## Review 001: Backend Sessions and Breaks APIs

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

### Findings

#### change-required: Field type annotation 'str' contradicts post-validator reality ('datetime')
**Location:** sessions.py:35-37, breaks.py:29-31
**Quote:**

```
class SessionCreate(BaseModel):
    start_time: str = Field(description="ISO 8601 datetime (UTC)")
    end_time: str = Field(description="ISO 8601 datetime (UTC)")
```

**Read:** The models declare start_time and end_time as str. The validators (with pre=True) convert these strings to datetime objects. Pydantic accepts the datetime values directly into str-annotated fields because pre=True validators bypass type coercion. The code works: after validation, payload.start_time is datetime, which is what SessionModel expects. But the field annotation is a lie—it claims str, but contains datetime.
**Concern:** Type annotations are a contract with future readers. This code violates that contract: the declaration promises str, but the actual value is datetime. A reader will assume string operations are possible and write incorrect code. IDEs and type checkers will be confused. This is exactly the kind of clarity bug the Caterpillar is paid to prevent.
**Request:** Declare the fields as 'datetime' to match what they actually contain after validation. Change the field declarations to: 'start_time: datetime = Field(description="ISO 8601 datetime (UTC)")' and same for end_time. The validators can stay as-is. This makes the contract honest: the annotation matches the actual type that the field contains.

#### suggestion: Unused import: Optional
**Location:** sessions.py:13
**Quote:**

```
from typing import Optional
```

**Read:** Imported but never used in the module.
**Concern:** Dead imports obscure actual dependencies and increase noise when reading.
**Request:** Remove line 13: 'from typing import Optional'.

#### suggestion: Duplicate SettingsSnapshot class definition
**Location:** sessions.py:26-28 and breaks.py:20-22
**Quote:**

```
class SettingsSnapshot(BaseModel):
    session_duration: int = Field(gt=0, description="Session duration in minutes")
    break_duration: int = Field(gt=0, description="Break duration in minutes")
```

**Read:** Identical class defined in both modules. Two separate class objects for the same schema.
**Concern:** Code duplication creates maintenance risk. If the settings schema changes (e.g., add a field), both must be updated. Forgetting one creates a divergence where sessions and breaks record different snapshots, violating the invariant that both should reflect identical settings at creation time.
**Request:** Move SettingsSnapshot to a shared module (e.g., 'src/backend/api/schemas.py') and import in both sessions.py and breaks.py. Single source of truth prevents drift.

### Approvals

- REST structure is clean: POST to create, GET by ID to retrieve one, GET without ID to list all. Sensible ordering (by start_time) on list endpoints.
- Immutability enforced: no PUT/PATCH/DELETE endpoints. Records are write-once. Matches the contract exactly.
- Settings snapshot captured at creation and stored as JSON. Reconstructed on retrieval. Preserves immutable snapshot of settings at creation time.
- Error handling explicit: HTTPException with 400 for validation failures (IntegrityError caught), 404 for not found. Includes error context.
- Database CheckConstraints (duration > 0, end >= start) enforce invariants at the data layer, independent of API validation.
- ISO 8601 timezone handling correct: 'Z' replaced with '+00:00' for fromisoformat compatibility. Timezone-aware datetime parsing.

### Cross-domain references

- Once type annotations are fixed, verify frontend contract alignment with Tweedledee — should the frontend send ISO strings or datetime objects?
