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diff --git a/experiments/snapshot_evolution_vit.py b/experiments/snapshot_evolution_vit.py
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+"""
+Snapshot evolution on a ViT-Mini (modern transformer-style architecture) trained
+with BP and block-level DFA on CIFAR-10. Logs ||h_l||, ||BP grad||, Γ per epoch.
+
+This is the P4 generalization test: does the residual-stream pathology + LayerNorm
+gradient collapse mechanism (verified on pre-LN ResMLP with terminal LN) also
+appear on an actual transformer architecture? If yes → strong P4 in modern setting.
+
+Block-level DFA: each TransformerBlock is a "layer". The DFA credit
+`a_l = e_T @ B_l^T` is broadcast across all tokens at that block's input. The
+local block loss is `<block_l(h_l), broadcast(a_l)>` summed over tokens.
+
+Usage:
+    CUDA_VISIBLE_DEVICES=2 nohup python experiments/snapshot_evolution_vit.py \
+        --output_dir results/snapshot_vit_v1 --epochs 60 --seed 42 \
+        > results/snapshot_vit_v1/run_s42.log 2>&1 &
+"""
+import os, sys, json, argparse
+import numpy as np
+import torch
+import torch.nn as nn
+import torch.nn.functional as F
+import torch.optim as optim
+from torch.utils.data import DataLoader
+import torchvision
+import torchvision.transforms as transforms
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+from models.vit_mini import ViTMini, TransformerBlock
+from metrics.credit_metrics import cosine_similarity_batch
+
+
+def get_cifar10(batch_size=128):
+    tv_train = transforms.Compose([
+        transforms.RandomCrop(32, padding=4),
+        transforms.RandomHorizontalFlip(),
+        transforms.ToTensor(),
+        transforms.Normalize((0.4914, 0.4822, 0.4465), (0.2470, 0.2435, 0.2616)),
+    ])
+    tv = transforms.Compose([
+        transforms.ToTensor(),
+        transforms.Normalize((0.4914, 0.4822, 0.4465), (0.2470, 0.2435, 0.2616)),
+    ])
+    tr = torchvision.datasets.CIFAR10('./data', True, download=True, transform=tv_train)
+    te = torchvision.datasets.CIFAR10('./data', False, download=True, transform=tv)
+    return (DataLoader(tr, batch_size=batch_size, shuffle=True, num_workers=2),
+            DataLoader(te, batch_size=batch_size, shuffle=False, num_workers=2))
+
+
+def fixed_eval_buffer(test_loader, device, n_samples=1024):
+    xs, ys = [], []
+    for x, y in test_loader:
+        xs.append(x); ys.append(y)
+        if sum(xb.size(0) for xb in xs) >= n_samples:
+            break
+    return torch.cat(xs)[:n_samples].to(device), torch.cat(ys)[:n_samples].to(device)
+
+
+def diagnose(model, x_eval, y_eval, dfa_Bs=None):
+    """Compute per-block ||h_l|| and ||BP grad at h_l||, plus optional Γ vs DFA credit."""
+    was_training = model.training
+    model.eval()
+    L = model.num_blocks
+
+    # Hidden states (no grad)
+    with torch.no_grad():
+        _, hiddens = model(x_eval, return_hidden=True)
+    # hiddens[l] is shape (B, n_tokens, d_model)
+    # Reduce to per-sample by taking the cls-token norm OR by flattening across tokens
+    # We'll report cls-token norm (the one that actually flows to the head)
+    hidden_norms_cls = [h[:, 0].norm(dim=-1).median().item() for h in hiddens]
+    hidden_norms_avg = [h.norm(dim=-1).mean().item() for h in hiddens]  # avg across tokens then over batch
+
+    # BP gradients
+    h0 = model.embed(x_eval.detach())
+    hs = [h0.clone().requires_grad_(True)]
+    for b in model.blocks:
+        hs.append(b(hs[-1]))
+    h_cls = model.out_ln(hs[-1][:, 0])
+    logits = model.out_head(h_cls)
+    loss = F.cross_entropy(logits, y_eval)
+    grads = torch.autograd.grad(loss, hs)
+    # grads[l] is shape (B, n_tokens, d_model)
+    # Per-sample L2 norm: take Frobenius over tokens × d_model
+    bp_grad_per_sample_l2 = [g.flatten(1).norm(dim=-1).median().item() for g in grads]
+    bp_grad_F = [g.norm().item() for g in grads]
+    bp_full = [g.detach() for g in grads]
+
+    acc = (logits.argmax(-1) == y_eval).float().mean().item()
+    loss_val = loss.item()
+
+    gamma_dfa = float('nan'); per_layer_gamma = []
+    if dfa_Bs is not None:
+        with torch.no_grad():
+            e_T = logits.softmax(-1); e_T[torch.arange(x_eval.size(0)), y_eval] -= 1
+        for l in range(L):
+            # Block-level DFA credit: per-sample (B, d_model), broadcast to (B, n_tokens, d_model)
+            a_dfa_per_sample = (e_T @ dfa_Bs[l].T).detach()  # (B, d_model)
+            a_dfa_broadcast = a_dfa_per_sample.unsqueeze(1).expand_as(bp_full[l])  # (B, n_tokens, d_model)
+            # Cosine using flattened (per-sample) representation
+            per_layer_gamma.append(cosine_similarity_batch(
+                a_dfa_broadcast.flatten(1), bp_full[l].flatten(1)))
+        gamma_dfa = float(np.mean(per_layer_gamma))
+
+    if was_training:
+        model.train()
+
+    return {
+        'hidden_norms_cls': hidden_norms_cls,
+        'hidden_norms_avg': hidden_norms_avg,
+        'bp_grad_per_sample_l2_med': bp_grad_per_sample_l2,
+        'bp_grad_F': bp_grad_F,
+        'gamma_dfa': gamma_dfa,
+        'gamma_dfa_per_layer': per_layer_gamma,
+        'acc_eval': acc,
+        'loss_eval': loss_val,
+    }
+
+
+def train_bp(model, train_loader, x_eval, y_eval, device, epochs, lr, wd):
+    opt = optim.AdamW(model.parameters(), lr=lr, weight_decay=wd)
+    sch = optim.lr_scheduler.CosineAnnealingLR(opt, T_max=epochs)
+    log = []
+    d0 = diagnose(model, x_eval, y_eval); d0['epoch'] = 0; log.append(d0)
+    print(f"  [BP-vit] Ep 0: ||h_L_cls||={d0['hidden_norms_cls'][-1]:.3e} ||g_2||={d0['bp_grad_per_sample_l2_med'][2]:.3e} acc={d0['acc_eval']:.4f}", flush=True)
+    for ep in range(1, epochs + 1):
+        model.train()
+        for x, y in train_loader:
+            x = x.to(device); y = y.to(device)
+            logits = model(x); loss = F.cross_entropy(logits, y)
+            opt.zero_grad(); loss.backward(); opt.step()
+        sch.step()
+        d = diagnose(model, x_eval, y_eval); d['epoch'] = ep; log.append(d)
+        if ep % 5 == 0 or ep == 1 or ep == epochs:
+            print(f"  [BP-vit] Ep {ep}: ||h_L_cls||={d['hidden_norms_cls'][-1]:.3e} ||g_2||={d['bp_grad_per_sample_l2_med'][2]:.3e} acc={d['acc_eval']:.4f}", flush=True)
+    return log
+
+
+def train_dfa_block_level(model, train_loader, x_eval, y_eval, device, epochs, lr, wd):
+    """Block-level DFA on ViT. Each TransformerBlock is treated as a unit; DFA credit
+    is broadcast across all tokens at the block's input.
+    """
+    d_model = model.d_hidden
+    L = model.num_blocks
+    C = 10
+    Bs = [torch.randn(d_model, C, device=device) / np.sqrt(C) for _ in range(L)]
+
+    block_opts = [optim.AdamW(b.parameters(), lr=lr, weight_decay=wd) for b in model.blocks]
+    embed_opt = optim.AdamW(
+        list(model.patch_embed.parameters()) + [model.cls_token, model.pos_embed],
+        lr=lr, weight_decay=wd)
+    head_opt = optim.AdamW(
+        list(model.out_head.parameters()) + list(model.out_ln.parameters()),
+        lr=lr, weight_decay=wd)
+    all_sch = [optim.lr_scheduler.CosineAnnealingLR(o, T_max=epochs) for o in block_opts] + \
+              [optim.lr_scheduler.CosineAnnealingLR(embed_opt, T_max=epochs),
+               optim.lr_scheduler.CosineAnnealingLR(head_opt, T_max=epochs)]
+    log = []
+    d0 = diagnose(model, x_eval, y_eval, dfa_Bs=Bs); d0['epoch'] = 0; log.append(d0)
+    print(f"  [DFA-vit] Ep 0: ||h_L_cls||={d0['hidden_norms_cls'][-1]:.3e} ||g_2||={d0['bp_grad_per_sample_l2_med'][2]:.3e} acc={d0['acc_eval']:.4f}", flush=True)
+    for ep in range(1, epochs + 1):
+        model.train()
+        for x, y in train_loader:
+            x = x.to(device); y = y.to(device)
+            batch = x.size(0)
+            with torch.no_grad():
+                logits, hiddens = model(x, return_hidden=True)
+                e_T = logits.softmax(-1); e_T[torch.arange(batch), y] -= 1
+            hL_det = hiddens[-1].detach()
+            # Head update via direct CE on cls token
+            h_cls = model.out_ln(hL_det[:, 0])
+            logits_out = model.out_head(h_cls)
+            loss_out = F.cross_entropy(logits_out, y)
+            head_opt.zero_grad(); loss_out.backward(); head_opt.step()
+            # Block updates: each block's local loss = <block(h_l), a_dfa_broadcast>
+            for l in range(L):
+                h_l = hiddens[l].detach()  # (B, n_tokens, d)
+                a_dfa = (e_T @ Bs[l].T).detach()  # (B, d)
+                a_dfa_broadcast = a_dfa.unsqueeze(1).expand_as(h_l)  # (B, n_tokens, d)
+                rms = (a_dfa_broadcast ** 2).mean(dim=-1, keepdim=True).sqrt() + 1e-6
+                a_norm = a_dfa_broadcast / rms
+                f_l = model.blocks[l](h_l)
+                local_loss = (f_l * a_norm).sum(dim=-1).mean()
+                block_opts[l].zero_grad(); local_loss.backward()
+                torch.nn.utils.clip_grad_norm_(model.blocks[l].parameters(), 1.0)
+                block_opts[l].step()
+            # Embed update (patch embed + cls + pos)
+            a_0 = (e_T @ Bs[0].T).detach()
+            rms_0 = (a_0 ** 2).mean(dim=-1, keepdim=True).sqrt() + 1e-6
+            h0 = model.embed(x)  # (B, n_tokens, d)
+            a_0_broadcast = a_0.unsqueeze(1).expand_as(h0)
+            embed_loss = (h0 * (a_0_broadcast / rms_0.unsqueeze(1))).sum(dim=-1).mean()
+            embed_opt.zero_grad(); embed_loss.backward(); embed_opt.step()
+        for s in all_sch: s.step()
+        d = diagnose(model, x_eval, y_eval, dfa_Bs=Bs); d['epoch'] = ep; log.append(d)
+        if ep % 5 == 0 or ep == 1 or ep == epochs:
+            print(f"  [DFA-vit] Ep {ep}: ||h_L_cls||={d['hidden_norms_cls'][-1]:.3e} ||g_2||={d['bp_grad_per_sample_l2_med'][2]:.3e} acc={d['acc_eval']:.4f} γ={d['gamma_dfa']:.4f}", flush=True)
+    return log
+
+
+def main():
+    p = argparse.ArgumentParser()
+    p.add_argument('--output_dir', type=str, default='results/snapshot_vit_v1')
+    p.add_argument('--epochs', type=int, default=60)
+    p.add_argument('--lr', type=float, default=1e-3)
+    p.add_argument('--wd', type=float, default=0.05)
+    p.add_argument('--seed', type=int, default=42)
+    p.add_argument('--depth', type=int, default=4)
+    p.add_argument('--d_model', type=int, default=128)
+    p.add_argument('--n_heads', type=int, default=4)
+    args = p.parse_args()
+
+    os.makedirs(args.output_dir, exist_ok=True)
+    device = torch.device('cuda:0')
+    print(f"ViT-MINI: depth={args.depth}, d_model={args.d_model}, n_heads={args.n_heads}, "
+          f"epochs={args.epochs}, seed={args.seed}", flush=True)
+
+    train_loader, test_loader = get_cifar10(batch_size=128)
+    x_eval, y_eval = fixed_eval_buffer(test_loader, device, n_samples=1024)
+
+    print("\n=== BP training (ViT-Mini) ===", flush=True)
+    torch.manual_seed(args.seed); np.random.seed(args.seed); torch.cuda.manual_seed_all(args.seed)
+    bp_model = ViTMini(num_blocks=args.depth, d_model=args.d_model, n_heads=args.n_heads).to(device)
+    print(f"  n_params={sum(p.numel() for p in bp_model.parameters())}", flush=True)
+    bp_log = train_bp(bp_model, train_loader, x_eval, y_eval, device, args.epochs, args.lr, args.wd)
+
+    print("\n=== DFA training (ViT-Mini, block-level DFA) ===", flush=True)
+    torch.manual_seed(args.seed); np.random.seed(args.seed); torch.cuda.manual_seed_all(args.seed)
+    dfa_model = ViTMini(num_blocks=args.depth, d_model=args.d_model, n_heads=args.n_heads).to(device)
+    dfa_log = train_dfa_block_level(dfa_model, train_loader, x_eval, y_eval, device, args.epochs, args.lr, args.wd)
+
+    out = {
+        'config': vars(args), 'depth': args.depth, 'd_model': args.d_model,
+        'architecture': 'ViTMini', 'bp_log': bp_log, 'dfa_log': dfa_log,
+    }
+    out_path = os.path.join(args.output_dir, f'snapshot_vit_s{args.seed}.json')
+    with open(out_path, 'w') as f:
+        json.dump(out, f, indent=2)
+    print(f"\nSaved {out_path}", flush=True)
+
+
+if __name__ == '__main__':
+    main()