# Backend

FastAPI backend for the Variant Risk Explainer research demo.

The backend loads a fine-tuned DNABERT-2 sequence-classification model once at
startup and exposes `POST /analyze` and `POST /api/analyze` for DNA sequence
risk prediction.
The response also includes an explanation generated from the model
probabilities, selected threshold, and prediction label. By default this is
rule-based. You can optionally enable an OpenAI-powered explanation paragraph.

This is for research/demo use only. It is not a clinical diagnostic system and
must not be used for medical decisions.

## Setup

From the repository root:

```bash
cd backend
python -m venv .venv
source .venv/bin/activate
python -m pip install --upgrade pip
pip install -r requirements.txt
cp .env.example .env
```

## Model Folder

The final local model folder is:

```text
training/training_model_files/
```

That folder is intentionally ignored by Git because it contains large model
files. For a self-contained Docker deployment, place a copy at:

```text
models/final_model/
```

Configure the model path in `backend/.env`:

```bash
MODEL_DIR=../training/training_model_files
MODEL_THRESHOLD=0.16
MODEL_MAX_LENGTH=512
MODEL_NAME=DNABERT-2 ClinVar 20k
DEVICE=auto
```

`DEVICE=auto` selects CUDA, then MPS, then CPU.

`MODEL_DIR` may also be a Hugging Face model repository ID:

```bash
MODEL_DIR=your-username/variant-risk-dnabert2-20k
```

For a private model repository, provide `HF_TOKEN` as a secret.

## Optional OpenAI Explanation

Do not paste your OpenAI API key into source code or `.env.example`.

Paste it only into your local `backend/.env` file:

```bash
USE_AI_EXPLANATION=true
OPENAI_API_KEY=your_openai_api_key_here
```

Then restart the backend. If the OpenAI key is missing, the package is not
installed, or the API call fails, the backend automatically falls back to the
local rule-based explanation.

## Run

From `backend/`:

```bash
uvicorn app.main:app --reload
```

Open:

```text
http://localhost:8000/docs
```

## Health Check

```bash
curl http://localhost:8000/health
```

The deployment alias is:

```bash
curl http://localhost:8000/api/health
```

The response shows whether the model loaded, selected device, model source,
threshold, and explanation availability.

## Analyze Example

```bash
curl -X POST http://localhost:8000/analyze \
  -H "Content-Type: application/json" \
  -d '{
    "variant_name": "GRCh38-example",
    "gene": "BRAF",
    "sequence": "ACGTACGTACGTACGTACGTACGTACGTACGT",
    "notes": "Demo request"
  }'
```

Python example:

```python
import requests

response = requests.post(
    "http://localhost:8000/analyze",
    json={
        "variant_name": "GRCh38-example",
        "gene": "BRAF",
        "sequence": "ACGTACGTACGTACGTACGTACGTACGTACGT",
    },
    timeout=30,
)
print(response.json())
```

## Response Fields

- `prediction_class`: `0` for benign/likely benign, `1` for pathogenic/likely pathogenic
- `prediction_label`: human-readable label
- `risk_level`: `Lower` or `Elevated`
- `benign_probability`: class 0 probability
- `pathogenic_probability`: class 1 probability
- `threshold`: decision threshold, currently `0.16`
- `sequence_length_used`: sequence length after optional center crop
- `explanation`: plain-language explanation of the model output
- `explanation_source`: `openai`, `rule-based`, or `rule-based-fallback`
- `confidence_level`: rough confidence category based on model probability
- `recommendation`: safe research/demo recommendation
- `limitations`: important limitations to show users

## Explanation Layer

The default explanation is generated by local backend rules. When
`USE_AI_EXPLANATION=true`, the backend asks OpenAI to rewrite only the
explanation paragraph in beginner-friendly language. The prediction,
probabilities, threshold, confidence level, recommendation, and limitations stay
controlled by backend logic.

The wording is intentionally cautious because it is derived only from the model
output, not from clinical review.

## Static Frontend

The Docker build creates the Next.js static export and copies it into
`backend/static/`. When that folder exists, FastAPI serves the frontend at `/`.
API routes are registered before static file serving so `/api/health` and
`/api/analyze` remain available.

## Safety Notice

Predictions are experimental model outputs. They can be wrong, incomplete,
biased by ClinVar labels, or invalid outside the training distribution. Do not
use this backend for diagnosis, treatment, or clinical decision-making.