## Review 004: Backend Sessions and Breaks APIs — Duration Validation and Contract

**Files reviewed:** src/backend/api/sessions.py, src/backend/api/breaks.py, src/backend/models.py
**Verdict:** accept

### Findings

#### suggestion: Remove unused to_dict() methods from models
**Location:** src/backend/models.py:35-42 and 73-80
**Quote:**

```
    def to_dict(self) -> dict:
        return {
            "id": self.id,
            "start_time": self.start_time.isoformat() if self.start_time else None,
            "end_time": self.end_time.isoformat() if self.end_time else None,
            "duration_seconds": self.duration_seconds,
            "settings_snapshot": self.settings_snapshot,
            "created_at": self.created_at.isoformat() if self.created_at else None,
        }
```

**Read:** Both Session and Break models define a to_dict() serialization method, but the API layer (sessions.py and breaks.py) uses Pydantic response models for serialization instead. These methods are never called.
**Concern:** Dead code creates ambiguity: a future reader may wonder whether to_dict() is the canonical serialization path (it isn't) or whether Pydantic responses are (they are). Maintaining two serialization paths creates unnecessary cognitive load.
**Request:** Remove the to_dict() methods from both Session and Break models. The Pydantic response models (SessionResponse, BreakResponse) are the canonical serialization path.

### Approvals

- Backend-computed duration is correct: the Pydantic validators parse ISO 8601 datetimes, then the endpoint computes duration_seconds = (end_time - start_time).total_seconds(). The backend is now authoritative on elapsed time, which eliminates the client-corruption risk.
- Temporal invariants are properly enforced: Pydantic validator checks end_time >= start_time (before DB write), and database CheckConstraints enforce duration_seconds > 0 and end_time >= start_time. Constraint violations surface as IntegrityError → 400 Bad Request.
- Contract is explicit and well-documented: both sessions.py and breaks.py include clear docstrings naming 'Contract v2 (backend-computed-duration)' and explaining what the client sends vs. what the backend computes. This is the right level of clarity for frontend pair-off.
- Write-once immutability is correctly enforced: only POST and GET endpoints exist (no PUT/PATCH). The settings_snapshot is captured at creation and serialized as-is in the response.
- Error handling is appropriate: client validation errors (e.g., end_time before start_time) raise Pydantic ValidationError → 422 Unprocessable Entity. Constraint violations at DB write raise IntegrityError → 400 Bad Request. Resource-not-found returns 404. The error detail messages include the underlying reason.
- DateTime handling is careful: using DateTime(timezone=True) in SQLAlchemy, isoformat() for serialization, and parsing with fromisoformat() (including the .replace('Z', '+00:00') edge case for RFC 3339 UTC indicator). ISO 8601 round-trips cleanly.

### Cross-domain references

- Frontend pair-off (M6) can now negotiate the SessionCreate and BreakCreate request shapes (start_time, end_time, settings_snapshot, skipped) against the contract docstrings.
- Test scenarios (Feature 001 and 002) are all marked pytest.skip and document the expected behavior. Once live, they will verify that duration validation and temporal invariants hold.
