Synthetic Data: Generation, Filtering, Distillation

Full detail backing SKILL.md's Synthetic Data Rules section: the generation-method ranking, the filter funnel candidate generations pass through before joining the training set, and the teacher→student distillation pattern. Base models are never named as recommendations here. Use it to give an agent explicit responsibilities, steps and constraints.

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Last verified: 2026-07-14

Synthetic Data: Generation, Filtering, Distillation

Full detail backing SKILL.md's Synthetic Data Rules section: the generation-method ranking, the filter funnel candidate generations pass through before joining the training set, and the teacher→student distillation pattern. Base models are never named as recommendations here — TEACHER and STUDENT are placeholders for whichever checkpoints a given run uses; see finetuning-method-selection's references/model-catalog.md for actual model choice.

Generation-Method Ranking

Methods below are ordered roughly weakest to strongest for sample efficiency and downstream quality at the same generation budget. Each level subsumes the previous — a rejection-sampling pipeline typically generates its candidates with Magpie or persona-conditioned prompts underneath, rather than replacing them:

1. Self-Instruct — bootstrap new prompts from a small seed set by having a model paraphrase and extend them. Cheapest, weakest: prompt diversity plateaus quickly and quality tracks the seed set closely.
2. Evol-Instruct — iteratively rewrite prompts to increase complexity (add constraints, deepen reasoning, broaden scope) across generations. Improves difficulty coverage over Self-Instruct but still seed-dependent.
3. Magpie — extract prompts directly from the target model's own chat-template prior by sampling from the template's user-turn position with no seed prompt at all. Removes seed-set bias entirely; this is why it's a workhorse rather than a niche technique.
4. Persona-conditioned generation — condition prompt generation on a sampled persona/role description to broaden style and topic coverage beyond what a single generation policy produces unconditioned.
5. Rejection sampling / verifier-filtered — generate multiple candidate completions per prompt and keep only those a filter, verifier, or judge accepts. This is the other workhorse from SKILL.md, and it composes with any of the prompt-generation methods above — it's a completion-side filter, not a prompt-generation method by itself.

Targeted, student-aware generation — steering prompt or persona selection toward the current student model's actual failure modes rather than sampling uniformly — layers on top of any method above and is what delivers the 1.3–2x sample efficiency gain cited in SKILL.md. It requires an eval signal on the student to know what its failure modes currently are; without that signal, generation defaults to untargeted/static.

Filter Funnel

Apply filters in this order — each stage is cheaper than the next, so cheap stages should eliminate volume before expensive stages run on what's left:

1. Exact dedup. Hash-based exact-match removal of identical rows (after normalization — whitespace/casing collapsed before hashing). Cheapest stage, run first, removes generation-loop repeats before anything downstream sees them.
2. Semantic dedup (~0.92 similarity threshold). Embed each candidate and drop rows whose nearest-neighbor cosine similarity to an already-kept row exceeds ~0.92. Catches paraphrase-level duplicates exact dedup misses.
3. Length filter. Drop candidates below a minimum or above a maximum token length for the task — too-short responses are usually degenerate, too-long ones are usually rambling or off-task.
4. Language ID filter. Drop candidates that fail a language-ID check against the target language(s) — generation occasionally drifts language, especially from multilingual base models on under-specified prompts.
5. Score-based top-30% filter. Score remaining candidates (reward model, heuristic, or self-consistency score) and keep roughly the top 30% — this is a coarse quality cut before the most expensive stage runs.
6. Judge ≥ threshold. Run the most expensive check last, on the smallest remaining set: an LLM-judge or human-equivalent quality check against a fixed threshold. Rows that fail here are dropped regardless of how they scored upstream.

The 10–30% typical accept rate from SKILL.md is the funnel's end-to-end yield across all six stages, not any single stage's pass rate — budget raw generation volume against the full-funnel yield, not against any one stage's rate.

Teacher→Student Distillation Pattern

1. Generate teacher traces. Sample completions (with reasoning traces, where applicable) from TEACHER against the target task's prompt distribution.
2. Verify. Run the traces through the Filter Funnel above — a distillation set is a synthetic dataset like any other and still needs the ≥25% real-data floor from SKILL.md respected in the final training mix, plus the same dedup and quality stages.
3. SFT the student. Train STUDENT on the verified traces using the standard SFT format and template rules from SKILL.md and references/formats-and-templates.md — a distillation dataset is not a special format, it's a provenance label on an otherwise-ordinary instruct or ChatML dataset.

Record TEACHER identity and generation configuration (sampling temperature, prompt template used to elicit traces) in the dataset card's provenance field — "distilled from TEACHER" is a provenance fact /finetune and downstream audits both expect to find there, not something to leave implicit.

Replay-Mix Construction

The implementation recipe behind checkpoint-promotion's catastrophic-forgetting escalation ladder (SKILL.md's owning document for when and how far to move the replay fraction — this section covers how to build the rows, the single most common REJECT remediation and the part most often improvised ad hoc under time pressure). Five decisions, in the order they come up:

1. General-Domain Source Selection

Pick a source that is genuinely general-domain for the capability being protected, not a narrow slice that happens to be convenient. Two failure modes to avoid:

• Don't teach to the gate. If the replay source is drawn from the exact same distribution as the drift suite's benchmarks (e.g. the same GSM8K train split the drift suite's test split comes from), the resulting drift score partially measures "did this model see similar items in training," not "did fine-tuning preserve the underlying capability." This is not automatically disallowed — see checkpoint-promotion's instruction-reuse disclosure rule — but it must be disclosed, and a broader source (not scoped to the drift suite's own benchmarks) is the more defensible default when one exists.
• Match the source to the forgetting signature. If error analysis on the failing checkpoint shows a specific lost capability (e.g. chain-of-thought math reasoning, not general knowledge), a replay source targeting that capability recovers it faster than a generic instruct-tuning mix — but narrows the "general-domain" claim; state in the dataset card which capability the replay mix targets and why.

2. Prompt Shape

Decide what shape replay rows take in the messages-shaped SFT set — this is a real choice, not a detail:

• Natural instruction — however the source data's own prompts are phrased. Lowest effort, least targeted.
• The drift harness's exact phrasing — matches the eval's instruction wording. Most directly addresses an instruction-following forgetting signature (e.g. "ignores the show-your-work instruction"), but triggers the instruction-reuse disclosure rule in checkpoint-promotion and inflates the post-replay score on that specific benchmark.
• Bare input, no instruction wrapper — closest to raw continued-pretraining signal; weakest at restoring instruction-following specifically.

Pick based on the forgetting signature from error analysis, not by default — and disclose the choice in the dataset card regardless of which one.

3. Answer Reformatting

Decide whether replay reference answers get reformatted toward the target task's output convention, or kept in the source format as-is. Example: rewriting a math dataset's #### N final-answer terminator to match the target task's own extraction convention. This is a judgment call that changes what the model learns to emit on replay-domain prompts — record the exact transformation applied (or "none — kept source format") in the dataset card, since it changes what a downstream error-analysis pass should expect to see.

4. Val-Split Treatment

Decide whether the validation split gains replay rows or stays task-only:

• Task-only val split keeps eval_loss directly comparable across runs that only differ in replay fraction — the training loop has zero visibility into replay fit, and replay recovery is only measurable at the next Phase 5 re-gate.
• Replay rows in val too gives in-loop visibility into replay fit, at the cost of eval_loss no longer being an apples-to-apples comparison against a prior run's task-only val split.

Neither is universally correct; state which was chosen and why in the dataset card, and don't compare eval_loss across runs that made different choices here without noting the confound.

5. Disjointness Verification

Before training, verify replay rows don't overlap the drift suite or the goldens set — required, not optional, regardless of which source was picked in step 1:

• Split-level separation — draw replay rows only from a source split (e.g. a train split) disjoint from whatever split the drift suite's items are drawn from.
• Exact-match text filter — normalize and exact-match replay row question/prompt text against the drift suite's selected items and eval/goldens.jsonl; drop any hit. Record the overlap count found (expect 0) in the dataset card — a nonzero count found and silently dropped is still worth recording, since it signals the source pool needs a tighter split boundary next time.

When a Later Run Changes the Replay Fraction: Swap, Don't Add

If a checkpoint-promotion gate calls for moving the replay fraction (see checkpoint-promotion's escalation ladder), implement the change by swapping rows, not adding them: drop target-task rows out of the training set as replay rows go in, so the total row/step count holds constant between the old and new run. Adding replay rows on top of the existing set changes replay fraction and total optimizer steps in the same move, making it impossible to attribute a later drift-score change to either variable alone — this confound has produced misleading run-to-run trajectories in practice, so treat swap-not-add as a hard rule for this recipe, not a style preference.

Row count is not token count. Swapping rows 1-for-1 holds the row count constant, but replay rows and target-task rows are rarely the same length — a swap can still shift total training tokens (and therefore max_steps under a fixed batch size and sequence-packing scheme) even though the row count didn't move. Hold total training tokens, or max_steps directly, constant between the old and new run — not just row count — and record the packed-token count for each run (not just the row count) in the dataset card before attributing a drift-score change to the replay-fraction change alone. A run that swapped rows but grew packed tokens 10% has the same attribution problem as one that added rows outright.

Synthetic-Only Datasets and the ≥25% Real Floor

SKILL.md's Synthetic Data Rules require ≥25% real data as a collapse guard. When a training set is 100% synthetic by construction (a greenfield task with no real-data pool to draw from at all — not merely a lot of synthetic augmentation on top of a real base), that floor is unmeetable by definition unless something in the mix counts as "real."

Resolution: general-domain replay rows count toward the ≥25% floor. "Real" in this rule means "not generated for this specific task from this specific student model" — a replay row pulled from an existing general-instruct dataset (human-authored or otherwise pre-existing, not freshly generated by the student or its teacher for this run) satisfies that definition even though the target-task rows around it are 100% synthetic. Build the replay mix per the five decisions above, then compute the synthetic/real ratio the dataset card requires treating replay rows as the "real" share — and state explicitly in the card that this is how the ratio was met, so a later audit doesn't misread an all-synthetic-target-data run as having silently skipped the collapse guard.

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