Dataset Curation

Dataset Curation

This skill assumes finetuning-method-selection

already routed here — the next step is preparing

data, not choosing a method. What follows: format

selection by target method, the template/packing

mechanics behind the most common silent training

failures, rules for mixing in synthetic data

without collapse, and the dataset card that closes

out Phase 2 before a run starts.

Input: raw examples (demonstrations, preference

judgments, or task prompts) plus a routing decision

from finetuning-method-selection.

Output format: a formatted, packed, validated

JSONL dataset plus a completed dataset card — the

Phase 2 artifact /finetune checks before launching

training.

Format Selection

| Method | Shape | Rows |

|---|---|---|

| SFT, single-turn | Instruct (instruction/response or prompt/completion) | ~1,000+ floor |

| SFT, multi-turn | Conversation / ChatML messages list | ~1,000+ floor |

| DPO / ORPO | Preference pair (prompt, chosen, rejected) | Method-dependent, see preference-optimization |

| KTO | Unpaired (prompt, completion, label) | Method-dependent, see preference-optimization |

| GRPO / RLVR | Prompt-only (prompt + verifier metadata) | Method-dependent, see grpo-rlvr-training |

• ~1,000+ rows is the recommended floor for SFT,

not a target. Below it, a handful of low-quality

or duplicate examples can dominate the gradient;

above it, quality over quantity — a smaller

verified, deduplicated set beats a larger noisy one.

• The ChatML shape, for orientation; the other four

formats plus a ShareGPT conversion note live in

references/formats-and-templates.md:

  {"messages": [
    {"role": "user", "content": "..."},
    {"role": "assistant", "content": "..."}
  ]}

Chat Templates and Loss Masking

Apply the target model's chat template before

any concatenation or packing, never after — packing

raw text and templating the packed blob afterward

corrupts turn boundaries, landing role markers in

the wrong place relative to each example.

• Train on assistant responses only. Mask the

loss (-100 in the labels tensor) over system/user

turns and the template's own role markers — only

assistant-turn content tokens contribute to loss.

• **Template/tokenizer mismatches are a top silent

failure mode.** A model trained against one chat

template but served or evaluated with a different

one degrades without erroring. Verify the same

template string used in training is applied at

inference and eval time.

• Keep the dataset in messages shape and let

the trainer template and mask it

(assistant_only_loss=True in current TRL) —

pre-rendering to a flat text field destroys the

turn boundaries masking needs. Full code sketch:

references/formats-and-templates.md. Sanity-check

before training — decode only unmasked positions;

expect only assistant text:

  keep = batch["labels"][0] != -100
  print(tokenizer.decode(batch["input_ids"][0][keep]))

Packing

**Without packing, 40–70% of compute is spent on

padding** — variable-length examples batched at a

fixed sequence length waste the gap between each

example's length and the batch's max. Packing

concatenates multiple examples into one sequence

up to the max length, cutting most of that waste.

• Packing changes batch semantics. A packed

sequence can contain several original examples, so

"steps per epoch" and any LR schedule keyed to

example count shift once packing is on — recompute

schedule milestones against packed-sequence count.

• **MANDATORY: decode and manually inspect 5–10

packed sequences before scaling to a full run.**

Confirm example boundaries land where expected,

template markers are intact per sub-example, and

the loss mask is still assistant-only within each

packed sequence. Not optional — packing bugs are

silent (the loss curve looks normal) and only

surface in eval quality, hours later:

  for seq in packed_dataset.select(range(10)):
      print(tokenizer.decode(seq["input_ids"]))

Synthetic Data Rules

• Keep ≥25% real data as a collapse guard.

Training on a growing share of model-generated

data without a real-data floor drives measurable

quality collapse over successive generations —

25% real is the minimum that holds the line.

**General-domain replay rows

count toward this floor** —

"real" means "not generated

for this task from this

student," not "human-authored."

An all-synthetic-by-construction

dataset can meet the ≥25% floor

through replay alone (see

references/synthetic-data.md's

Replay-Mix Construction recipe);

state which rows count as "real"

in the dataset card rather than

leaving the floor structurally

unmeetable.

• **Magpie and rejection sampling are the

workhorses.** Magpie extracts prompts from the

model's own template prior; rejection sampling

generates several candidates per prompt and keeps

only the ones a filter passes. Both beat naive

single-shot generation.

• **Targeted, student-aware generation beats static

generation by 1.3–2x sample efficiency** — aiming

at the student's actual failure modes hits a

quality bar with fewer filtered examples.

• **Typical accept rates after filtering run

10–30%.** Plan volume accordingly — a 10,000-row

target at 15% accept needs ~65,000+ raw generations.

• Generation-method ranking, filter funnel, replay-

mix construction, and distillation pattern:

references/synthetic-data.md.

The Dataset Card

Every dataset that reaches training gets a card —

the required Phase 2 artifact /finetune checks

before launching. The card is not free-form

documentation; it MUST carry these fields:

• Provenance — where every row came from (real

source(s), synthetic method(s), or both),

traceable to trace-to-training-data output.

• Counts — total rows, and rows per split

(train/eval/held-out) if split.

• Synthetic/real ratio — the measured ratio,

checked against the ≥25% real floor above.

• Dedup method — exact-match, semantic

(embedding threshold), or both; see the filter

funnel in references/synthetic-data.md.

• Template used — the exact chat template

string/identifier, kept consistent through

inference and eval — this is what ties an

eval-harness-first run back to the checkpoint.

• Packing config — whether packing was used,

max sequence length, and confirmation the

5–10-sequence manual inspection above was done.

A dataset missing any of these six fields isn't

ready for /finetune — the card is a gate, not a

summary written after the fact.

Phase 2 Exit Checklist

Before handing off to /finetune, confirm:

• Format matches the method (table above).
• Template applied before concatenation.
• Loss masked to assistant turns only.
• 5–10 packed sequences decoded and read.
• ≥25% real data in the final mix.
• Dataset card complete — all six fields.

References

• references/formats-and-templates.md — JSONL

examples per format, current-TRL masking code,

and the ShareGPT conversion note.

• references/synthetic-data.md — generation-method

ranking, filter funnel, replay-mix construction,

and teacher→student distillation pattern.

Related skills: finetuning-method-selection routes

here; lora-qlora-recipes, vision-sft, and

preference-optimization consume the datasets this

skill produces; trace-to-training-data is the

provenance source for graded-trajectory datasets;

eval-harness-first grades the resulting checkpoint.