training_artifacts/sft_train.py

"""Full-parameter assistant-only SFT for NedoLM 0.8B."""

from __future__ import annotations

import argparse
from concurrent.futures import ThreadPoolExecutor
import contextlib
import hashlib
import json
import math
import os
import signal
import sys
import time
from pathlib import Path
from typing import Any, Optional

import torch
import torch.distributed as dist
import torch.nn.functional as F
import yaml
from torch.nn.parallel import DistributedDataParallel as DDP

from .checkpoint import latest_checkpoint, save_checkpoint, save_model_archive, write_run_metadata
from .config import EOS_ID, OptimConfig, RunConfig
from .distributed import Logger, cleanup, gather_object, setup_distributed
from .model import build_model, packed_flash_attn_available
from .optim import build_optimizer, lr_at_step, set_lr
from .sft_data import build_sft_loader, corpus_stats, discover_sft_shards, validate_roles

_STOP_REQUESTED = False


def _request_stop(signum, frame) -> None:  # noqa: ANN001
    global _STOP_REQUESTED
    _STOP_REQUESTED = True
    print(f"[signal] received {signum}; will checkpoint and exit", flush=True)


def parse_args() -> argparse.Namespace:
    p = argparse.ArgumentParser(description="NedoLM 0.8B assistant-only SFT")
    p.add_argument("--config", required=True, type=Path)
    p.add_argument("--smoke-steps", type=int, default=0)
    p.add_argument("--no-compile", action="store_true")
    return p.parse_args()


def set_seed(seed: int, rank: int) -> None:
    import random
    import numpy as np
    torch.manual_seed(seed)
    if torch.cuda.is_available():
        torch.cuda.manual_seed_all(seed)
    random.seed(seed + rank)
    np.random.seed((seed + rank) % (2**32 - 1))


def load_yaml(path: Path) -> dict[str, Any]:
    raw = yaml.safe_load(path.read_text(encoding="utf-8")) or {}
    required = {"run_dir", "base_config", "base_run_dir", "data_dir", "optim", "train"}
    missing = sorted(required - set(raw))
    if missing:
        raise ValueError(f"missing SFT config keys: {missing}")
    return raw


def _load_torch(path: Path) -> dict[str, Any]:
    try:
        return torch.load(path, map_location="cpu", weights_only=False, mmap=True)
    except TypeError:
        return torch.load(path, map_location="cpu", weights_only=False)


def _sum_all(value: torch.Tensor) -> torch.Tensor:
    if dist.is_initialized():
        dist.all_reduce(value, op=dist.ReduceOp.SUM)
    return value


def _main_rank_call(info, action: str, fn):  # noqa: ANN001
    """Run a rank-0-only action and propagate success/failure to every rank.

    This replaces the unsafe pattern ``if rank0: action(); barrier()`` where a
    Python exception on rank 0 can strand all other ranks inside a barrier.
    The return value is broadcast as a small Python object when distributed.
    """
    result = None
    error = ""
    if info.is_main:
        try:
            result = fn()
        except Exception as exc:  # noqa: BLE001
            error = f"{action} failed on rank 0: {type(exc).__name__}: {exc}"
    if dist.is_initialized():
        payload = [result, error]
        dist.broadcast_object_list(payload, src=0)
        result, error = payload
    if error:
        raise RuntimeError(str(error))
    return result


def _broadcast_module_state(module: torch.nn.Module, src: int = 0) -> None:
    """Synchronize model parameters/buffers before compile warmup and DDP.

    Fresh SFT reads the large base checkpoint on rank 0 only.  Compile warmup
    deliberately runs before DDP so failures cannot deadlock DDP collectives;
    therefore the bare module must first be made identical on every rank.
    """
    if not dist.is_initialized():
        return
    with torch.no_grad():
        for param in module.parameters():
            dist.broadcast(param.data, src=src)
        for buf in module.buffers():
            dist.broadcast(buf.data, src=src)


def _collect_rank_errors(info, local_error: str) -> list[str]:
    if not dist.is_initialized():
        return [local_error] if local_error else []
    errors = gather_object(local_error, info.world_size)
    return [f"rank {rank}: {msg}" for rank, msg in enumerate(errors) if msg]


def _all_rank_call(info, action: str, fn):  # noqa: ANN001
    """Run local non-collective work on every rank, then make failure global."""
    result = None
    local_error = ""
    try:
        result = fn()
    except Exception as exc:  # noqa: BLE001
        local_error = f"{type(exc).__name__}: {exc}"
    errors = _collect_rank_errors(info, local_error)
    if errors:
        raise RuntimeError(f"{action} failed: " + " | ".join(errors))
    return result


def _sync_control_flags(
    control_flags: torch.Tensor,
    *,
    rank0_archive_due: bool,
    local_stop: bool,
) -> tuple[bool, bool]:
    """Synchronize archive/stop decisions so every rank takes the same branch."""
    control_flags[0] = 1 if rank0_archive_due else 0
    control_flags[1] = 1 if local_stop else 0
    if dist.is_initialized():
        dist.all_reduce(control_flags, op=dist.ReduceOp.MAX)
    return bool(control_flags[0].item()), bool(control_flags[1].item())


def _sha256(path: Path) -> str:
    h = hashlib.sha256()
    with path.open("rb") as handle:
        for chunk in iter(lambda: handle.read(1 << 20), b""):
            h.update(chunk)
    return h.hexdigest()


def _validate_dataset_contract(data_dir: Path, vocab_size: int, seq_len: int) -> dict[str, Any]:
    verify_path = data_dir / "FINAL_BINARY_VERIFY_REPORT.json"
    special_path = data_dir / "SPECIAL_TOKENS.json"
    packing_path = data_dir / "PACKING_POLICY_ADAPTIVE_AUDIT.json"
    manifest_path = data_dir / "SHARD_MANIFEST.json"
    required = (verify_path, special_path, packing_path, manifest_path)
    missing_contract = [path.name for path in required if not path.exists()]
    if missing_contract:
        raise RuntimeError(f"verified SFT contract missing: {missing_contract}")

    verify = json.loads(verify_path.read_text(encoding="utf-8"))
    special = json.loads(special_path.read_text(encoding="utf-8"))
    packing = json.loads(packing_path.read_text(encoding="utf-8"))
    manifest = json.loads(manifest_path.read_text(encoding="utf-8"))

    if verify.get("status") != "PASS":
        raise RuntimeError("SFT final binary verifier is not PASS")
    if packing.get("status") != "PASS":
        raise RuntimeError("SFT adaptive packing audit is not PASS")
    if packing.get("schema") != "nedolm_sft_adaptive_packing_audit_v2":
        raise RuntimeError(f"unexpected adaptive packing schema: {packing.get('schema')}")
    if int(packing.get("seq_len", -1)) != seq_len:
        raise RuntimeError(
            f"adaptive packing audit seq_len={packing.get('seq_len')} but train seq_len={seq_len}"
        )
    if int(packing.get("read_span_tokens", -1)) != seq_len + 1:
        raise RuntimeError("adaptive packing read span is not seq_len+1")
    if packing.get("packing_policy") != "adaptive_document_turn_overlap_v2":
        raise RuntimeError("unexpected adaptive packing policy")
    if packing.get("target_policy") != "exact_once_non_tool_assistant_targets":
        raise RuntimeError("adaptive packing is not exact-once target policy")
    if packing.get("continuation_policy") != (
        "prefer_latest_user_or_system_boundary_else_assistant_message_boundary"
    ):
        raise RuntimeError("unexpected adaptive continuation policy")
    if int(packing.get("hard_continuations", -1)) != 0:
        raise RuntimeError("adaptive packing contains unsafe hard continuations")

    if manifest.get("status") != "PASS" or manifest.get("algorithm") != "sha256":
        raise RuntimeError("SFT shard manifest is not a PASS sha256 manifest")
    if manifest.get("schema") != "nedolm_sft_shard_manifest_v1":
        raise RuntimeError(f"unexpected shard manifest schema: {manifest.get('schema')}")
    entries = manifest.get("files")
    if not isinstance(entries, list) or not entries:
        raise RuntimeError("SFT shard manifest has no files")
    if int(manifest.get("file_count", -1)) != len(entries):
        raise RuntimeError("SFT shard manifest file_count mismatch")

    canonical_entries = json.dumps(
        entries, sort_keys=True, separators=(",", ":")
    ).encode("utf-8")
    if hashlib.sha256(canonical_entries).hexdigest() != str(
        manifest.get("aggregate_sha256", "")
    ):
        raise RuntimeError("SFT shard manifest aggregate digest is invalid")

    actual_binary = sorted(
        path.relative_to(data_dir).as_posix()
        for pattern in ("*.tokens.bin", "*.segments.bin", "*.index.bin")
        for path in (data_dir / "shards").glob(pattern)
    )
    listed_binary = sorted(str(item.get("path", "")) for item in entries)
    if listed_binary != actual_binary:
        raise RuntimeError("SFT shard manifest file set differs from on-disk binary shard set")

    # Verify the actual binaries, not only the verifier JSON. This runs on rank
    # 0 only (via _main_rank_call) and hashes independent files in parallel so
    # the integrity gate remains cheap without multiplying Lustre traffic by
    # world_size.
    def _check_manifest_entry(item: dict[str, Any]) -> None:
        rel = str(item["path"])
        path = data_dir / rel
        expected_size = int(item["size"])
        if path.stat().st_size != expected_size:
            raise RuntimeError(f"SFT shard size changed after verification: {rel}")
        digest = _sha256(path)
        if digest != str(item["sha256"]):
            raise RuntimeError(f"SFT shard sha256 changed after verification: {rel}")

    with ThreadPoolExecutor(max_workers=min(8, len(entries))) as pool:
        list(pool.map(_check_manifest_entry, entries))

    if int(verify.get("vocab_size", -1)) != vocab_size:
        raise RuntimeError(f"SFT vocab={verify.get('vocab_size')} but model vocab={vocab_size}")
    if int(verify.get("message_start_id", -1)) != 32000:
        raise RuntimeError("unexpected message_start_id")
    if int(verify.get("message_end_id", -1)) != 32001:
        raise RuntimeError("unexpected message_end_id")
    if int(verify.get("eos_id", -1)) != EOS_ID:
        raise RuntimeError("unexpected EOS id in verified corpus")
    if int(verify.get("max_token_id", vocab_size)) >= vocab_size:
        raise RuntimeError("verified corpus contains token id outside model vocab")

    required_true = (
        "global_doc_keys_unique",
        "content_tokens_exact_vs_source",
        "offsets_lengths_exact",
        "turkish_user_header_verified",
        "assistant_only_trainable_contract_verified",
        "header_schema_verified",
        "structural_control_counts_verified",
        "token_id_range_verified",
    )
    missing = [key for key in required_true if verify.get(key) is not True]
    if missing:
        raise RuntimeError(f"SFT verifier contract missing PASS fields: {missing}")

    controls = {int(x["id"]): x["token"] for x in special.get("control_tokens", [])}
    if controls != {32000: "<|mesaj_basla|>", 32001: "<|mesaj_bitir|>"}:
        raise RuntimeError(f"unexpected SFT control-token registry: {controls}")
    roles = special.get("roles")
    expected_roles = {"system": "sistem", "user": "kullanici", "assistant": "asistan"}
    if roles != expected_roles:
        raise RuntimeError(f"unexpected SFT role registry: {roles}")

    fingerprint = hashlib.sha256(
        verify_path.read_bytes()
        + b"\0"
        + special_path.read_bytes()
        + b"\0"
        + packing_path.read_bytes()
        + b"\0"
        + manifest_path.read_bytes()
    ).hexdigest()
    return {
        "fingerprint": fingerprint,
        "verify_sha256": _sha256(verify_path),
        "special_tokens_sha256": _sha256(special_path),
        "packing_policy_sha256": _sha256(packing_path),
        "shard_manifest_sha256": _sha256(manifest_path),
        "shard_manifest_files": len(entries),
        "training_windows": int(packing["windows"]),
        "assistant_target_tokens_per_epoch": int(packing["assistant_target_tokens"]),
        "valid_context_tokens_per_epoch": int(packing["valid_context_tokens"]),
        "padded_tokens_per_epoch": int(packing["padded_tokens"]),
        "turn_aligned_continuations": int(packing["turn_aligned_continuations"]),
        "assistant_aligned_continuations": int(
            packing["assistant_aligned_continuations"]
        ),
        "documents": int(verify["documents"]),
        "messages": int(verify["messages"]),
        "tokens": int(verify["tokens_including_eos"]),
        "vocab_size": int(verify["vocab_size"]),
    }

def _packed_layout(input_ids: torch.Tensor) -> tuple[torch.Tensor, torch.Tensor]:
    """Return FlashAttention boundaries and RoPE positions for packed rows.

    EOS and every batch-row start are hard conversation boundaries.  Attention
    isolation alone is not enough: rotary positions must also restart at zero
    after each boundary, otherwise later conversations inherit arbitrary packed
    row offsets (for example position 5000 instead of position 0).
    """
    if input_ids.ndim != 2 or input_ids.size(1) == 0:
        raise ValueError("packed attention expects non-empty [B,S] input_ids")
    starts = torch.zeros_like(input_ids, dtype=torch.bool)
    starts[:, 0] = True
    starts[:, 1:] = input_ids[:, :-1].eq(EOS_ID)

    offsets = torch.nonzero(starts.reshape(-1), as_tuple=False).flatten().to(torch.int32)
    end = torch.full((1,), input_ids.numel(), device=input_ids.device, dtype=torch.int32)
    cu_seqlens = torch.cat((offsets, end), dim=0).contiguous()

    cols = torch.arange(input_ids.size(1), device=input_ids.device, dtype=torch.long)
    cols = cols.unsqueeze(0).expand(input_ids.size(0), -1)
    boundary_cols = torch.where(starts, cols, torch.zeros_like(cols))
    last_boundary = torch.cummax(boundary_cols, dim=1).values
    position_ids = (cols - last_boundary).contiguous()
    return cu_seqlens, position_ids


def _packed_cu_seqlens(input_ids: torch.Tensor) -> torch.Tensor:
    """Compatibility helper used by older tests/tools."""
    return _packed_layout(input_ids)[0]


def _shard_id(shard) -> int:
    name = shard.token_path.name
    try:
        return int(name.split("-")[1])
    except (IndexError, ValueError) as exc:
        raise RuntimeError(f"cannot parse SFT shard id from {name}") from exc


def _split_train_val_shards(shards, val_ids: list[int]):
    wanted = set(int(x) for x in val_ids)
    if not wanted:
        return shards, []
    train = [sh for sh in shards if _shard_id(sh) not in wanted]
    val = [sh for sh in shards if _shard_id(sh) in wanted]
    found = {_shard_id(sh) for sh in val}
    missing = sorted(wanted - found)
    if missing:
        raise RuntimeError(f"validation shard ids not found: {missing}")
    if not train:
        raise RuntimeError("validation split consumed every SFT shard")
    return train, val


def _training_contract_fingerprint(
    dataset_fingerprint: str,
    seq_len: int,
    val_shard_ids: list[int],
    recipe: dict[str, Any],
) -> str:
    """Fingerprint every setting that can change the optimization trajectory."""
    payload = json.dumps(
        {
            "schema": "nedolm_sft_training_contract_v2",
            "dataset_fingerprint": dataset_fingerprint,
            "seq_len": int(seq_len),
            "read_span": int(seq_len) + 1,
            "val_shard_ids": sorted(int(x) for x in val_shard_ids),
            "packing_policy": "adaptive_document_turn_overlap_v2",
            "target_policy": "exact_once_non_tool_assistant_targets",
            "continuation_policy": (
                "prefer_latest_user_or_system_boundary_else_assistant_message_boundary"
            ),
            "attention_policy": "eos_plus_row_boundaries_varlen",
            "position_policy": "eos_plus_row_boundaries_reset_rope_to_zero",
            "recipe": recipe,
        },
        sort_keys=True,
        separators=(",", ":"),
    ).encode("utf-8")
    return hashlib.sha256(payload).hexdigest()


@torch.no_grad()
def _run_validation(
    base_model: torch.nn.Module,
    val_loader,
    info,
    *,
    seq_len: int,
    batches: int,
    assistant_only_head: bool,
    loss_fp32_logits: bool,
    use_amp: bool,
    device_type: str,
) -> dict[str, float | int]:
    """Run held-out validation without allowing one rank to strand its peers."""
    if batches <= 0:
        raise ValueError("validation batches must be positive")
    was_training = base_model.training
    base_model.eval()
    local_loss = torch.zeros((), device=info.device, dtype=torch.float32)
    local_targets = torch.zeros((), device=info.device, dtype=torch.int64)
    local_context = torch.zeros((), device=info.device, dtype=torch.int64)
    local_error = ""

    try:
        try:
            it = iter(val_loader)
            for _ in range(batches):
                batch = next(it)
                raw_tokens = batch["tokens"].to(
                    info.device, dtype=torch.long, non_blocking=True
                )
                inputs = raw_tokens[:, :-1]
                labels = raw_tokens[:, 1:].clone()
                trainable_mask = batch["trainable_mask"][:, 1:].to(
                    info.device, non_blocking=True
                )
                labels.masked_fill_(~trainable_mask, -100)
                target_count = int(trainable_mask.sum().item())
                if target_count <= 0:
                    raise RuntimeError("validation batch contains zero assistant targets")

                cu_seqlens, position_ids = _packed_layout(inputs)
                with torch.autocast(device_type, dtype=torch.bfloat16, enabled=use_amp):
                    if assistant_only_head:
                        loss_sum, _ = base_model(
                            inputs,
                            labels=labels,
                            return_logits=False,
                            loss_reduction="sum",
                            loss_fp32_logits=loss_fp32_logits,
                            cu_seqlens=cu_seqlens,
                            max_seqlen=seq_len,
                            position_ids=position_ids,
                        )
                    else:
                        _, logits = base_model(
                            inputs,
                            return_logits=True,
                            cu_seqlens=cu_seqlens,
                            max_seqlen=seq_len,
                            position_ids=position_ids,
                        )
                        loss_sum = F.cross_entropy(
                            logits.reshape(-1, logits.size(-1)).float(),
                            labels.reshape(-1),
                            ignore_index=-100,
                            reduction="sum",
                        )
                        del logits

                if not torch.isfinite(loss_sum):
                    raise RuntimeError("non-finite validation loss")
                local_loss += loss_sum.float()
                local_targets += target_count
                local_context += inputs.numel()
                del (
                    raw_tokens,
                    inputs,
                    labels,
                    trainable_mask,
                    cu_seqlens,
                    position_ids,
                    loss_sum,
                )
        except Exception as exc:  # noqa: BLE001
            local_error = f"{type(exc).__name__}: {exc}"

        # Every rank reaches this object collective before any metric all-reduce.
        # A recoverable Python/data/model error on one rank therefore becomes a
        # uniform exception instead of leaving healthy ranks blocked in SUM.
        validation_errors = _collect_rank_errors(info, local_error)
        if validation_errors:
            raise RuntimeError(
                "distributed validation failed: " + " | ".join(validation_errors)
            )

        _sum_all(local_loss)
        _sum_all(local_targets)
        _sum_all(local_context)
        targets = int(local_targets.item())
        if targets <= 0:
            raise RuntimeError("global validation target count is zero")
        mean_loss = float(local_loss.item() / targets)
        if not math.isfinite(mean_loss):
            raise RuntimeError(f"non-finite validation mean loss: {mean_loss}")
        return {
            "loss": mean_loss,
            "ppl": math.exp(min(20.0, mean_loss)),
            "assistant_tokens": targets,
            "context_tokens": int(local_context.item()),
        }
    finally:
        if was_training:
            base_model.train()

def main() -> int:
    args = parse_args()
    sft = load_yaml(args.config)
    base_cfg = RunConfig.load(sft["base_config"])
    tcfg = dict(sft["train"])
    ocfg = OptimConfig(**dict(sft["optim"]))

    run_dir = Path(sft["run_dir"])
    data_dir = Path(sft["data_dir"])
    seq_len = int(tcfg.get("seq_len", base_cfg.model.max_seq_len))
    micro_batch = int(tcfg.get("micro_batch_size", 8))
    grad_accum = int(tcfg.get("grad_accum_steps", 4))
    num_workers = int(tcfg.get("num_workers", 4))
    prefetch_factor = int(tcfg.get("prefetch_factor", 4))
    persistent_workers = bool(tcfg.get("persistent_workers", True))
    seed = int(tcfg.get("seed", 20260907))
    compile_model = bool(tcfg.get("compile", True)) and not args.no_compile
    compile_mode = str(tcfg.get("compile_mode", "default"))
    grad_checkpoint = bool(tcfg.get("grad_checkpoint", False))
    morph_dispatch = str(tcfg.get("morph_dispatch", base_cfg.model.morph_dispatch))
    if morph_dispatch not in ("masked", "sparse"):
        raise ValueError("train.morph_dispatch must be 'masked' or 'sparse'")
    base_cfg.model.morph_dispatch = morph_dispatch
    assistant_only_head = bool(tcfg.get("assistant_only_head", True))
    loss_fp32_logits = bool(tcfg.get("loss_fp32_logits", True))
    ddp_static_graph = bool(tcfg.get("ddp_static_graph", True))
    ddp_bucket_cap_mb = int(tcfg.get("ddp_bucket_cap_mb", 100))
    control_interval = max(1, int(tcfg.get("control_interval", 20)))
    log_interval = int(tcfg.get("log_interval", 10))
    ckpt_interval = int(tcfg.get("ckpt_interval", 250))
    keep_last = int(tcfg.get("keep_last", 3))
    keep_every = int(tcfg.get("keep_every", 1000))
    archive_interval_hours = float(tcfg.get("archive_interval_hours", 4.0))
    max_hours = float(tcfg.get("max_hours", 71.0))
    save_final_checkpoint = bool(tcfg.get("save_final_checkpoint", True))

    vcfg = dict(sft.get("validation", {}))
    val_enabled = bool(vcfg.get("enabled", True))
    val_shard_ids = [int(x) for x in vcfg.get("shard_ids", [53])] if val_enabled else []
    val_interval = max(1, int(vcfg.get("interval", 250)))
    val_batches = max(1, int(vcfg.get("batches", 8)))
    val_micro_batch = max(1, int(vcfg.get("micro_batch_size", micro_batch)))
    val_workers = max(0, int(vcfg.get("num_workers", min(2, num_workers))))
    val_seed = int(vcfg.get("seed", 20260920))
    val_at_start = bool(vcfg.get("at_start", True))

    info = setup_distributed()
    signal.signal(signal.SIGUSR1, _request_stop)
    signal.signal(signal.SIGTERM, _request_stop)

    def _validate_local_runtime():
        if seq_len > base_cfg.model.max_seq_len:
            raise ValueError(
                f"SFT seq_len={seq_len} exceeds model max_seq_len={base_cfg.model.max_seq_len}"
            )
        if base_cfg.model.attn_impl != "flash":
            raise ValueError("packed SFT isolation requires model.attn_impl='flash'")
        if not packed_flash_attn_available():
            raise RuntimeError(
                "packed SFT isolation requires flash_attn_func and flash_attn_varlen_func"
            )
        return True

    _all_rank_call(info, "validate local SFT runtime", _validate_local_runtime)
    _main_rank_call(
        info, "create run directory", lambda: run_dir.mkdir(parents=True, exist_ok=True)
    )
    log = _all_rank_call(
        info, "initialize SFT logger", lambda: Logger(run_dir, info)
    )
    dataset_contract = _main_rank_call(
        info,
        "validate SFT binary/dataset contract",
        lambda: _validate_dataset_contract(data_dir, base_cfg.model.vocab_size, seq_len),
    )
    set_seed(seed, info.rank)

    if torch.cuda.is_available():
        torch.backends.cuda.matmul.allow_tf32 = True
        torch.backends.cudnn.allow_tf32 = True
        # H200 benefits from the reduced-precision BF16 accumulation paths;
        # model numerics remain BF16 under autocast with fp32 master weights.
        torch.backends.cuda.matmul.allow_bf16_reduced_precision_reduction = True
    torch.set_float32_matmul_precision("high")

    # Discover/build the adaptive plan on every rank, but synchronize any
    # local filesystem/data failure before later collectives can deadlock.
    def _discover_runtime_data():
        local_all_shards = discover_sft_shards(data_dir, seq_len)
        local_all_stats = corpus_stats(local_all_shards, seq_len)
        if int(local_all_stats["windows"]) != int(dataset_contract["training_windows"]):
            raise RuntimeError(
                "runtime adaptive sample count differs from audited packing contract: "
                f"{local_all_stats['windows']} != {dataset_contract['training_windows']}"
            )
        if int(local_all_stats["assistant_target_tokens"]) != int(
            dataset_contract["assistant_target_tokens_per_epoch"]
        ):
            raise RuntimeError(
                "runtime adaptive assistant-target count differs from audited contract: "
                f"{local_all_stats['assistant_target_tokens']} != "
                f"{dataset_contract['assistant_target_tokens_per_epoch']}"
            )
        if int(local_all_stats["hard_continuations"]) != 0:
            raise RuntimeError("runtime adaptive plan contains hard continuations")

        local_shards, local_val_shards = _split_train_val_shards(
            local_all_shards, val_shard_ids
        )
        if val_enabled and not local_val_shards:
            raise RuntimeError("validation enabled but validation shard set is empty")
        local_stats = corpus_stats(local_shards, seq_len)
        local_val_stats = (
            corpus_stats(local_val_shards, seq_len) if local_val_shards else None
        )
        return local_all_shards, local_shards, local_val_shards, local_stats, local_val_stats

    all_shards, shards, val_shards, stats, val_stats = _all_rank_call(
        info, "discover/validate adaptive SFT plan", _discover_runtime_data
    )
    def _rank0_role_audit():
        role_stats = validate_roles(shards)
        val_role_stats = validate_roles(val_shards) if val_stats is not None else None
        return {"train": role_stats, "validation": val_role_stats}

    role_audit = _main_rank_call(info, "SFT role audit", _rank0_role_audit)
    if info.is_main:
        log.info_msg(
            f"SFT train: {stats['shards']} shards, {stats['raw_tokens']/1e9:.3f}B raw tokens, "
            f"{stats['windows']:,} adaptive samples, "
            f"valid_context={100*stats['valid_context_ratio']:.3f}% "
            f"assistant_target={100*stats['assistant_target_ratio']:.3f}% "
            f"turn_cont={stats['turn_aligned_continuations']:,} "
            f"assistant_cont={stats['assistant_aligned_continuations']:,}"
        )
        log.info_msg(f"train role audit: {role_audit['train']}")
        if val_stats is not None:
            log.info_msg(
                f"SFT held-out validation: shard_ids={val_shard_ids} "
                f"{val_stats['raw_tokens']/1e6:.1f}M raw tokens, "
                f"{val_stats['windows']:,} windows"
            )
            log.info_msg(f"validation role audit: {role_audit['validation']}")

    sequences_per_step = micro_batch * grad_accum * info.world_size
    tokens_per_step = sequences_per_step * seq_len
    auto_steps = math.ceil(int(stats["windows"]) / sequences_per_step)
    configured_steps = int(tcfg.get("max_steps", 0))
    max_steps = configured_steps if configured_steps > 0 else auto_steps
    if args.smoke_steps:
        max_steps = args.smoke_steps
    training_recipe = {
        "max_steps": int(max_steps),
        "micro_batch_size": micro_batch,
        "grad_accum_steps": grad_accum,
        "seed": seed,
        "num_workers": num_workers,
        "morph_dispatch": morph_dispatch,
        "grad_checkpoint": grad_checkpoint,
        "assistant_only_head": assistant_only_head,
        "loss_fp32_logits": loss_fp32_logits,
        "compile_requested": bool(tcfg.get("compile", True)),
        "compile_mode_requested": compile_mode,
        "optim": {
            "lr": ocfg.lr,
            "min_lr_ratio": ocfg.min_lr_ratio,
            "betas": list(ocfg.betas),
            "eps": ocfg.eps,
            "weight_decay": ocfg.weight_decay,
            "grad_clip": ocfg.grad_clip,
            "schedule": ocfg.schedule,
            "warmup_steps": ocfg.warmup_steps,
            "decay_fraction": ocfg.decay_fraction,
        },
        "model_config": base_cfg.model.__dict__,
    }
    split_fingerprint = _training_contract_fingerprint(
        dataset_contract["fingerprint"], seq_len, val_shard_ids, training_recipe
    )
    log.info_msg(
        f"world_size={info.world_size} device={info.device} seq_len={seq_len} "
        f"micro_batch={micro_batch} accum={grad_accum} tokens/step={tokens_per_step:,} "
        f"steps={max_steps} (auto_one_epoch={auto_steps})"
    )

    # Build architecture exactly from the pretraining config. Fail together
    # if a rank has a local construction/runtime problem.
    base_model = _all_rank_call(
        info, "build SFT model", lambda: build_model(base_cfg.model)
    )
    base_model.grad_checkpoint = grad_checkpoint

    start_step = 0
    context_tokens_seen = 0
    assistant_tokens_seen = 0
    resume_state: Optional[dict[int, list[int]]] = None
    source_checkpoint = ""

    sft_ckpt_value = _main_rank_call(
        info,
        "resolve latest SFT checkpoint",
        lambda: str(latest_checkpoint(run_dir) or ""),
    )
    sft_ckpt = Path(str(sft_ckpt_value)) if sft_ckpt_value else None
    state: Optional[dict[str, Any]] = None
    if sft_ckpt is not None:
        def _load_and_validate_local_resume():
            local_state = _load_torch(sft_ckpt)
            base_model.load_state_dict(local_state["model"])
            saved_fingerprint = str(local_state.get("dataset_fingerprint", ""))
            if not saved_fingerprint:
                raise RuntimeError(
                    "refusing legacy SFT resume without dataset_fingerprint; use a fresh run_dir"
                )
            if saved_fingerprint != dataset_contract["fingerprint"]:
                raise RuntimeError(
                    "SFT dataset contract changed since checkpoint; refusing unsafe resume"
                )
            saved_split = str(local_state.get("training_contract_fingerprint", ""))
            if not saved_split:
                raise RuntimeError(
                    "refusing legacy SFT resume without training_contract_fingerprint"
                )
            if saved_split != split_fingerprint:
                raise RuntimeError(
                    "SFT train/validation split or packing/recipe contract changed; "
                    "refusing unsafe resume"
                )
            return local_state

        state = _all_rank_call(
            info, "load/validate local SFT resume checkpoint", _load_and_validate_local_resume
        )
        start_step = int(state["step"])
        context_tokens_seen = int(state.get("context_tokens_seen", 0))
        assistant_tokens_seen = int(state.get("assistant_tokens_seen", 0))
        source_checkpoint = str(state.get("base_checkpoint", ""))
        log.info_msg(f"resuming SFT from {sft_ckpt} at step {start_step}")
    else:
        def _resolve_base_checkpoint():
            explicit = str(sft.get("base_checkpoint", "")).strip()
            if explicit:
                candidate = Path(explicit)
            else:
                candidate = latest_checkpoint(Path(sft["base_run_dir"]))
                if candidate is None:
                    raise FileNotFoundError(
                        f"no pretraining checkpoint under {sft['base_run_dir']}"
                    )
            if not candidate.exists():
                raise FileNotFoundError(candidate)
            return str(candidate)

        base_ckpt_value = _main_rank_call(
            info, "resolve base pretraining checkpoint", _resolve_base_checkpoint
        )
        base_ckpt = Path(str(base_ckpt_value))
        source_checkpoint = str(base_ckpt)

        # Fresh multi-GPU SFT only needs one Lustre read of the large base
        # checkpoint. DDP construction below broadcasts rank-0 parameters to
        # every other rank anyway. Resume checkpoints still load on every rank
        # because optimizer/data state is rank-local and must be restored.
        def _load_base_on_rank0():
            base_state = _load_torch(base_ckpt)
            try:
                base_model.load_state_dict(base_state["model"])
            finally:
                del base_state

        if info.world_size == 1:
            _load_base_on_rank0()
        else:
            _main_rank_call(info, "load base checkpoint", _load_base_on_rank0)
        log.info_msg(f"starting NEW SFT from latest pretraining weights: {base_ckpt}")
        if info.world_size > 1:
            log.info_msg("fresh distributed init: base checkpoint read on rank 0 only; bare model state broadcast before compile")
        log.info_msg("pretraining optimizer/scheduler/data state intentionally NOT loaded")

    base_model.to(info.device)
    device_type = info.device.type
    use_amp = device_type == "cuda"

    # Fresh SFT only read the base checkpoint on rank 0. Synchronize the bare
    # module before compile warmup; this also verifies resume ranks are bitwise
    # aligned before optimizer construction.
    if info.world_size > 1:
        _broadcast_module_state(base_model, src=0)
        log.info_msg("bare model state synchronized from rank 0 before compile warmup")

    model: torch.nn.Module = base_model
    if compile_model:
        compiled_candidate: Optional[torch.nn.Module] = None
        local_compile_error = ""
        warm_x = warm_y = warm_cu = warm_pos = None
        try:
            compiled_candidate = torch.compile(base_model, mode=compile_mode)
            warm_x = torch.randint(
                0, base_cfg.model.vocab_size, (micro_batch, seq_len), device=info.device
            )
            warm_y = warm_x.clone()
            warm_y[:, : seq_len // 3] = -100
            warm_x[:, seq_len // 4] = EOS_ID
            warm_x[:, seq_len // 2] = EOS_ID
            warm_x[:, (3 * seq_len) // 4] = EOS_ID
            warm_cu, warm_pos = _packed_layout(warm_x)
            warm_start = time.time()
            with torch.autocast(device_type, dtype=torch.bfloat16, enabled=use_amp):
                if assistant_only_head:
                    warm_loss, _ = compiled_candidate(
                        warm_x, labels=warm_y, return_logits=False,
                        loss_reduction="sum", loss_fp32_logits=loss_fp32_logits,
                        cu_seqlens=warm_cu, max_seqlen=seq_len,
                        position_ids=warm_pos,
                    )
                    warm_loss = warm_loss / max(1, int((warm_y != -100).sum().item()))
                else:
                    _, warm_logits = compiled_candidate(
                        warm_x, return_logits=True,
                        cu_seqlens=warm_cu, max_seqlen=seq_len,
                        position_ids=warm_pos,
                    )
                    warm_loss = F.cross_entropy(
                        warm_logits.reshape(-1, warm_logits.size(-1)).float(),
                        warm_y.reshape(-1), ignore_index=-100, reduction="sum",
                    ) / max(1, int((warm_y != -100).sum().item()))
            warm_loss.backward()
            log.info_msg(f"local SFT compile warmup ok ({time.time()-warm_start:.1f}s)", all_ranks=True)
        except Exception as exc:  # noqa: BLE001
            local_compile_error = f"{type(exc).__name__}: {exc}"
        finally:
            base_model.zero_grad(set_to_none=True)
            del warm_x, warm_y, warm_cu, warm_pos

        compile_errors = _collect_rank_errors(info, local_compile_error)
        if compile_errors:
            compile_model = False
            compiled_candidate = None
            model = base_model
            if info.is_main:
                log.info_msg(
                    "torch.compile disabled globally because at least one rank failed warmup: "
                    + " | ".join(compile_errors)
                )
        else:
            model = compiled_candidate
            log.info_msg(f"torch.compile globally accepted mode={compile_mode}")

    # DDP is constructed only after every rank has made the same compile/eager
    # decision. Compile failures therefore cannot split the world into mixed
    # execution modes or strand a rank inside a DDP backward collective.
    if info.world_size > 1:
        model = DDP(
            model,
            device_ids=[info.device.index] if info.device.type == "cuda" else None,
            broadcast_buffers=False,
            gradient_as_bucket_view=True,
            static_graph=ddp_static_graph,
            bucket_cap_mb=ddp_bucket_cap_mb,
        )

    effective_compile = bool(compile_model)
    if state is not None:
        saved_effective_compile = state.get("effective_compile")
        if saved_effective_compile is None:
            raise RuntimeError(
                "refusing legacy SFT resume without effective_compile runtime contract"
            )
        if bool(saved_effective_compile) != effective_compile:
            raise RuntimeError(
                "SFT execution mode changed since checkpoint: "
                f"saved effective_compile={bool(saved_effective_compile)} "
                f"current effective_compile={effective_compile}. "
                "Resume with the same compile/eager mode or use a fresh run_dir."
            )

    optimizer = _all_rank_call(
        info, "build SFT optimizer", lambda: build_optimizer(base_model, ocfg)
    )
    if state is not None:
        def _restore_local_optimizer_and_data_state():
            optimizer.load_state_dict(state["optimizer"])
            saved_world = int(state.get("world_size", info.world_size))
            saved_workers = int(state.get("num_workers", num_workers))
            if saved_world != info.world_size or saved_workers != num_workers:
                raise RuntimeError(
                    "refusing SFT resume with changed world_size/num_workers: "
                    f"saved world={saved_world}, workers={saved_workers}; "
                    f"current world={info.world_size}, workers={num_workers}. "
                    "Use the original topology or start a fresh run_dir."
                )
            local_resume_state: Optional[dict[int, list[int]]] = None
            per_rank = state.get("data_state", [])
            if info.rank < len(per_rank) and per_rank[info.rank]:
                local_resume_state = {
                    int(k): list(v) for k, v in per_rank[info.rank].items()
                }
            return local_resume_state

        restored_resume = _all_rank_call(
            info,
            "restore local optimizer/data resume state",
            _restore_local_optimizer_and_data_state,
        )
        if restored_resume is not None:
            resume_state = restored_resume
        del state

    def _build_local_loaders():
        local_train_loader, _ = build_sft_loader(
            data_dir=data_dir,
            seq_len=seq_len,
            micro_batch_size=micro_batch,
            rank=info.rank,
            world_size=info.world_size,
            num_workers=num_workers,
            prefetch_factor=prefetch_factor,
            seed=seed,
            resume_state=resume_state,
            shards=shards,
            persistent_workers=persistent_workers,
        )
        local_train_iter = iter(local_train_loader)

        local_val_loader = None
        if val_shards:
            local_val_loader, _ = build_sft_loader(
                data_dir=data_dir,
                seq_len=seq_len,
                micro_batch_size=val_micro_batch,
                rank=info.rank,
                world_size=info.world_size,
                num_workers=val_workers,
                prefetch_factor=max(1, prefetch_factor),
                seed=val_seed,
                resume_state=None,
                shards=val_shards,
                persistent_workers=False,
            )
            # Force worker construction now so startup failures are synchronized.
            if val_workers > 0:
                probe = iter(local_val_loader)
                del probe
        return local_train_loader, local_train_iter, local_val_loader

    train_loader, train_iter, val_loader = _all_rank_call(
        info, "build SFT data loaders", _build_local_loaders
    )

    def _write_rank0_metadata():
        resolved = {
            "sft_config": sft,
            "base_model_config": base_cfg.to_dict(),
            "resolved_base_checkpoint": source_checkpoint,
            "world_size": info.world_size,
            "tokens_per_step": tokens_per_step,
            "auto_one_epoch_steps": auto_steps,
            "corpus_stats": stats,
            "validation_stats": val_stats,
            "validation_shard_ids": val_shard_ids,
            "dataset_contract": dataset_contract,
            "training_recipe": training_recipe,
            "training_contract_fingerprint": split_fingerprint,
            "effective_compile": effective_compile,
        }
        (run_dir / "config.resolved.json").write_text(
            json.dumps(resolved, indent=2, ensure_ascii=False) + "\n", encoding="utf-8"
        )
        write_run_metadata(
            run_dir,
            {
                "job_id": os.environ.get("SLURM_JOB_ID", ""),
                "nodes": os.environ.get("SLURM_JOB_NODELIST", ""),
                "world_size": info.world_size,
                "start_step": start_step,
                "base_checkpoint": source_checkpoint,
                "raw_sft_tokens": stats["raw_tokens"],
                "packed_windows": stats["windows"],
                "dataset_fingerprint": dataset_contract["fingerprint"],
                "training_contract_fingerprint": split_fingerprint,
                "validation_shard_ids": val_shard_ids,
                "effective_compile": effective_compile,
                "torch": torch.__version__,
            },
        )

    _main_rank_call(info, "write SFT run metadata", _write_rank0_metadata)

    control_flags = torch.zeros(2, device=info.device, dtype=torch.int32)
    data_state: dict[int, list[int]] = dict(resume_state or {})
    job_start = time.time()
    step_timer = time.time()
    last_archive = time.time()
    done = start_step
    loss_log_sum = torch.zeros((), device=info.device, dtype=torch.float32)
    last_log_step = start_step
    last_log_assistant_tokens = assistant_tokens_seen
    assistant_tokens_seen_device = (
        torch.tensor(assistant_tokens_seen, device=info.device, dtype=torch.int64)
        if info.world_size > 1 else None
    )

    def assistant_seen_value() -> int:
        if assistant_tokens_seen_device is not None:
            return int(assistant_tokens_seen_device.item())
        return int(assistant_tokens_seen)


    if val_loader is not None and val_at_start:
        val_t0 = time.time()
        metrics = _run_validation(
            base_model, val_loader, info,
            seq_len=seq_len,
            batches=val_batches,
            assistant_only_head=assistant_only_head,
            loss_fp32_logits=loss_fp32_logits,
            use_amp=use_amp,
            device_type=device_type,
        )
        def _log_initial_validation():
            log.metric({
                "step": start_step,
                "kind": "validation",
                "val_loss": round(float(metrics["loss"]), 6),
                "val_ppl": round(float(metrics["ppl"]), 4),
                "val_assistant_tokens": int(metrics["assistant_tokens"]),
                "val_context_tokens": int(metrics["context_tokens"]),
            })
            log.info_msg(
                f"VAL step={start_step} loss={metrics['loss']:.4f} "
                f"ppl={metrics['ppl']:.2f} assistant={int(metrics['assistant_tokens']):,} "
                f"context={int(metrics['context_tokens']):,}"
            )

        _main_rank_call(info, "log initial validation", _log_initial_validation)

    # Benchmark/throughput timing starts after optional compile/JIT warmup and
    # initial validation.
    step_timer = time.time()
    log.info_msg(f"SFT training steps {start_step} -> {max_steps}")
    model.train()

    for step in range(start_step, max_steps):
        lr = lr_at_step(step, ocfg, max_steps)
        set_lr(optimizer, lr)
        optimizer.zero_grad(set_to_none=True)

        # Pull one whole optimizer step first so loss can be normalized by the
        # exact number of assistant tokens across all ranks/microbatches.
        micro_batches = []
        local_target_tokens = 0
        batch_fetch_error = ""
        try:
            for _ in range(grad_accum):
                batch = next(train_iter)
                micro_batches.append(batch)
                local_target_tokens += int(batch["trainable_tokens"].sum().item())
                row = batch["state"][-1].tolist()
                data_state[int(row[0])] = [int(v) for v in row]
            if local_target_tokens <= 0:
                raise RuntimeError(
                    f"optimizer step contains zero assistant trainable tokens on rank {info.rank}"
                )
        except Exception as exc:  # noqa: BLE001
            batch_fetch_error = f"{type(exc).__name__}: {exc}"

        if info.world_size > 1:
            # Fuse the per-rank batch-fetch status with the target-count SUM.
            # The hot path therefore keeps exactly one tiny collective here.
            # Expensive object gathering happens only on the exceptional path.
            target_status = torch.tensor(
                [local_target_tokens, 1 if batch_fetch_error else 0],
                device=info.device,
                dtype=torch.int64,
            )
            _sum_all(target_status)
            if int(target_status[1].item()) > 0:
                batch_fetch_errors = _collect_rank_errors(info, batch_fetch_error)
                raise RuntimeError(
                    f"SFT batch fetch failed at optimizer step {step}: "
                    + " | ".join(batch_fetch_errors)
                )
            global_target = target_status[0]
            # Count targets exactly in int64; only the normalization scalar is
            # converted to float on-device.
            grad_scale = info.world_size / global_target.to(torch.float32)
        else:
            if batch_fetch_error:
                raise RuntimeError(
                    f"SFT batch fetch failed at optimizer step {step}: {batch_fetch_error}"
                )
            global_target = None
            grad_scale = 1.0 / float(local_target_tokens)
        local_loss_sum = torch.zeros((), device=info.device, dtype=torch.float32)

        for micro, batch in enumerate(micro_batches):
            raw_tokens = batch["tokens"].to(
                info.device, dtype=torch.long, non_blocking=True
            )
            inputs = raw_tokens[:, :-1]
            labels = raw_tokens[:, 1:].clone()
            cu_seqlens, position_ids = _packed_layout(inputs)
            trainable_mask = batch["trainable_mask"][:, 1:].to(
                info.device, non_blocking=True
            )
            labels.masked_fill_(~trainable_mask, -100)
            is_last = micro == grad_accum - 1
            sync_ctx = model.no_sync() if (info.world_size > 1 and not is_last) else contextlib.nullcontext()
            with sync_ctx:
                with torch.autocast(device_type, dtype=torch.bfloat16, enabled=use_amp):
                    if assistant_only_head:
                        # Exact assistant-only objective. The LM head + CE live
                        # inside model.forward so tied-embedding gradients are
                        # covered by DDP and torch.compile can optimize the full
                        # training graph.
                        loss_sum, _ = model(
                            inputs,
                            labels=labels,
                            return_logits=False,
                            loss_reduction="sum",
                            loss_fp32_logits=loss_fp32_logits,
                            cu_seqlens=cu_seqlens,
                            max_seqlen=seq_len,
                            position_ids=position_ids,
                        )
                    else:
                        _, logits = model(
                            inputs, return_logits=True,
                            cu_seqlens=cu_seqlens, max_seqlen=seq_len,
                            position_ids=position_ids,
                        )
                        loss_sum = F.cross_entropy(
                            logits.reshape(-1, logits.size(-1)).float(),
                            labels.reshape(-1), ignore_index=-100, reduction="sum"
                        )
                (loss_sum * grad_scale).backward()
            local_loss_sum += loss_sum.detach()
            if not assistant_only_head:
                del logits
            del loss_sum, raw_tokens, inputs, labels, trainable_mask, cu_seqlens, position_ids

        grad_norm = torch.nn.utils.clip_grad_norm_(
            base_model.parameters(), ocfg.grad_clip, foreach=True,
            error_if_nonfinite=True,
        )
        optimizer.step()
        done = step + 1
        context_tokens_seen += tokens_per_step
        if assistant_tokens_seen_device is not None:
            assistant_tokens_seen_device.add_(global_target.detach())
        else:
            assistant_tokens_seen += local_target_tokens
        loss_log_sum += local_loss_sum

        # Loss is metrics-only. Reduce/synchronize only when a log is emitted.
        if done % log_interval == 0 or done == start_step + 1:
            global_loss_sum = loss_log_sum.clone()
            _sum_all(global_loss_sum)
            assistant_seen_log = assistant_seen_value()
            target_log_tokens = assistant_seen_log - last_log_assistant_tokens
            mean_loss = float(global_loss_sum.item() / max(1, target_log_tokens))
            if use_amp:
                torch.cuda.synchronize()
            elapsed = time.time() - step_timer
            span = max(1, done - last_log_step)
            sec_per_step = elapsed / span
            tok_per_s = tokens_per_step / sec_per_step
            target_ratio = target_log_tokens / (tokens_per_step * span)
            rec = {
                "step": done,
                "loss": round(mean_loss, 6),
                "ppl": round(math.exp(min(20.0, mean_loss)), 4),
                "lr": lr,
                "grad_norm": round(float(grad_norm), 4),
                "context_tokens_seen": context_tokens_seen,
                "assistant_tokens_seen": assistant_seen_log,
                "assistant_ratio_step": round(target_ratio, 5),
                "sec_per_step": round(sec_per_step, 3),
                "tokens_per_s": round(tok_per_s, 1),
                "peak_mem_gib": round(
                    torch.cuda.max_memory_allocated(info.device) / (1024**3), 3
                    if use_amp else 0.0
                ) if use_amp else 0.0,
                "hours": round((time.time() - job_start) / 3600.0, 3),
            }
            def _log_training_step():
                log.metric(rec)
                log.info_msg(
                    f"SFT {done}/{max_steps} loss {mean_loss:.4f} ppl {rec['ppl']:.2f} "
                    f"lr {lr:.3e} gn {rec['grad_norm']:.2f} target={target_ratio*100:.1f}% "
                    f"{tok_per_s/1e3:.0f}k tok/s context={context_tokens_seen/1e9:.2f}B "
                    f"assistant={assistant_seen_log/1e9:.2f}B"
                )

            _main_rank_call(info, f"log training metrics at step {done}", _log_training_step)
            loss_log_sum.zero_()
            last_log_assistant_tokens = assistant_seen_log
            last_log_step = done
            step_timer = time.time()

        if val_loader is not None and (done % val_interval == 0 or done == max_steps):
            val_wall = time.time()
            metrics = _run_validation(
                base_model, val_loader, info,
                seq_len=seq_len,
                batches=val_batches,
                assistant_only_head=assistant_only_head,
                loss_fp32_logits=loss_fp32_logits,
                use_amp=use_amp,
                device_type=device_type,
            )
            def _log_validation():
                log.metric({
                    "step": done,
                    "kind": "validation",
                    "val_loss": round(float(metrics["loss"]), 6),
                    "val_ppl": round(float(metrics["ppl"]), 4),
                    "val_assistant_tokens": int(metrics["assistant_tokens"]),
                    "val_context_tokens": int(metrics["context_tokens"]),
                })
                log.info_msg(
                    f"VAL step={done} loss={metrics['loss']:.4f} "
                    f"ppl={metrics['ppl']:.2f} assistant={int(metrics['assistant_tokens']):,} "
                    f"context={int(metrics['context_tokens']):,}"
                )

            _main_rank_call(info, f"log validation at step {done}", _log_validation)
            # Keep validation wall time out of training throughput estimates.
            step_timer += time.time() - val_wall

        # All ranks enter control collectives on deterministic step boundaries.
        # Rank 0 alone decides wall-clock archive cadence; MAX-reduction then
        # broadcasts that decision while also OR-ing stop requests from any rank.
        # This removes the former per-rank time.time() branch that could send one
        # rank into a barrier while its peers skipped it.
        local_stop = _STOP_REQUESTED or (
            bool(max_hours) and (time.time() - job_start) / 3600.0 >= max_hours
        )
        control_due = (
            done % control_interval == 0
            or done % ckpt_interval == 0
            or done == max_steps
        )
        archive_due = False
        must_stop = False
        if control_due:
            rank0_archive_due = (
                info.is_main
                and archive_interval_hours > 0
                and (time.time() - last_archive) / 3600.0 >= archive_interval_hours
            )
            archive_due, must_stop = _sync_control_flags(
                control_flags,
                rank0_archive_due=rank0_archive_due,
                local_stop=local_stop,
            )

        if archive_due:
            def _write_archive():
                path = save_model_archive(
                    run_dir,
                    done,
                    {
                        "step": done,
                        "base_checkpoint": source_checkpoint,
                        "context_tokens_seen": context_tokens_seen,
                        "assistant_tokens_seen": assistant_seen_value(),
                        "dataset_fingerprint": dataset_contract["fingerprint"],
                        "training_contract_fingerprint": split_fingerprint,
                        "validation_shard_ids": val_shard_ids,
                        "effective_compile": effective_compile,
                        "model": base_model.state_dict(),
                    },
                )
                log.info_msg(f"SFT model archive written: {path.name}")
                return path

            _main_rank_call(info, f"write model archive at step {done}", _write_archive)
            # Keep every rank's local cadence anchor aligned to the same event.
            last_archive = time.time()
            step_timer = time.time()

        checkpoint_due = (
            must_stop
            or done % ckpt_interval == 0
            or (done == max_steps and save_final_checkpoint)
        )
        if checkpoint_due:
            gathered = gather_object({str(k): v for k, v in data_state.items()}, info.world_size)

            def _write_checkpoint():
                payload = {
                    "step": done,
                    "base_checkpoint": source_checkpoint,
                    "context_tokens_seen": context_tokens_seen,
                    "assistant_tokens_seen": assistant_seen_value(),
                    "model": base_model.state_dict(),
                    "optimizer": optimizer.state_dict(),
                    "data_state": [{int(k): v for k, v in d.items()} for d in gathered],
                    "world_size": info.world_size,
                    "num_workers": num_workers,
                    "dataset_fingerprint": dataset_contract["fingerprint"],
                    "training_contract_fingerprint": split_fingerprint,
                    "validation_shard_ids": val_shard_ids,
                    "effective_compile": effective_compile,
                    "dataset_contract": dataset_contract,
                    "training_recipe": training_recipe,
                    "sft_config": sft,
                }
                path = save_checkpoint(
                    run_dir, done, payload,
                    keep_last=keep_last, keep_every=keep_every,
                )
                log.info_msg(f"SFT checkpoint written: {path.name}")
                return path

            _main_rank_call(info, f"write SFT checkpoint at step {done}", _write_checkpoint)
            step_timer = time.time()

        if must_stop:
            log.info_msg("stop requested; exiting after SFT checkpoint")
            break

    final_assistant_seen = assistant_seen_value()

    def _finalize_log_rank0():
        log.info_msg(
            f"SFT finished at step {done}: context={context_tokens_seen/1e9:.3f}B, "
            f"assistant={final_assistant_seen/1e9:.3f}B, "
            f"hours={(time.time()-job_start)/3600:.2f}"
        )
        log.close()

    _main_rank_call(info, "finalize SFT log", _finalize_log_rank0)
    cleanup()
    return 0


if __name__ == "__main__":
    sys.exit(main())