# AGENTS.md - Agent Development Guide

This document provides context and guidelines for agentic coding agents working on the zugferd-service repository.

## Project Overview

ZUGFeRD-Service is a REST API for extracting and validating ZUGFeRD/Factur-X invoice data from PDF files. Built with FastAPI and Python 3.11+.

**Tech Stack:**
- FastAPI >= 0.109.0 (web framework)
- Uvicorn >= 0.27.0 (ASGI server)
- Pydantic >= 2.5.0 (data validation)
- factur-x >= 2.5 (ZUGFeRD/Factur-X library)
- pypdf >= 4.0.0 (PDF text extraction)
- lxml >= 5.0.0 (XML processing)

## Commands

### Development
```bash
# Install dependencies
pip install -e .

# Run the service (default: 0.0.0.0:5000)
python -m src.main
zugferd-service  # entry point

# With environment variables
HOST=127.0.0.1 PORT=8000 LOG_LEVEL=DEBUG python -m src.main
```

### Testing
```bash
# Run all tests
pytest

# Run specific test file
pytest tests/test_extract.py

# Run specific test function
pytest tests/test_api.py::test_health_check

# Run with coverage
pytest --cov=src

# Run with verbose output
pytest -v
```

### Building
```bash
# Docker build
docker build -t zugferd-service .

# Nix build
nix build .#zugferd-service

# Nix development shell
nix develop
```

## Code Style Guidelines

### Type Hints (Python 3.11+)
Use modern union syntax (`|`) instead of `Optional` or `Union`:
```python
# Good
field: str | None
numbers: list[int] | None

# Avoid
from typing import Optional, Union
field: Optional[str]
numbers: Union[list[int], None]
```

All public functions must have type hints:
```python
def extract_zugferd(pdf_bytes: bytes) -> ExtractResponse:
    """Extract ZUGFeRD data from PDF bytes.

    Args:
        pdf_bytes: Raw PDF file content

    Returns:
        ExtractResponse with extraction results
    """
```

### Imports
- Group imports: standard library, third-party, local modules
- Use `from typing import Any` only when needed
- Avoid star imports (`from module import *`)

```python
# Standard library
import io
import time
from typing import Any

# Third-party
from fastapi import FastAPI
from lxml import etree
from pydantic import BaseModel

# Local modules
from src.models import ExtractResponse
from src.utils import amounts_match
```

### Naming Conventions
- **Classes**: `PascalCase` (e.g., `ExtractionMeta`, `ValidateRequest`)
- **Functions/variables**: `snake_case` (e.g., `extract_text_from_pdf`, `pdf_bytes`)
- **Constants**: `SCREAMING_SNAKE_CASE` (e.g., `NAMESPACES`, `UNECE_UNIT_CODES`)
- **Private**: `_leading_underscore` (e.g., `_parse_internal`)

### Pydantic Models
All models defined in `src/models.py` using Pydantic v2:
```python
from pydantic import BaseModel, Field

class Supplier(BaseModel):
    """Supplier/seller information."""

    name: str = Field(description="Supplier name")
    vat_id: str | None = Field(default=None, description="VAT ID")
```
- Use `Field()` for all fields with descriptions
- Use `default=None` for optional fields (not `None` in type hint)
- Use `default_factory=list` for mutable defaults

### Error Handling
**Custom exceptions** for domain-specific errors:
```python
class ExtractionError(Exception):
    """Error during PDF extraction."""

    def __init__(self, error_code: str, message: str, details: str = ""):
        self.error_code = error_code
        self.message = message
        self.details = details
        super().__init__(message)
```

**FastAPI exception handlers** defined in `src/main.py`:
- `ExtractionError` → 400 with error code/message
- `HTTPException` → preserves status_code
- Generic `Exception` → 500 internal error

**Raise HTTPException for validation errors:**
```python
from fastapi import HTTPException

raise HTTPException(
    status_code=400,
    detail={"error": "invalid_base64", "message": "Invalid base64 encoding"},
)
```

### Docstrings
Use Google-style docstrings:
```python
def parse_supplier(xml_root: etree._Element) -> Supplier:
    """Parse supplier information from XML.

    Args:
        xml_root: XML root element

    Returns:
        Supplier model with parsed data
    """
```

### XML Parsing
Use `lxml.etree` with namespace-aware XPath:
```python
from lxml import etree

NAMESPACES = {
    "rsm": "urn:un:unece:uncefact:data:standard:CrossIndustryInvoice:100",
    "ram": "urn:un:unece:uncefact:data:standard:ReusableAggregateBusinessInformationEntity:100",
}

# Use namespaces in all XPath queries
name = xml_root.xpath(
    "//ram:ApplicableHeaderTradeAgreement/ram:SellerTradeParty/ram:Name/text()",
    namespaces=NAMESPACES,
)
```

### Logging
Structured JSON logging via custom `JSONFormatter` in `src/main.py`:
```python
logger = logging.getLogger(__name__)
logger.info("Extraction completed", extra={"data": {"profile": "EN16931"}})
```

### Testing
- Use `pytest` with `pytest-asyncio` for async tests
- Use `TestClient` from `fastapi.testclient` for API tests
- Define fixtures in `tests/conftest.py`
- Test PDFs in `tests/fixtures/`

```python
import pytest
from fastapi.testclient import TestClient
from src.main import app

@pytest.fixture
def client():
    return TestClient(app)

def test_health_check(client):
    response = client.get("/health")
    assert response.status_code == 200
    assert response.json()["status"] == "healthy"
```

### File Structure
```
src/
├── __init__.py
├── main.py          # FastAPI app, endpoints, exception handlers
├── models.py        # Pydantic models (all requests/responses)
├── extractor.py     # ZUGFeRD XML extraction logic
├── validator.py     # Invoice validation logic
├── pdf_parser.py    # PDF text extraction
└── utils.py         # Utility functions (constants, helpers)

tests/
├── conftest.py      # Pytest fixtures
├── test_api.py      # API endpoint tests
├── test_extractor.py # Extraction logic tests
├── test_validator.py # Validation logic tests
└── fixtures/        # Test PDF files
```

## Validation Checks

Four validation checks supported:
1. **pflichtfelder** - Required fields present and non-empty
2. **betraege** - Amount calculations correct (tolerance: 0.01)
3. **ustid** - VAT ID format (DE, AT, CH)
4. **pdf_abgleich** - XML vs PDF text comparison

## Environment Variables
- `HOST` (default: `0.0.0.0`)
- `PORT` (default: `5000`)
- `LOG_LEVEL` (default: `INFO`)

## Additional Notes
- Python 3.11+ required
- No type suppression (`# type: ignore`) allowed
- File size limit: 10MB for PDF uploads
- Returns warnings (not errors) for non-critical issues
- Uses Decimal rounding with ROUND_HALF_UP for monetary values