diff --git a/nanochat/checkpoint_manager.py b/nanochat/checkpoint_manager.py
index 5a95fbf..e24533a 100644
--- a/nanochat/checkpoint_manager.py
+++ b/nanochat/checkpoint_manager.py
@@ -170,3 +170,22 @@ def load_model(source, *args, **kwargs):
     base_dir = get_base_dir()
     checkpoints_dir = os.path.join(base_dir, model_dir)
     return load_model_from_dir(checkpoints_dir, *args, **kwargs)
+
+def load_optimizer_state(source, device, rank, model_tag=None, step=None):
+    """Load just the optimizer shard for a given rank, without re-loading the model."""
+    model_dir = {
+        "base": "base_checkpoints",
+        "sft": "chatsft_checkpoints",
+        "rl": "chatrl_checkpoints",
+    }[source]
+    base_dir = get_base_dir()
+    checkpoints_dir = os.path.join(base_dir, model_dir)
+    if model_tag is None:
+        model_tag = find_largest_model(checkpoints_dir)
+    checkpoint_dir = os.path.join(checkpoints_dir, model_tag)
+    if step is None:
+        step = find_last_step(checkpoint_dir)
+    optimizer_path = os.path.join(checkpoint_dir, f"optim_{step:06d}_rank{rank:d}.pt")
+    log0(f"Loading optimizer state from {optimizer_path}")
+    optimizer_data = torch.load(optimizer_path, map_location=device)
+    return optimizer_data
diff --git a/scripts/base_train.py b/scripts/base_train.py
index 996b2ba..bb76e90 100644
--- a/scripts/base_train.py
+++ b/scripts/base_train.py
@@ -468,6 +468,7 @@ while True:
                 "user_config": user_config, # inputs to the training script
                 "device_batch_size": args.device_batch_size,
                 "max_seq_len": args.max_seq_len,
+                "total_batch_size": total_batch_size,
                 "dataloader_state_dict": dataloader_state_dict,
                 "loop_state": { # all loop state (other than step) so that we can resume training
                     "min_val_bpb": min_val_bpb,
diff --git a/scripts/chat_sft.py b/scripts/chat_sft.py
index 4c81f06..edac3d8 100644
--- a/scripts/chat_sft.py
+++ b/scripts/chat_sft.py
@@ -9,6 +9,7 @@ Or torchrun for training:
 torchrun --standalone --nproc_per_node=8 -m scripts.chat_sft -- --device-batch-size=16
 """
 
+import gc
 import argparse
 import os
 os.environ["PYTORCH_ALLOC_CONF"] = "expandable_segments:True"
@@ -16,12 +17,14 @@ import time
 import wandb
 import torch
 from contextlib import nullcontext
-from nanochat.common import compute_init, compute_cleanup, print0, DummyWandb, get_base_dir, autodetect_device_type
+from nanochat.common import compute_init, compute_cleanup, print0, DummyWandb, get_base_dir, autodetect_device_type, get_peak_flops
 from nanochat.tokenizer import get_token_bytes
-from nanochat.checkpoint_manager import save_checkpoint
+from nanochat.checkpoint_manager import save_checkpoint, load_model, load_optimizer_state
 from nanochat.loss_eval import evaluate_bpb
-from nanochat.checkpoint_manager import load_model
 import torch.distributed as dist
+from nanochat.flash_attention import HAS_FA3
+from nanochat.engine import Engine
+from scripts.chat_eval import run_chat_eval
 
 from tasks.common import TaskMixture
 from tasks.gsm8k import GSM8K
@@ -37,27 +40,30 @@ parser = argparse.ArgumentParser(description="Supervised fine-tuning (SFT) the m
 parser.add_argument("--run", type=str, default="dummy", help="wandb run name ('dummy' disables wandb logging)")
 # Runtime
 parser.add_argument("--device-type", type=str, default="", help="cuda|cpu|mps (empty = autodetect)")
-parser.add_argument("--dtype", type=str, default="bfloat16", help="float32|bfloat16")
 # Model loading
 parser.add_argument("--model-tag", type=str, default=None, help="model tag to load from")
 parser.add_argument("--model-step", type=int, default=None, help="model step to load from")
+parser.add_argument("--load-optimizer", type=int, default=0, help="warm-start optimizer from pretrained checkpoint (0=no, 1=yes)")
 # Training horizon
 parser.add_argument("--num-iterations", type=int, default=-1, help="number of optimization steps (-1 = full epoch)")
-# Batch sizes
-parser.add_argument("--max-seq-len", type=int, default=2048, help="max context length")
-parser.add_argument("--device-batch-size", type=int, default=32, help="per-device batch size")
-parser.add_argument("--total-batch-size", type=int, default=524288, help="total batch size in tokens")
-# Optimization
-parser.add_argument("--embedding-lr", type=float, default=0.3, help="learning rate for embedding parameters (Adam)")
-parser.add_argument("--unembedding-lr", type=float, default=0.004, help="learning rate for unembedding parameters (Adam)")
-parser.add_argument("--matrix-lr", type=float, default=0.02, help="learning rate for matrix parameters (Muon)")
-parser.add_argument("--weight-decay", type=float, default=0.0, help="weight decay for embedding/unembedding parameters (Adam)")
-parser.add_argument("--init-lr-frac", type=float, default=1.0, help="initial LR as fraction of base LR")
+# Batch sizes (default: inherit from pretrained checkpoint)
+parser.add_argument("--max-seq-len", type=int, default=None, help="max context length (default: inherit from pretrain)")
+parser.add_argument("--device-batch-size", type=int, default=None, help="per-device batch size (default: inherit from pretrain)")
+parser.add_argument("--total-batch-size", type=int, default=None, help="total batch size in tokens (default: inherit from pretrain)")
+# Optimization (default: inherit from pretrained checkpoint)
+parser.add_argument("--embedding-lr", type=float, default=None, help="learning rate for embedding parameters (Adam) (default: inherit from pretrain)")
+parser.add_argument("--unembedding-lr", type=float, default=None, help="learning rate for unembedding parameters (Adam) (default: inherit from pretrain)")
+parser.add_argument("--matrix-lr", type=float, default=None, help="learning rate for matrix parameters (Muon) (default: inherit from pretrain)")
+parser.add_argument("--init-lr-frac", type=float, default=0.8, help="initial LR as fraction of base LR")
+parser.add_argument("--warmup-ratio", type=float, default=0.0, help="ratio of iterations for LR warmup")
+parser.add_argument("--warmdown-ratio", type=float, default=0.5, help="ratio of iterations for LR warmdown")
+parser.add_argument("--final-lr-frac", type=float, default=0.0, help="final LR as fraction of initial LR")
 # Evaluation
-parser.add_argument("--eval-every", type=int, default=150, help="evaluate val bpb every N steps (-1 = disable)")
-parser.add_argument("--eval-tokens", type=int, default=20*524288, help="number of tokens to evaluate val loss on")
-# Output
-parser.add_argument("--dry-run", action="store_true", help="log to wandb but skip checkpoints/report")
+parser.add_argument("--eval-every", type=int, default=200, help="evaluate val bpb every N steps (-1 = disable)")
+parser.add_argument("--eval-tokens", type=int, default=40*524288, help="number of tokens to evaluate val loss on")
+parser.add_argument("--chatcore-every", type=int, default=200, help="evaluate ChatCORE metric every N steps (-1 = disable)")
+parser.add_argument("--chatcore-max-cat", type=int, default=-1, help="max problems per categorical task for ChatCORE")
+parser.add_argument("--chatcore-max-sample", type=int, default=24, help="max problems per generative task for ChatCORE")
 args = parser.parse_args()
 user_config = vars(args).copy()
 # -----------------------------------------------------------------------------
@@ -66,20 +72,48 @@ user_config = vars(args).copy()
 device_type = autodetect_device_type() if args.device_type == "" else args.device_type
 ddp, ddp_rank, ddp_local_rank, ddp_world_size, device = compute_init(device_type)
 master_process = ddp_rank == 0
-ptdtype = torch.float32 if args.dtype == 'float32' else torch.bfloat16
-autocast_ctx = torch.amp.autocast(device_type=device_type, dtype=ptdtype) if device_type == "cuda" else nullcontext()
+autocast_ctx = torch.amp.autocast(device_type=device_type, dtype=torch.bfloat16) if device_type == "cuda" else nullcontext()
 synchronize = torch.cuda.synchronize if device_type == "cuda" else lambda: None
 get_max_memory = torch.cuda.max_memory_allocated if device_type == "cuda" else lambda: 0
+if device_type == "cuda":
+    gpu_device_name = torch.cuda.get_device_name(0)
+    gpu_peak_flops = get_peak_flops(gpu_device_name)
+    print0(f"GPU: {gpu_device_name} | Peak FLOPS (BF16): {gpu_peak_flops:.2e}")
+else:
+    gpu_peak_flops = float('inf')  # MFU not meaningful for CPU/MPS
 
 # wandb logging init
 use_dummy_wandb = args.run == "dummy" or not master_process
 wandb_run = DummyWandb() if use_dummy_wandb else wandb.init(project="nanochat-sft", name=args.run, config=user_config)
 
+# Flash Attention status
+if not HAS_FA3:
+    print0("WARNING: Flash Attention 3 not available, using PyTorch SDPA fallback. Training will be less efficient.")
+
 # Load the model and tokenizer
 model, tokenizer, meta = load_model("base", device, phase="train", model_tag=args.model_tag, step=args.model_step)
-pretrain_batch_size = meta.get("device_batch_size", None)
-if pretrain_batch_size is not None and args.device_batch_size > pretrain_batch_size:
-    print0(f"FOOTGUN WARNING: base model training used device_batch_size {pretrain_batch_size}, did you pass in a good --device-batch-size to this script?")
+
+# Inherit training hyperparameters from pretrained checkpoint (None = inherit, explicit value = override)
+pretrain_user_config = meta.get("user_config", {})
+for name, fallback, source in [
+    ("max_seq_len",       2048,  meta),
+    ("device_batch_size", 32,    meta),
+    ("total_batch_size",  524288, meta),
+    ("embedding_lr",      0.3,   pretrain_user_config),
+    ("unembedding_lr",    0.004, pretrain_user_config),
+    ("matrix_lr",         0.02,  pretrain_user_config),
+]:
+    arg_val = getattr(args, name)
+    pretrain_val = source.get(name)
+    if arg_val is None:
+        resolved = pretrain_val if pretrain_val is not None else fallback
+        setattr(args, name, resolved)
+        print0(f"Inherited {name}={resolved} from pretrained checkpoint")
+    elif pretrain_val is not None and arg_val != pretrain_val:
+        print0(f"NOTE: --{name.replace('_', '-')}={arg_val} overrides pretrained value of {pretrain_val}")
+    else:
+        print0(f"Using {name}={arg_val}")
+
 orig_model = model
 model = torch.compile(model, dynamic=False)
 depth = model.config.n_layer
@@ -94,14 +128,23 @@ print0(f"Total batch size {args.total_batch_size:,} => gradient accumulation ste
 token_bytes = get_token_bytes(device=device)
 
 # Initialize the Optimizer (combined MuonAdamW: Muon for matrix params, AdamW for rest)
-optimizer = model.setup_optimizer(unembedding_lr=args.unembedding_lr, embedding_lr=args.embedding_lr, matrix_lr=args.matrix_lr, weight_decay=args.weight_decay)
+# Note that pretraining ramps weight_decay to zero by end of pretraining, so SFT continues with zero
+optimizer = model.setup_optimizer(unembedding_lr=args.unembedding_lr, embedding_lr=args.embedding_lr, matrix_lr=args.matrix_lr, weight_decay=0.0)
+
+# Optionally warm-start optimizer from pretrained checkpoint (momentum buffers etc.)
+base_dir = get_base_dir()
+if args.load_optimizer:
+    optimizer_data = load_optimizer_state("base", device, rank=ddp_rank, model_tag=args.model_tag, step=args.model_step)
+    optimizer.load_state_dict(optimizer_data)
+    del optimizer_data
+    print0("Loaded optimizer state from pretrained checkpoint")
+
 # Override the initial learning rate as a fraction of the base learning rate
 for group in optimizer.param_groups:
     group["lr"] = group["lr"] * args.init_lr_frac
     group["initial_lr"] = group["lr"]
 
 # SFT data mixture and DataLoader
-base_dir = get_base_dir()
 identity_conversations_filepath = os.path.join(base_dir, "identity_conversations.jsonl")
 train_dataset = TaskMixture([
     SmolTalk(split="train"), # 460K rows of general conversations
@@ -236,10 +279,17 @@ train_loader = sft_data_generator_bos_bestfit("train")
 build_val_loader = lambda: sft_data_generator_bos_bestfit("val")
 progress = 0 # will go from 0 to 1 over the course of the epoch
 
-# Learning rate scheduler
+# Learning rate schedule (linear warmup, constant, linear warmdown)
+# Same shape as base_train but uses progress (0→1) instead of absolute step counts,
+# because SFT doesn't always know num_iterations in advance (dataset-driven stopping).
 def get_lr_multiplier(progress):
-    # first 80% of training: no decay, then linearly ramp down to 0.
-    return 1 if progress < 0.8 else 1 - (progress - 0.8) / 0.2
+    if progress < args.warmup_ratio:
+        return (progress + 1e-8) / args.warmup_ratio
+    elif progress <= 1.0 - args.warmdown_ratio:
+        return 1.0
+    else:
+        decay = (progress - (1.0 - args.warmdown_ratio)) / args.warmdown_ratio
+        return (1 - decay) * 1.0 + decay * args.final_lr_frac
 
 # Momentum scheduler for Muon optimizer
 def get_muon_momentum(it):
@@ -282,8 +332,44 @@ while True:
         })
         model.train()
 
-    # save checkpoint at the end of the run (only on master process)
-    if master_process and last_step and not args.dry_run:
+    # once in a while: estimate the ChatCORE metric (all ranks participate)
+    # use the original uncompiled model because the inputs keep changing shape
+    chatcore_results = {}
+    if args.chatcore_every > 0 and (last_step or (step > 0 and step % args.chatcore_every == 0)):
+        model.eval()
+        engine = Engine(orig_model, tokenizer)
+        all_tasks = ['ARC-Easy', 'ARC-Challenge', 'MMLU', 'GSM8K', 'HumanEval', 'SpellingBee']
+        categorical_tasks = {'ARC-Easy', 'ARC-Challenge', 'MMLU'}
+        baseline_accuracies = {
+            'ARC-Easy': 0.25, 'ARC-Challenge': 0.25, 'MMLU': 0.25,
+            'GSM8K': 0.0, 'HumanEval': 0.0, 'SpellingBee': 0.0,
+        }
+        task_results = {}
+        for task_name in all_tasks:
+            limit = args.chatcore_max_cat if task_name in categorical_tasks else args.chatcore_max_sample
+            max_problems = None if limit < 0 else limit  # -1 means no limit
+            with autocast_ctx:
+                acc = run_chat_eval(task_name, orig_model, tokenizer, engine,
+                                    batch_size=args.device_batch_size, max_problems=max_problems)
+            task_results[task_name] = acc
+            print0(f"  {task_name}: {100*acc:.2f}%")
+        # Compute ChatCORE metrics (mean centered accuracy, ranges from 0=random to 1=perfect)
+        def centered_mean(tasks):
+            return sum((task_results[t] - baseline_accuracies[t]) / (1.0 - baseline_accuracies[t]) for t in tasks) / len(tasks)
+        chatcore = centered_mean(all_tasks)
+        chatcore_cat = centered_mean(categorical_tasks)
+        print0(f"Step {step:05d} | ChatCORE: {chatcore:.4f} | ChatCORE_cat: {chatcore_cat:.4f}")
+        wandb_run.log({
+            "step": step,
+            "total_training_flops": flops_so_far,
+            "chatcore_metric": chatcore,
+            "chatcore_cat": chatcore_cat,
+            **{f"chatcore/{task_name}": acc for task_name, acc in task_results.items()},
+        })
+        model.train()
+
+    # save checkpoint at the end of the run (all ranks participate so each saves its optimizer shard)
+    if last_step:
         output_dirname = args.model_tag if args.model_tag else f"d{depth}" # e.g. d12
         checkpoint_dir = os.path.join(base_dir, "chatsft_checkpoints", output_dirname)
         save_checkpoint(
@@ -304,7 +390,8 @@ while True:
                     "window_pattern": model.config.window_pattern,
                 },
                 "user_config": user_config, # inputs to the training script
-            }
+            },
+            rank=ddp_rank,
         )
 
     if last_step:
@@ -346,8 +433,7 @@ while True:
     pct_done = 100 * progress
     tok_per_sec = int(args.total_batch_size / dt)
     flops_per_sec = num_flops_per_token * args.total_batch_size / dt
-    promised_flops_per_sec_h100 = 989e12 * ddp_world_size # bfloat16 H100 SXM and without 2:4 sparsity
-    mfu = 100 * flops_per_sec / promised_flops_per_sec_h100 # in %
+    mfu = 100 * flops_per_sec / (gpu_peak_flops * ddp_world_size)
     if step > 10:
         total_training_time += dt # only count the time after the first 10 steps
     print0(f"step {step:05d} ({pct_done:.2f}%) | loss: {debiased_smooth_loss:.6f} | lrm: {lrm:.2f} | dt: {dt * 1000:.2f}ms | tok/sec: {tok_per_sec:,} | mfu: {mfu:.2f} | epoch: {current_epoch} | total time: {total_training_time/60:.2f}m")
@@ -364,24 +450,32 @@ while True:
             "train/epoch": current_epoch,
         })
 
+    # The garbage collector spends ~500ms scanning for cycles quite frequently.
+    # We manually manage it to avoid these pauses during training.
+    if step == 1:
+        gc.collect() # manually collect a lot of garbage from setup
+        gc.freeze() # freeze all currently surviving objects and exclude them from GC
+        gc.disable() # disable GC entirely except:
+    elif step % 5000 == 0: # every 5000 steps...
+        gc.collect() # manually collect, just to be safe for very long runs
+
 # print a few more stats
 print0(f"Peak memory usage: {get_max_memory() / 1024 / 1024:.2f}MiB")
 print0(f"Total training time: {total_training_time/60:.2f}m")
 print0(f"Minimum validation bpb: {min_val_bpb:.4f}")
 
 # Log to report
-if not args.dry_run:
-    from nanochat.report import get_report
-    get_report().log(section="SFT", data=[
-        user_config, # CLI args
-        { # stats about the training setup
-            "Number of iterations": step,
-            "DDP world size": ddp_world_size,
-        },
-        { # stats about training outcomes
-            "Minimum validation bpb": min_val_bpb,
-        }
-    ])
+from nanochat.report import get_report
+get_report().log(section="SFT", data=[
+    user_config, # CLI args
+    { # stats about the training setup
+        "Number of iterations": step,
+        "DDP world size": ddp_world_size,
+    },
+    { # stats about training outcomes
+        "Minimum validation bpb": min_val_bpb,
+    }
+])
 
 # cleanup
 wandb_run.finish() # wandb run finish