## Review 002: Backend Sessions and Breaks APIs

**Files reviewed:** src/backend/models.py, src/backend/api/sessions.py, src/backend/api/breaks.py, src/backend/api/__init__.py
**Verdict:** request-changes

### Findings

#### change-required: Missing duration semantic validation
**Location:** src/backend/api/sessions.py:41-44 (SessionCreate validators)
**Quote:**

```
@validator('end_time')
def validate_end_after_start(cls, v, values):
    if 'start_time' in values and v < values['start_time']:
        raise ValueError('end_time must be >= start_time')
    return v
```

**Read:** The validator checks that end_time >= start_time, but does not validate that duration_seconds matches the calculated span. A client can POST start=09:00, end=09:25 (25 min span) with duration_seconds=9999 and the backend accepts it as a persisted fact.
**Concern:** The test docstring explicitly flags this: 'Without duration validation, malicious client could corrupt history.' The feature claim is 'see it logged in history'—which requires the logged data to be accurate. A client that claims false durations corrupts the history. The backend currently trusts the client without verification.
**Request:** Add a validator to SessionCreate that enforces `duration_seconds == (end_time - start_time).total_seconds()`. If clock skew is a concern, allow a small tolerance (±5 seconds), but do not accept arbitrary durations. The constraint should fire before the record reaches the database.

#### change-required: Missing duration semantic validation (Breaks)
**Location:** src/backend/api/breaks.py:41-44 (BreakCreate validators)
**Quote:**

```
@validator('end_time')
def validate_end_after_start(cls, v, values):
    if 'start_time' in values and v < values['start_time']:
        raise ValueError('end_time must be >= start_time')
    return v
```

**Read:** Same pattern as sessions: end_time >= start_time is checked, but duration_seconds is accepted without verification against the elapsed time span.
**Concern:** Breaks are part of history. Inaccurate durations corrupt the record in the same way as sessions. The issue is not specific to focus sessions.
**Request:** Add the same duration validation to BreakCreate. Breaks and Sessions should apply the same constraint: duration_seconds must match the calculated span (end_time - start_time).

#### note: Dead code: to_dict() methods unused
**Location:** src/backend/models.py:22-26 (SessionModel.to_dict); line 57-62 (BreakModel.to_dict)
**Quote:**

```
def to_dict(self):
    return {
        'id': self.id,
        'start_time': self.start_time.isoformat(),
        ...
    }
```

**Read:** SessionModel and BreakModel define to_dict() methods that serialize the model to a dictionary. The API layer does not call these methods; instead, it uses Pydantic response models (SessionResponse, BreakResponse) for serialization. The to_dict() methods are not referenced anywhere.
**Concern:** Dead code increases maintenance surface. If the response format needs to change, there are now two places to update (response model and to_dict). Having one source of truth is cleaner.
**Request:** Remove the to_dict() methods from both models. The Pydantic response models are the canonical serialization path and should be the only one. If to_dict() is intended for future use (e.g., debugging), document that intent explicitly; otherwise delete.

### Approvals

- Persistence and retrieval are correctly implemented—sessions and breaks are created with 201, retrieved with 404 for missing, and listed in chronological order.
- Immutability constraint (write-once) is correctly enforced—no update or delete endpoints exist.
- Settings snapshots are captured at creation and stored as JSON—future changes to settings won't corrupt historical records.
- DB constraints (duration > 0, end_time >= start_time) are properly enforced and surfaced as 400 errors.
- Pydantic validators correctly parse ISO 8601 datetimes and handle the Z/+00:00 UTC suffix edge case.
- Error handling is sound—IntegrityError caught, 404 on not-found, 422 on validation failure.
- Router registration and dependency injection (get_db) are correct.
- Code structure mirrors the pattern between sessions and breaks—consistency is clear.

### Cross-domain references

- The duration validation issue implicates the frontend contract: the client must now calculate duration_seconds and the backend will verify it. This should be documented as a Contract Note before Tweedledee begins M6 frontend work.
- The duration validation also touches on data integrity—Queen of Hearts may want to rule on whether duration mismatch should be logged as an audit event (potential fraud signal).
