dump-bisect-debug

name	dump-bisect-debug
description	Locate forward numerical bugs by dumping intermediate tensors from a target implementation and a known-good reference, then bisecting layer by layer. Also covers batch-invariance bisect (the same token at any batch position should produce a bitwise-identical output, per DeepSeek V4 paper §3.3). Use when "the output is wrong but I don't know where" — model produces gibberish, degenerates, or picks the wrong token, but code review reveals nothing.
version	3.1.0
scope	ATOM (generalizable to any PyTorch forward debug)
last_updated	"2026-05-20T00:00:00.000Z"

ATOM built-in infrastructure (v3.0+ — read first)

Use these instead of hand-rolling hooks / compare scripts.

Module	Purpose
atom/utils/debug_helper/dump.py	Env-gated forward / weight / sampler dump; multi-class hooks; multi-call counter.
atom/utils/debug_helper/compare.py	cos_max (double-precision — avoids the fp32 cos > 1.0 trap) + slot_split + pick_prefill_call + CLI.
atom/utils/debug_helper/ref_patch.py	Monkey-patch context managers for instrumenting read-only reference implementations.
atom/utils/envs.py "Debug Dump" section	9 env vars (ATOM_FWD_DUMP_*, ATOM_WEIGHT_DUMP_*, ATOM_DEBUG_TOPK*), all no-op by default.

Public API: from atom.utils.debug_helper import .... CLI: python -m atom.utils.debug_helper.compare <subcommand>.

Once these are wired up, debugging a new model never requires editing model_runner.py, writing a cos_max again, or re-inventing the warmup-vs-prefill heuristic.

Dump-Bisect Debug Methodology

Looking at logits or output text only tells you that it's wrong, not where it's wrong. This methodology compresses where down to O(log N) dump-and-compare iterations.

Quick-start decision tree

Is the symptom reproducible at temperature=0.0 / single prompt?
├─ NO  → batch / sampling-stochastic
│   └─ Same prompt yields different tokens across runs OR across batch slots?
│       ├─ across runs    → you ALREADY BROKE determinism — fix sampling seed or env
│       └─ across slots   → likely NOT a bug; jump to Phase 8 (batch invariance)
└─ YES → deterministic forward bug, run the linear pipeline:
    ├─ Have a reference impl?  ──NO──► build one (Phase 0)
    ├─ Schema/shape mismatch?  ──YES─► fix loader (Phase 1, do NOT skip)
    ├─ Single layer dump cos<1? ───── Phase 3 → Phase 4 → Phase 5
    └─ Pinpoint sub-stage cos drop ── Phase 6 (standalone GPU isolation)
                                   └─ Phase 7 (fix + isolation revert)

Phase-at-a-glance

#	Goal	Time	Move on when
0	Establish reference	once per project	reference reproduces user-expected output end-to-end
1	Rule out weight loading	≤ 1h, 1–2 GPU runs	all dumped params byte-equal or cos > 0.9999
2	Define dump protocol	once per model	both sides agree on stage names + tensor contract
3	First single-layer comparison	≤ 30min, 2 GPU runs	layer 0 cos table drawn
4	Layer-level bisect	1–2h	first layer with cos < 0.99 (or rel > 10%) located
5	Intra-layer sub-stage bisect	1–3h per bug	sub-stage with single-step cos drop > 0.001 located
6	Standalone GPU kernel isolation	1–2h	path A reproduces ATOM dump cos > 0.9999; cross-experiment confirms quant vs GEMM
7	Fix + isolation revert	~1h	e2e byte-equal vs ref AND each fix's necessity verified
8	Batch invariance bisect (parallel mode)	2–3h	slot-vs-slot cos vs spec — if broken, classify as kernel-stack issue, NOT model bug

Total per root cause: 4–6h. Multiple root causes stack. 5–10× faster than "stare at the code and guess" with no missed diagnoses.

When to use

Trigger conditions (any one applies):

• Model output is gibberish, degenerates, or picks the wrong token, but code review finds nothing.
• Output is correct on some prompts and wrong on others ("corner case").
• Single prompt OK / batch fails; prefill OK / decode fails; short prompt OK / long prompt fails.
• A correct reference implementation exists (HF transformers, official inference repo, a previously-verified commit).

Don't use when:

• No reference ground truth (only "I think it's wrong") — build a reference first.
• Difference of just a few tokens — could be numerical noise; confirm it's a real bug first.
• Symptom is "non-deterministic across runs at temp=0.0" — that's broken sampling determinism (env / RNG / kernel), not a forward bug; this skill won't help.

Core principles

1. Read the model paper before assuming a bug. Some divergence is expected by design of the model architecture, not a bug to fix. V4 paper §3.3 explicitly assumes batch-invariant kernels; running on a non-batch-invariant inference stack will flip edge-confidence tokens and that is not an ATOM bug. Always check the model's reproducibility / determinism claims first — if the runtime stack doesn't meet them, classify as a kernel-stack limitation and document, don't bisect.
2. The reference must actually run end-to-end, not "the code looks right" — references can have bugs too.
3. Reference may be batch=1 only. Many official inference/model.py files (V4, parts of DeepSeek family) hardcode max_batch_size=1. You cannot directly compare batch>1 against them. Either modify the ref to support batches OR design experiments that work within the bsz=1 constraint (e.g. multiple seeded runs).
4. Share as much code as possible: reference and target use the same tokenizer / kernel / inputs, so the only variable is the code under investigation.
5. Rule out weight loading first: confirm byte-equality before any forward bisect. Schema diff (names + shapes + dtypes) BEFORE numerical comparison — a missing param prefix masquerades as a "forward bug".
6. Fix isolation revert is mandatory after multi-bug fixes. When 2+ fixes land together, revert each one in turn to identify which are critical (output-changing) vs fine-tuning (cosmetic / perf-cost). This decides PR split granularity — critical fixes merge fast, fine-tuning fixes can wait.
7. Cross-check against a second reference: besides the canonical reference, look at how sglang / vLLM implemented and fixed the same model.
8. Dump names must be semantic: intra_attn_norm_in / intra_ffn_out, never tensor_5.
9. Dump one layer / one prompt at a time: avoids file explosion and cross-contamination.
10. Align input_ids first: tokenizer mismatch is the most common false-positive bug source.
11. Same stage name ≠ numerically equivalent: confirm both sides have applied the same number of ops at dump time. The most common trap: ATOM dumps pre-all-reduce, ref dumps post-all-reduce.

Eight-phase flow

Phases 0–7 are the linear pipeline for ref-vs-target bisect. Phase 8 is a parallel mode for batch-invariance investigation; trigger it independently when the symptom is "single prompt OK, batch fails".

Phase 0: Establish a reference

If no reference exists yet, build one. Priority:

1. The official repo's inference/generate.py: run with torchrun (e.g. /data/DeepSeek-V4-Pro/inference/generate.py).
2. HF transformers: AutoModelForCausalLM.from_pretrained(...).generate().
3. A known-good prior commit: git checkout <commit> and run.

Reference requirements:

• Same weights (stream from safetensors directly; do not convert).
• Same tokenizer / chat template.
• Same GPU kernel when investigating numerical drift — use the same aiter / cuBLAS kernels in the reference, otherwise you can't distinguish "algorithm bug" from "kernel numerical drift".
• End-to-end verified: the reference output must match user expectation (e.g. "1+2+3=?" answers "6").

Output: ref_full_generate.py or similar — capable of producing a ground-truth token sequence.

Phase 1: Rule out weight loading (before any forward bisect)

Why first: if weights load wrong, every subsequent forward comparison will mis-attribute the cause to the forward path. 10 minutes of weight comparison saves a day of forward bisect.

Use maybe_dump_weights_and_exit(self.model) from atom.utils.debug_helper, already wired in model_runner.py. It dumps params + buffers per rank and sys.exit(0):

ATOM_WEIGHT_DUMP_DIR=/path/to/dump \
ATOM_WEIGHT_DUMP_LAYERS=0,2 \
python -m atom.examples.simple_inference --model ... -tp 8

Comparison checklist:

Check type	What to do
Schema diff first	List ATOM-only / ref-only / shape mismatch / dtype diff before any numerical comparison.
FP8 weight	Compare byte-equality after aiter.ops.shuffle.shuffle_weight(ref_w, layout=(16,16)).
FP8 scale (e8m0)	Cast via aiter.utility.fp4_utils.e8m0_to_f32(ref_s) to fp32, then compare.
TP-replicated layer	Per-rank ATOM byte-equal vs ref.
TP-sharded layer	torch.cat(ref_rank0..7, dim=tp_dim) vs full ATOM (or per-rank ATOM vs ref slice + shuffle).
Norm weight (BF16 vs FP32)	Different dtype, equal value — cast then cos.
MoE expert weights	Volume is huge; skip initially — assume the expert loader is consistent with other weights.

Conclusion patterns:

• ✓ All byte-equal / cos > 0.9999 → weight loading is OK; proceed to Phase 2.
• ✗ Any byte mismatch → fix the loader first (WeightsMapper / shuffle / TP shard / quant_type).

Phase 2: Define the forward dump protocol

Both sides agree on the same checkpoint names and tensor shape contract.

Minimum set (dump per layer):

Stage	Meaning	Use
embed.input_ids	Input token ids	Confirm tokenization consistency
embed.embed_out	Embedding output	Confirm lookup consistency
layer{L}.hidden_in	Hidden entering the layer	Confirm previous layer's output
layer{L}.attn_norm_out	After attention norm	Isolate norm differences
layer{L}.attn_out	Attention output	Whole attention block
layer{L}.ffn_norm_out	After FFN norm	Isolate norm
layer{L}.ffn_out	FFN output	Whole FFN block
layer{L}.hidden_out	Hidden leaving the layer	Feeds the next layer's comparison
embed.final_h	Pre-lm_head hidden	Accumulated diff vs lm_head amplification
embed.final_logits	Final logits	Vocab-space difference

Use the built-in dump infrastructure (do not write hooks by hand):

# ATOM side
ATOM_FWD_DUMP_DIR=/path/to/dump \
ATOM_FWD_DUMP_LAYERS=0 \
ATOM_FWD_DUMP_BLOCK_CLASS=Block \
python -m atom.examples.simple_inference --prompt "1+2+3=?" --max-tokens 1

For the reference side (often a read-only /data/<model>/inference/model.py):

from atom.utils.debug_helper import patch_block_forward, patch_module_dump

with patch_block_forward(ref_Block, layer_attr="layer_id", side_prefix="ref"):
    ref_model.forward(...)
    # writes ref_layer{LL}_Block__{stage}_rank{R}.pt to ATOM_FWD_DUMP_DIR

For deeper sub-stages (RoPE, q_norm, sparse_attn output …), patch the relevant method directly with patch_method and insert named dump(stage, tensor) calls — copy the original forward body verbatim and insert the dumps in between to avoid losing side effects.

Phase 3: First comparison (single layer, single prompt)

# Reference
ATOM_FWD_DUMP_DIR=$DIR ATOM_FWD_DUMP_LAYERS=0 \
  torchrun --nproc-per-node=8 ref_full_generate.py --prompt "1+2+3=?" --max-new-tokens 1

# Target (ATOM)
ATOM_FWD_DUMP_DIR=$DIR ATOM_FWD_DUMP_LAYERS=0 \
  python -m atom.examples.simple_inference --prompt "1+2+3=?" --max-tokens 1

# Compare
python -m atom.utils.debug_helper.compare ref-vs-target --dir $DIR

The CLI uses the project's standard cos_max (double precision). It prints a per-stage table with severity flags and asserts input_ids match before doing anything else.

Severity thresholds (look at both cos and rel; rel is more sensitive):

cos	rel	Meaning	Action
> 0.9999	< 1%	Bit-equal range	✓ OK
0.99 ~ 0.9999	1 ~ 10%	Numerical drift (kernel/dtype)	? Flag — watch for accumulation
0.9 ~ 0.99	10 ~ 30%	Mild algorithmic drift / partial heads wrong	✗ Bisect to sub-stage
< 0.9	> 30%	Real bug	✗ Locate immediately
≈ 0 or negative	> 50%	Total scramble / sign flip	✗ Usually weight loading / shuffle bug

Important: when cos and rel disagree, trust rel:

• When hidden values span large ranges (e.g. max_abs = 1e5), cos is dominated by a few outliers and may read 0.9998 even though rel = 57%.
• A 50%+ per-element error → after lm_head amplification, logits are completely wrong.

Phase 4: Layer-level bisect — find the first cos drop, with layer-class awareness

The first layer with cos < 0.99 or rel > 10% = the layer where the bug lives.

Key observation: layer class. When dumping multiple layers (0 / N / 2N / 3N) to look at the decay curve, focus on which layer's cos suddenly drops and correlate with that layer's class:

Model	Layer-class examples	Investigation direction
DeepSeek-V4	First N layers use hash routing; rest use sqrtsoftplus routing	Did the non-hash path miss a fix?
DeepSeek-V4	compress_ratio=4 (sparse) vs =128 (window)	Inside the sparse path?
Qwen3-Next	Hybrid attention vs full attention	Attention-type branch?
MTP	Base layer vs MTP block	MTP path needs an independent fix?

V4 example: layer 0/2 hidden_out cos = 1.0 ✓, but layer 3 suddenly drops to cos = 0.98. Layers 0/1/2 are hash routing (layer_id < n_hash_layers); layer 3+ takes select_experts(sqrtsoftplus). → Compare the hash path vs the sqrtsoftplus path → discover the latter is missing * routed_scaling_factor.

Accumulated drift vs algorithmic bug:

• Each layer cos = 0.999 but compounded to layer 60 it's 0.94 → kernel drift.
• One layer suddenly drops from 0.999 to 0.5 → algorithmic bug.
• Some type of layer (e.g. layer 3, 5, 7, …) consistently has poor cos while others are fine → layer-class branch bug.

Phase 5: Intra-layer sub-stage bisect → component-level root cause

Once the layer is located, dump finer checkpoints inside it. Each arrow = one dump:

Attention:
  x_in → wq_a → q_norm → qr → wq_b → q_pre_norm → q_post_norm → q_post_rope → ┐
                                                                              ├ → sparse_attn → o_pre_invrope → o_post_invrope → wo_a → wo_b
  x_in → wkv → kv_pre_norm → kv_post_norm → kv_post_rope → kv_after_quant → ┘

FFN (MoE):
  x_in → ffn_norm → norm_out → gate(routing) → topk → ┐
                                                       ├ → expert_out → all_reduce → + shared_out → combined_out
                                                       ┘

Each sub-stage's single-step cos should be > 0.999. If a sub-stage drops by more than 0.001 in a single step, that op is the culprit.

Common MoE trap: dump stages out of sync

• ATOM expert_out is pre-all-reduce (per-rank partial).
• Ref expert_out is post-all-reduce (full).
• The two will read cos = 0.36, looking like a bug — but the dump stage isn't aligned.
• Fix: move ATOM's dump after the all-reduce, OR move ref's dump before — pick one, keep both sides aligned.

Phase 6: Standalone GPU kernel isolation (key innovation)

Is cos = 0.998 from input quantization noise or a GEMM kernel difference? Re-run the kernel offline using dumped data to isolate:

# standalone_<op>_test.py — runs on a single GPU
qr = atom_dump["layer0.attn.qr"].cuda()                  # input
atom_w = atom_weight_dump["layers.0.attn.wq_b.weight"]   # ATOM-loaded weight (shuffled)
ref_w = ref_weight_dump["layers.0.attn.wq_b.weight"]     # ref-loaded weight (unshuffled)

# Path A: ATOM kernel = ATOM input quant + ATOM GEMM
qr_q_A, qr_s_A = atom_quant_func(qr)
out_A = atom_gemm(qr_q_A, atom_w, qr_s_A, atom_w_scale)

# Path B: REF kernel = ref input quant + ref GEMM
qr_q_B, qr_s_B = ref_quant_func(qr)
out_B = ref_gemm(qr_q_B, ref_w, qr_s_B, ref_w_scale)

# Sanity: A must perfectly reproduce the ATOM dump
assert cos_max(out_A, atom_dump["layer0.attn.q_pre_rope_pre_norm"])[0] > 0.9999

# Cross experiments isolate quant vs GEMM:
# C1: ref quant + ATOM GEMM   → measure quant impact
# C2: ATOM quant + ref GEMM   → measure GEMM impact
out_C1 = atom_gemm(qr_q_B, atom_w, qr_s_B, atom_w_scale)  # may have layout incompat
out_C2 = ref_gemm(qr_q_A, ref_w, qr_s_A, ref_w_scale)
print(cos_max(out_C1, ref_dump))
print(cos_max(out_C2, ref_dump))

Validating a fix candidate:

• Write a PyTorch reference quant function (e.g. my_ue8m0_quant).
• Confirm its FP8 / scale outputs are byte-equal vs the ref kernel (100%).
• Feed into the ref GEMM, confirm cos vs ref dump = 1.0.
• This step doesn't need the model — takes seconds to confirm the fix direction.

Phase 7: Fix implementation + isolation revert

1. Implement the fix env-gated when its correctness is uncertain or it carries a perf cost: e.g. V4_USE_REF_QUANT=1 toggles a PoC fix without changing default behavior. For high-confidence fixes with no perf cost, land them as the new default.
2. Re-run the forward dump and verify cos: confirm the fix is real and the sub-stage cos actually improved.
3. Run e2e and look at token output: from rambling → coherent output → byte-equal vs ref.
4. Multiple bugs may need fixing: one fix is often not enough (e.g. fixing V4 attention still leaves FFN wrong) → continue bisecting the next one.
5. Fix isolation revert ★ Once all fixes pass, revert each one in turn to identify the critical ones (this is core principle #6). A fix may be "fine-tuning" (byte-equal vs ref but other fixes alone already produce correct output) and carry a perf cost — those can ship separately or wait. This step decides PR split granularity: critical fixes merge immediately; fine-tuning fixes can wait.

Phase 8: Batch-invariance bisect (DeepSeek V4 paper §3.3)

Trigger conditions:

• Single prompt is correct; batch=N produces a wrong answer in some slot.
• The same prompt run multiple times yields different results (determinism is broken).
• The model paper / design assumes "bitwise reproducibility across pre-training, post-training, inference" (V4, Qwen3+).

Core design (V4 paper §3.3):

"Batch invariance ensures that the output of any given token remains bitwise identical, regardless of its position within a batch."

The V4 team built three kinds of batch-invariant kernel for this:

• Attention: dual-kernel (single-SM full-seq + multi-SM single-seq, bitwise-identical).
• MatMul: replace cuBLAS with DeepGEMM, abandon split-K.
• MoE backward: token-order pre-processing + per-rank buffer isolation.

Running on a non-batch-invariant inference stack (aiter / cuBLAS / default vLLM) → slot-to-slot cos < 1.0 → edge-confidence prompts flip tokens. This is not a bug; it's a violation of the model's architectural assumption.

Experiment design (4 identical prompts + temp = 0.0):

EXPERIMENTS = [
    ("E0_single",   [P]),                              # baseline
    ("E1_4xsame",   [P, P, P, P]),                     # all same → tests batch invariance
    ("E2_pos0",     [P, OTHER, OTHER, OTHER]),
    ("E3_pos1",     [OTHER, P, OTHER, OTHER]),
    ("E4_pos2",     [OTHER, OTHER, P, OTHER]),
    ("E5_pos3",     [OTHER, OTHER, OTHER, P]),
]
# Single model load; sequentially run all batches and look at slot-vs-slot output.

Reading the results:

• E1's four slots all agree → batch invariance is OK.
• E1 has divergent slots → batch invariance is broken (a kernel's reduction order depends on batch position).
• E2–E5 results vary with P's position → non-commutative path.

Dump + bisect recipe:

# 1. Run with multi-class hooks + multi-call (Compressor / Indexer live in a per-seq loop)
ATOM_FWD_DUMP_DIR=$DIR \
ATOM_FWD_DUMP_LAYERS="0,1,2,3" \
ATOM_FWD_DUMP_BLOCK_CLASS="Block,DeepseekV4Attention,MoE,Compressor,Indexer" \
ATOM_FWD_DUMP_ONE_SHOT=0 \
python debug_e1_4xsame.py

# 2. Compare per-slot
python -m atom.utils.debug_helper.compare slot-invariance --dir $DIR --n-slots 4
# Output: per (layer, stage) cos(slot 0, slot N) — find the first stage where it drops below 1.0.

Critical trap: warmup vs prefill vs decode

A forward hook with one_shot=True (the default) captures the warmup forward (max_num_batched_tokens, typically a power-of-2 dummy input), not the real prefill.

Set ATOM_FWD_DUMP_ONE_SHOT=0 so each call writes its own file (layer{LL}_{Cls}_rank{R}_call{NNN}.pt), then use pick_prefill_call() to pick the real prefill:

• n_tok % n_slots == 0 and n_tok > n_slots (excludes 1-tok-per-seq decode).
• Not a power-of-2 (excludes warmup dummies).
• Lowest call_idx among matches (earliest match).

Per-seq dispatch loop trap (specific to V4 Attention):

• Compressor / Indexer are called inside a for seq in range(num_seqs) loop.
• A single prefill triggers N module calls, one per slot.
• one_shot=True only captures seq 0 — slot drift is invisible.
• Required: ATOM_FWD_DUMP_ONE_SHOT=0 + use call_idx to disambiguate.

Root-cause classification:

Stage cos drop	Candidates
Layer 0 attention cos < 1.0	sparse_attn split-KV / per-slot KV-cache memory layout.
Layer 0 MoE drops more than attn	Fused expert kernel split-K / silu+clamp boundary amplifies 1e-4 → 1e-3.
Layer 1+ attn cos explodes (0.97)	Hash routing / sqrtsoftplus cascades small upstream drift.
Whole model cos ≈ 0.999 yet final logits diverge	Edge-confidence token + sampling noise.

Diagnostic pattern — when batch=N flips a token that single-prompt gets right:

• E1 with 4× the same prompt at temp=0.0 shows slots disagreeing (e.g. slot 2 produces different tokens than 0/1/3).
• Layer 0 attention cos = 0.99988 (near identical) → MoE 0.998 (10× amplification) → layer 1 attention 0.97 (cascaded).
• The single-step cos drop is in the GEMM / MoE reduction, not the model code.

Fix direction:

• Not fixable at the ATOM layer — needs aiter to ship batch-invariant kernels (DeepGEMM-style + dual-kernel attention).
• Mitigation: document as a known limitation; only single-prompt and high-confidence batch paths are guaranteed correct.

Common root-cause categories (by frequency)

1. Weight loading bug: shape correct, content wrong (cos near 0 or negative).
   • WeightsMapper mismatch.
   • Wrong TP shard direction (ColumnParallel vs RowParallel vs Replicated).
   • Wrong FP8 shuffle (quant_type).
2. Quant scale path differences: each GEMM cos ≈ 0.998; compounded over 60 layers, final logits diverge.
   • HIP kernel doesn't support ue8m0 round but the reference does → scale value disagreement.
   • PyTorch ceil vs aiter round-to-even, 1-ulp difference.
3. Routing scale missing a multiply: a type of layer consistently has lower cos, others are fine.
   • One of hash path / non-hash path is missing * routed_scaling_factor.
4. Layout-triple mismatch: weight prep / input quant scale layout / GEMM kernel must all agree.
   • Changing one but not the other two → cos drops sharply.
5. Cumulative kernel drift: each layer cos 0.999, by layer 30 it's 0.94.
   • aiter mhc_post / different GEMM kernels with diverging numerical precision.
6. Activation / algorithmic difference: cos suddenly drops.
   • Swiglu vs Silu implementation mismatch.
   • Missing clamp / scale factor.
7. TP reduce missing or duplicated: cos = 1/tp_size off.
   • reduce_results=True/False mismatch.
   • shared_expert reduced twice.
8. KV cache contamination: first token correct, later tokens drift.
   • Warmup not reset.
   • start_pos misaligned.
9. Batch=N vs batch=1 inconsistency: single OK, batch fails (or vice versa).
   • cu_seqlens / padding / block_table has a bug specific to the multi-seq path.
   • After fixing the single prompt, always re-run a batch test.

When to stop / accept divergence

Not every cos drop is a bug worth fixing. Stop bisecting and document as a known limitation when:

Signal	Likely diagnosis	Action
Slot-vs-slot cos < 1.0 with identical inputs	Non-batch-invariant kernels (V4 paper §3.3); model spec assumes invariance, runtime stack doesn't provide it	Document; do not try to "fix" the model code — fix is at the kernel/aiter layer
Per-layer cos = 0.999 stable across many runs, never drops further	Floating-point non-associativity baseline	Accept; verify final task accuracy (lm_eval / GSM8K) instead
Reference itself disagrees with user expectation	Bad reference	Build a better reference (Phase 0) before continuing
Different models, same kernel: same drift magnitude	Kernel-level noise floor	Accept; this is the fp8/fp4 budget
Symptom only at temp > 0 stochastic, never at temp=0.0	Sampling RNG / borderline confidence	Verify with task accuracy; not a forward bug

Rule of thumb: a divergence is worth chasing if (a) it changes greedy decode output, (b) it tanks task accuracy by > model-card noise, OR (c) the model paper explicitly calls out reproducibility/invariance as a design property.

Anti-patterns (don't do this)

• ❌ Only look at output tokens, never dump intermediate tensors: you'll never find the bug.
• ❌ Skip Phase 1 weight check, jump straight to forward bisect: a day later you'll discover it was the loader.
• ❌ Dump a pile of un-named dicts: three days later even you can't read them.
• ❌ Don't verify the reference itself is correct: a day of "matching the reference" later, you discover the reference was wrong too.
• ❌ Change two things at once and re-dump: bisect breaks down.
• ❌ Dump multiple prompts / multiple layers in one shot: 100MB+ files, slow loads, tangled comparisons.
• ❌ Look at cos only, not rel: cos = 0.997 looks fine, but rel = 11% will compound across 60 layers.
• ❌ Compare stages without checking the op count on each side: ATOM pre-all-reduce vs ref post-all-reduce, cos = 0.36 misjudgment.
• ❌ Stop after fixing one prompt: must run batch + multiple prompts; there may be independent bugs.
• ❌ Fix with a hardcoded change instead of env-gated: production runs of other models will explode.
• ❌ Look at a single reference implementation: sglang / vLLM also implement the same model — use a second reference to confirm the fix direction.
• ❌ Use F.cosine_similarity on bf16 / fp32: cos can return > 1.0 (rounding artifact) — use cos_max() (double precision) instead.
• ❌ Use one_shot=True for batch-invariance bisect: you capture the warmup forward, not the real prefill; modules inside per-seq dispatch loops only get seq 0 captured.
• ❌ Mistake batch invariance for an ATOM bug: model papers explicitly assume "bitwise reproducibility" (V4, Qwen3+) — running on a non-batch-invariant inference stack will flip edge-confidence tokens; that's expected, not a single-point bug.

Companion skills

• Pair with /debug-guide to handle server / GPU residual issues.
• Pair with /capture-trace to grab a GPU kernel trace for performance analysis.
• Pre-commit cleanup: env-gated dump hooks can stay (disabled by default); hardcoded debug prints must be removed.

Case studies

Bug	Key location step	Time
weights_mapping prefix conflict	Phase 1 weight comparison cos = 0.3 → grep substring matches	~2h
wo_a FP8 shuffle	Layer 0 attn_out cos = -0.002 → check quant_type → set to No	~1h
Swiglu activation 9× loss	Layer 0 ffn_out magnitude off by 9× → bisect activation → fix with silu+clamp	~3h
aiter mhc_post cumulative drift	Single prompt: first 30 tokens OK, then drifts → disable mhc_post to confirm	~4h
V4 input quant missing ue8m0	Phase 1 weight OK → Phase 5 wq_b GEMM cos = 0.998 → Phase 6 standalone test isolates quant as the cause (FIX-ref-GEMM cos = 1.0)	~3h
V4 act_quant_inplace ceil vs round-to-even	kv_after_quant cos = 0.998 → diff against ref_kernels.py → ceil vs f32_to_e8m0 1-ulp difference	~1h
V4 select_experts missing routed_scaling_factor	Phase 4 layer 0/2 cos = 1.0 but layer 3 jumps to cos = 0.98 → check layer-class difference (hash vs sqrtsoftplus) → diff ATOM select_experts vs ref Gate.forward → discover the missing *= route_scale at the end	~2h
Fix isolation revert	After three fixes pass e2e, revert linear.py → Bug 9+10 alone produce "6" → decide PR split (Bug 8 deferred)	~30min
Bug 11 batch invariance	Phase 8 E1 4×P3 + temp=0.0 → slot 2 differs → CLI slot-invariance shows layer 0 attn cos 0.99988 → MoE 0.998 → layer 1 attn 0.97 → match V4 paper §3.3 → not an ATOM bug	~3h

Key utility library

Use the ATOM built-ins (import from atom.utils.debug_helper):

from atom.utils.debug_helper import (
    # numerical primitives
    cos_max,            # returns (cos, max_abs, rel) — DOUBLE precision, avoids fp32 cos > 1.0 bug
    flag_for,           # ✓ / ? / ✗ / ✗✗ severity
    byte_equal_pct,     # bytewise overlap fraction
    # batch invariance
    slot_split,         # flat tensor → per-slot list
    compare_slots,      # one-shot slot 0 vs others
    pick_prefill_call,  # warmup-vs-prefill-vs-decode heuristic
    # schema
    schema_diff,        # only_a / only_b / common
    # dump hooks
    install_block_forward_hooks, maybe_dump_weights_and_exit, maybe_log_topk,
    # ref monkey-patching
    patch_method, patch_block_forward, patch_module_dump,
    # thresholds
    COS_BIT_EQUAL, COS_NUM_DRIFT, COS_ALGO_DIFF, COS_BUG,
)

CLI:

python -m atom.utils.debug_helper.compare slot-invariance --dir DIR --n-slots 4
python -m atom.utils.debug_helper.compare ref-vs-target  --dir DIR
python -m atom.utils.debug_helper.compare schema --a A.pt --b B.pt

Project-specific helpers (aiter / TP gather):

from aiter.utility.fp4_utils import e8m0_to_f32, f32_to_e8m0  # FP8 e8m0 ↔ FP32
from aiter.ops.shuffle import shuffle_weight                  # FP8 weight CK layout

# TP gather
ref_full = torch.cat([ref_rank[r]["weight"] for r in range(8)], dim=tp_dim)

# TP-replicated check
for r in range(1, 8):
    assert (ref_rank0["weight"].view(uint8) == ref_rank_r["weight"].view(uint8)).all()

File location convention

/app/logs_claude/{project}/dumps/
  ├── weights/                  # atom_rank{R}_layer{L}.pt + ref_rank{R}_layer{L}.pt
  ├── fwd/                      # atom_rank{R}_fwd.pt + ref_rank{R}_fwd.pt (single-layer)
  ├── fwd_multi/                # multi-layer dumps (0,15,30,45,60)
  └── fwd_v4ref/                # one dir per fix variant
/app/logs_claude/{project}/standalone_<op>_test.py  # GPU isolation experiments
/app/logs_claude/{project}/notes/NN_*.md             # bisect notes, numbered sequentially

Documentation discipline

After each phase, write a notes/NN_*.md containing:

1. Current hypothesis (what the bug is).
2. Experiment design (how to verify it).
3. Data (cos table / command output / key log).
4. Conclusion (hypothesis confirmed / refuted / rewritten).
5. Next step.

Cost of skipping notes: three days later you can't tell which phase you're in; handing over to someone else means redoing the work.