake_run)
+    h = report.generate_header()
+    assert "# nanochat training report" in h
+    assert "Branch: b" in h
+    assert "Hourly Rate: $1.23/hour" in h
+    assert "Dependencies (uv.lock lines): 2" in h
+
+    # No GPU and no uv.lock branch.
+    os.remove(tmp_path / "uv.lock")
+    monkeypatch.setattr(report, "get_gpu_info", lambda: {"available": False})
+    monkeypatch.setattr(report, "estimate_cost", lambda _gpu: None)
+    h2 = report.generate_header()
+    assert "GPUs: None available" in h2
+
+
+def test_slug_extract_timestamp_helpers():
+    assert report.slugify("Hello World") == "hello-world"
+    section = "a: 1\nb: 2\nc: 3"
+    assert report.extract(section, "b") == {"b": "2"}
+    assert report.extract(section, ["a", "c"]) == {"a": "1", "c": "3"}
+
+    content = "x\ntimestamp: 2026-01-02 03:04:05\nz"
+    dt = report.extract_timestamp(content, "timestamp:")
+    assert dt == datetime.datetime(2026, 1, 2, 3, 4, 5)
+    assert report.extract_timestamp("timestamp: not-a-time", "timestamp:") is None
+    assert report.extract_timestamp("bad", "timestamp:") is None
+
+
+def test_report_log_generate_reset_and_get_report(monkeypatch, tmp_path):
+    r = report.Report(str(tmp_path))
+    p = r.log("My Section", [{"i": 12345, "f": 1.23456, "s": "x"}, "", None, "plain\n"])
+    text = (tmp_path / "my-section.md").read_text(encoding="utf-8")
+    assert p.endswith("my-section.md")
+    assert "- i: 12,345" in text
+    assert "- f: 1.2346" in text
+    assert "plain" in text
+
+    # Prepare files for generate().
+    header = tmp_path / "header.md"
+    header.write_text(
+        "Run started: 2026-01-01 00:00:00\n\n### Bloat\n- Lines: 10\n\n",
+        encoding="utf-8",
+    )
+    (tmp_path / "base-model-evaluation.md").write_text(
+        "timestamp: 2026-01-01 01:00:00\nCORE: 0.5\n",
+        encoding="utf-8",
+    )
+    (tmp_path / "chat-evaluation-sft.md").write_text(
+        "timestamp: 2026-01-01 02:00:00\nARC-Easy: 0.7\nChatCORE: 0.8\n",
+        encoding="utf-8",
+    )
+    (tmp_path / "chat-evaluation-rl.md").write_text(
+        "timestamp: 2026-01-01 03:00:00\nGSM8K: 0.9\n",
+        encoding="utf-8",
+    )
+    cwd = tmp_path / "cwd"
+    cwd.mkdir()
+    monkeypatch.chdir(cwd)
+    out = r.generate()
+    rep = (tmp_path / "report.md").read_text(encoding="utf-8")
+    assert out.endswith("report.md")
+    assert "## Summary" in rep
+    assert "Total wall clock time: 2h0m" in rep
+    assert "| CORE" in rep
+
+    # Missing header path + reset.
+    (tmp_path / "header.md").unlink()
+    out2 = r.generate()
+    rep2 = (tmp_path / "report.md").read_text(encoding="utf-8")
+    assert out2.endswith("report.md")
+    assert "Total wall clock time: unknown" in rep2
+
+    monkeypatch.setattr(report, "generate_header", lambda: "H\n")
+    r.reset()
+    assert (tmp_path / "header.md").exists()
+    assert "Run started:" in (tmp_path / "header.md").read_text(encoding="utf-8")
+
+    d = report.DummyReport()
+    assert d.log() is None
+    assert d.reset() is None
+
+    # get_report() rank 0 and non-zero branches.
+    monkeypatch.setattr("nanochat.common.get_dist_info", lambda: (True, 0, 0, 2))
+    monkeypatch.setattr("nanochat.common.get_base_dir", lambda: str(tmp_path))
+    got = report.get_report()
+    assert isinstance(got, report.Report)
+
+    monkeypatch.setattr("nanochat.common.get_dist_info", lambda: (True, 1, 1, 2))
+    got2 = report.get_report()
+    assert isinstance(got2, report.DummyReport)
+
+
+def test_report_main_block_generate_and_reset(monkeypatch, tmp_path):
+    class FakeParser:
+        def __init__(self, *a, **k):
+            pass
+
+        def add_argument(self, *a, **k):
+            return None
+
+        def parse_args(self):
+            return self.args
+
+    parser = FakeParser()
+    parser.args = types.SimpleNamespace(command="generate")
+    monkeypatch.setattr("argparse.ArgumentParser", lambda *a, **k: parser)
+    monkeypatch.setenv("NANOCHAT_BASE_DIR", str(tmp_path / "base"))
+    runpy.run_module("nanochat.report", run_name="__main__")
+
+    parser.args = types.SimpleNamespace(command="reset")
+    runpy.run_module("nanochat.report", run_name="__main__")
diff --git a/tests/test_tokenizer_new.py b/tests/test_tokenizer_new.py
new file mode 100644
index 0000000..f1fdaf5
--- /dev/null
+++ b/tests/test_tokenizer_new.py
@@ -0,0 +1,281 @@
+import os
+import pickle
+from types import SimpleNamespace
+
+import pytest
+import torch
+
+import nanochat.tokenizer as tokmod
+
+
+class _FakeHFCore:
+    def __init__(self, *args, **kwargs):
+        del args, kwargs
+        self.saved = None
+        self.normalizer = None
+        self.pre_tokenizer = None
+        self.decoder = None
+        self.post_processor = None
+        self.trained = False
+        self.special = {"<|bos|>": 100, "<|endoftext|>": 101}
+
+    @classmethod
+    def from_pretrained(cls, _path):
+        return cls()
+
+    @classmethod
+    def from_file(cls, _path):
+        return cls()
+
+    def train_from_iterator(self, text_iterator, trainer):
+        del trainer
+        list(text_iterator)
+        self.trained = True
+
+    def get_vocab_size(self):
+        return 123
+
+    def get_added_tokens_decoder(self):
+        return {0: SimpleNamespace(content="<|bos|>"), 1: SimpleNamespace(content="<|assistant_start|>")}
+
+    def id_to_token(self, id):
+        return f"tok-{id}"
+
+    def encode(self, text, add_special_tokens=False):
+        del add_special_tokens
+        return SimpleNamespace(ids=[len(text), len(text) + 1])
+
+    def token_to_id(self, text):
+        return self.special.get(text)
+
+    def decode(self, ids, skip_special_tokens=False):
+        del skip_special_tokens
+        return "|".join(map(str, ids))
+
+    def save(self, path):
+        self.saved = path
+        with open(path, "w", encoding="utf-8") as f:
+            f.write("{}")
+
+
+def test_huggingface_tokenizer_wrapper(monkeypatch, tmp_path):
+    monkeypatch.setattr(tokmod, "HFTokenizer", _FakeHFCore)
+    monkeypatch.setattr(tokmod, "BPE", lambda **kwargs: kwargs)
+    monkeypatch.setattr(tokmod, "Regex", lambda x: f"re:{x}")
+
+    class _ByteLevel:
+        @staticmethod
+        def alphabet():
+            return [b"a"]
+
+        def __init__(self, **kwargs):
+            self.kw = kwargs
+
+    fake_pre = SimpleNamespace(
+        Split=lambda **kwargs: ("split", kwargs),
+        ByteLevel=_ByteLevel,
+        Sequence=lambda xs: ("seq", xs),
+    )
+    monkeypatch.setattr(tokmod, "pre_tokenizers", fake_pre)
+    monkeypatch.setattr(tokmod, "decoders", SimpleNamespace(ByteLevel=lambda: "decoder"))
+    monkeypatch.setattr(tokmod, "BpeTrainer", lambda **kwargs: ("trainer", kwargs))
+
+    h1 = tokmod.HuggingFaceTokenizer.from_pretrained("gpt2")
+    assert isinstance(h1, tokmod.HuggingFaceTokenizer)
+
+    d = tmp_path / "hf"
+    d.mkdir()
+    (d / "tokenizer.json").write_text("{}", encoding="utf-8")
+    h2 = tokmod.HuggingFaceTokenizer.from_directory(str(d))
+    assert isinstance(h2, tokmod.HuggingFaceTokenizer)
+
+    h3 = tokmod.HuggingFaceTokenizer.train_from_iterator(iter(["hello", "world"]), vocab_size=300)
+    assert isinstance(h3, tokmod.HuggingFaceTokenizer)
+    assert h3.tokenizer.trained is True
+
+    assert h3.get_vocab_size() == 123
+    assert "<|bos|>" in h3.get_special_tokens()
+    assert h3.id_to_token(7) == "tok-7"
+    assert h3.encode_special("<|bos|>") == 100
+    assert h3.get_bos_token_id() == 100
+
+    h3.tokenizer.special["<|bos|>"] = None
+    assert h3.get_bos_token_id() == 101
+    h3.tokenizer.special["<|endoftext|>"] = None
+    with pytest.raises(AssertionError):
+        h3.get_bos_token_id()
+    h3.tokenizer.special["<|bos|>"] = 100
+
+    one = h3._encode_one("abc", prepend="<|bos|>", append=999)
+    assert one[0] == 100 and one[-1] == 999
+    assert isinstance(h3.encode("abc"), list)
+    assert isinstance(h3.encode(["a", "bb"]), list)
+    with pytest.raises(ValueError):
+        h3.encode(123)  # type: ignore[arg-type]
+
+    assert h3("abc") == h3.encode("abc")
+    assert h3.decode([1, 2]) == "1|2"
+    save_dir = tmp_path / "save_hf"
+    h3.save(str(save_dir))
+    assert (save_dir / "tokenizer.json").exists()
+
+
+class _FakeRustTokenizerCore:
+    def __init__(self):
+        self.pattern = "pat"
+        self.mergeable = [(b"a", 0), (b"b", 1)]
+        self.trained = None
+
+    def train_from_iterator(self, it, vocab_size_no_special, pattern):
+        self.trained = (list(it), vocab_size_no_special, pattern)
+
+    def get_pattern(self):
+        return self.pattern
+
+    def get_mergeable_ranks(self):
+        return self.mergeable
+
+
+class _FakeEnc:
+    def __init__(self):
+        self.n_vocab = 300
+        self.special_tokens_set = set(tokmod.SPECIAL_TOKENS)
+        self.special = {name: 256 + i for i, name in enumerate(tokmod.SPECIAL_TOKENS)}
+        self.special["<|endoftext|>"] = 999
+
+    def encode_single_token(self, text):
+        return self.special[text]
+
+    def encode_ordinary(self, text):
+        return [ord(c) % 50 for c in text]
+
+    def encode_ordinary_batch(self, text, num_threads=8):
+        del num_threads
+        return [self.encode_ordinary(t) for t in text]
+
+    def decode(self, ids):
+        return ",".join(str(x) for x in ids)
+
+
+def test_rust_bpe_wrapper_and_helpers(monkeypatch, tmp_path):
+    monkeypatch.setattr(tokmod.rustbpe, "Tokenizer", _FakeRustTokenizerCore)
+    monkeypatch.setattr(tokmod.tiktoken, "Encoding", lambda **kwargs: _FakeEnc())
+    monkeypatch.setattr(tokmod.tiktoken, "get_encoding", lambda _name: _FakeEnc())
+
+    r1 = tokmod.RustBPETokenizer.train_from_iterator(iter(["hello", "world"]), vocab_size=300)
+    assert isinstance(r1, tokmod.RustBPETokenizer)
+
+    with pytest.raises(AssertionError):
+        tokmod.RustBPETokenizer.train_from_iterator(iter(["x"]), vocab_size=10)
+
+    assert r1.get_vocab_size() == 300
+    assert "<|bos|>" in r1.get_special_tokens()
+    assert isinstance(r1.id_to_token(3), str)
+    assert r1.get_bos_token_id() == r1.encode_special("<|bos|>")
+    assert r1.encode_special("<|bos|>") == r1.encode_special("<|bos|>")  # cache path
+
+    s = r1.encode("abc", prepend="<|bos|>", append="<|assistant_end|>")
+    assert s[0] == r1.encode_special("<|bos|>")
+    assert s[-1] == r1.encode_special("<|assistant_end|>")
+    b = r1.encode(["a", "bc"], prepend=1, append=2, num_threads=1)
+    assert b[0][0] == 1 and b[0][-1] == 2
+    with pytest.raises(ValueError):
+        r1.encode(123)  # type: ignore[arg-type]
+    assert r1("abc") == r1.encode("abc")
+    assert isinstance(r1.decode([1, 2]), str)
+
+    save_dir = tmp_path / "save_rust"
+    r1.save(str(save_dir))
+    assert (save_dir / "tokenizer.pkl").exists()
+    r2 = tokmod.RustBPETokenizer.from_directory(str(save_dir))
+    assert isinstance(r2, tokmod.RustBPETokenizer)
+    r3 = tokmod.RustBPETokenizer.from_pretrained("gpt2")
+    assert isinstance(r3, tokmod.RustBPETokenizer)
+
+
+def test_render_conversation_and_completion(monkeypatch):
+    r = tokmod.RustBPETokenizer(_FakeEnc(), "<|bos|>")
+
+    conv = {
+        "messages": [
+            {"role": "system", "content": "system msg"},
+            {"role": "user", "content": "hello"},
+            {
+                "role": "assistant",
+                "content": [
+                    {"type": "text", "text": "hi"},
+                    {"type": "python", "text": "1+1"},
+                    {"type": "python_output", "text": "2"},
+                ],
+            },
+        ]
+    }
+    ids, mask = r.render_conversation(conv, max_tokens=128)
+    assert len(ids) == len(mask)
+    assert any(m == 1 for m in mask)
+    vis = r.visualize_tokenization(ids[:5], mask[:5], with_token_id=True)
+    assert "|" in vis
+
+    conv_str_assistant = {
+        "messages": [
+            {"role": "user", "content": "hello"},
+            {"role": "assistant", "content": "plain answer"},
+        ]
+    }
+    ids2, mask2 = r.render_conversation(conv_str_assistant, max_tokens=64)
+    assert len(ids2) == len(mask2)
+    assert any(m == 1 for m in mask2)
+
+    comp_conv = {
+        "messages": [
+            {"role": "user", "content": "Q"},
+            {"role": "assistant", "content": "A"},
+        ]
+    }
+    out = r.render_for_completion(comp_conv)
+    assert out[-1] == r.encode_special("<|assistant_start|>")
+
+    bad_role = {"messages": [{"role": "assistant", "content": "x"}]}
+    with pytest.raises(AssertionError):
+        r.render_conversation(bad_role)
+
+    bad_content = {
+        "messages": [
+            {"role": "user", "content": "x"},
+            {"role": "assistant", "content": 123},
+        ]
+    }
+    with pytest.raises(ValueError):
+        r.render_conversation(bad_content)
+
+    bad_part = {
+        "messages": [
+            {"role": "user", "content": "x"},
+            {"role": "assistant", "content": [{"type": "bad", "text": "x"}]},
+        ]
+    }
+    with pytest.raises(ValueError):
+        r.render_conversation(bad_part)
+
+    with pytest.raises(AssertionError):
+        r.render_for_completion({"messages": [{"role": "user", "content": "x"}]})
+
+
+def test_get_tokenizer_and_token_bytes(monkeypatch, tmp_path):
+    tok_dir = tmp_path / "tokenizer"
+    tok_dir.mkdir()
+    token_bytes = torch.tensor([0, 1, 2], dtype=torch.int64)
+    torch.save(token_bytes, tok_dir / "token_bytes.pt")
+
+    monkeypatch.setattr("nanochat.common.get_base_dir", lambda: str(tmp_path))
+    monkeypatch.setattr(tokmod.RustBPETokenizer, "from_directory", classmethod(lambda cls, d: ("tok", d)))
+    got = tokmod.get_tokenizer()
+    assert got[0] == "tok"
+    assert got[1].endswith("tokenizer")
+
+    tb = tokmod.get_token_bytes(device="cpu")
+    assert torch.equal(tb, token_bytes)
+
+    os.remove(tok_dir / "token_bytes.pt")
+    with pytest.raises(AssertionError):
+        tokmod.get_token_bytes()
diff --git a/uv.lock b/uv.lock
index bbc9519..85e76fc 100644
--- a/uv.lock
+++ b/uv.lock
@@ -556,6 +556,124 @@ wheels = [
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 ]
 
+[[package]]
+name = "coverage"
+version = "7.13.4"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/24/56/95b7e30fa389756cb56630faa728da46a27b8c6eb46f9d557c68fff12b65/coverage-7.13.4.tar.gz", hash = "sha256:e5c8f6ed1e61a8b2dcdf31eb0b9bbf0130750ca79c1c49eb898e2ad86f5ccc91", size = 827239, upload-time = "2026-02-09T12:59:03.86Z" }
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+]
+
+[package.optional-dependencies]
+toml = [
+    { name = "tomli", marker = "python_full_version <= '3.11' or (extra == 'extra-8-nanochat-cpu' and extra == 'extra-8-nanochat-gpu')" },
+]
+
 [[package]]
 name = "cycler"
 version = "0.12.1"
@@ -1527,6 +1645,8 @@ gpu = [
 [package.dev-dependencies]
 dev = [
     { name = "pytest" },
+    { name = "pytest-cov" },
+    { name = "pytest-mock" },
 ]
 
 [package.metadata]
@@ -1556,7 +1676,11 @@ requires-dist = [
 provides-extras = ["cpu", "gpu"]
 
 [package.metadata.requires-dev]
-dev = [{ name = "pytest", specifier = ">=8.0.0" }]
+dev = [
+    { name = "pytest", specifier = ">=8.0.0" },
+    { name = "pytest-cov", specifier = ">=7.0.0" },
+    { name = "pytest-mock", specifier = ">=3.15.1" },
+]
 
 [[package]]
 name = "nest-asyncio"
@@ -2336,6 +2460,32 @@ wheels = [
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 ]
 
+[[package]]
+name = "pytest-cov"
+version = "7.0.0"
+source = { registry = "https://pypi.org/simple" }
+dependencies = [
+    { name = "coverage", extra = ["toml"] },
+    { name = "pluggy" },
+    { name = "pytest" },
+]
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+wheels = [
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+]
+
+[[package]]
+name = "pytest-mock"
+version = "3.15.1"
+source = { registry = "https://pypi.org/simple" }
+dependencies = [
+    { name = "pytest" },
+]
+sdist = { url = "https://files.pythonhosted.org/packages/68/14/eb014d26be205d38ad5ad20d9a80f7d201472e08167f0bb4361e251084a9/pytest_mock-3.15.1.tar.gz", hash = "sha256:1849a238f6f396da19762269de72cb1814ab44416fa73a8686deac10b0d87a0f", size = 34036, upload-time = "2025-09-16T16:37:27.081Z" }
+wheels = [
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+]
+
 [[package]]
 name = "python-dateutil"
 version = "2.9.0.post0"