# Purpose classifier

This directory is the reproducible data and training pipeline for
`docs/PURPOSE_CLASSIFIER.md`. The current slice covers work item 2 and the first part of
work item 3: deterministic curation/splitting, a frozen v1 eval set, MiniLM fine-tuning,
temperature calibration, shared confidence thresholds, and the frozen-set accuracy,
recall, hard-slice, calibration, and latency report.

## Data contract

The canonical generated sources are listed in `data/generation-manifest.json`.
`round2-NN.jsonl` files are retained generation batches and intentionally duplicate
`purpose-prompts-round2.jsonl`; they are provenance, not additional training input.

`prepare_data.py`:

- validates the strict generated-record schema;
- removes exact and high-overlap word-trigram duplicates;
- fails for review if a high-overlap pair has conflicting labels;
- keeps shipped fixtures completely outside source data;
- holds every `vague-eval` record out of training;
- stratifies by primary purpose, slice, and primary language; and
- verifies that the deterministic test partition still matches the versioned
  `data/frozen-test-v1.jsonl`.

The frozen test set is the synthetic JSONL plus the 87 classifiable records in
`Tests/NucleicCoreTests/Fixtures/purpose-prompts.json`. The fixture file's five `general`
records are excluded because `general` is deliberately not a model label. The exact
membership and hashes are locked in `data/dataset-v1-manifest.json`.

## Prepare

From the repository root:

```bash
python3 ml/purpose-classifier/validate-data.py
python3 ml/purpose-classifier/prepare_data.py
python3 -m unittest discover -s ml/purpose-classifier/tests -p 'test_*.py'
```

For a newly generated raw 200-record batch, enable batch-shape checks explicitly with
`validate-data.py path/to/batch.jsonl --batch-size 200 --expected-total 200`.

The generated train/validation copies land under `.artifacts/dataset-v1/` and are
gitignored. A source, curation, seed, or split-policy change that moves the frozen test
set fails closed. After reviewing such a change, intentionally version it with:

```bash
python3 ml/purpose-classifier/prepare_data.py --refresh-frozen-test
```

## Train purpose-lite

Use a dedicated virtual environment. The base model is pinned to a specific
`sentence-transformers/all-MiniLM-L6-v2` commit: a 6-layer, 384-dimensional encoder. The
training collator always pads/truncates to 128 tokens so the later ONNX/Core ML export
can expose a fixed `1 x 128` runtime shape.

```bash
python3 -m venv ml/purpose-classifier/.venv
ml/purpose-classifier/.venv/bin/pip install -r ml/purpose-classifier/requirements.txt
ml/purpose-classifier/.venv/bin/python ml/purpose-classifier/train.py
```

The default requirements use PyTorch's CPU-only wheel on Linux, avoiding an accidental
multi-gigabyte CUDA install in CI and development containers. For NVIDIA, AMD, or Intel
accelerator training, install the platform's `torch==2.13.0` build using PyTorch's
platform selector, then install `requirements-base.txt`.

Training writes a local checkpoint, `calibration.json`, and `metrics.json` under
`outputs/purpose-lite-v1/`. It fits one validation-only temperature and derives nested
HIGH/MEDIUM/LOW cutoffs from calibrated top-one probability plus top-two margin.

For a wiring smoke test, use a small deterministic prefix:

```bash
ml/purpose-classifier/.venv/bin/python ml/purpose-classifier/train.py \
  --epochs 1 --max-train-records 64 --max-validation-records 64 \
  --output-dir ml/purpose-classifier/outputs/smoke --overwrite-output
```

## Evaluate

```bash
ml/purpose-classifier/.venv/bin/python ml/purpose-classifier/eval.py
```

The command returns failure unless frozen accuracy is at least 95%, every purpose recall
is at least 85%, and measured batch-one p95 is at most 20 ms. Use `--no-gate` only for
diagnostic runs. Accelerator residency, ONNX export/quantization, tokenizer golden tests,
tier-drift evaluation, and Core ML parity remain follow-on work. Before calling dataset
work item 2 complete, also run a semantic embedding duplicate audit and record the planned
10% human label spot-check; the current dependency-free word-trigram pass is deliberately
conservative.