# Review: Backend Sessions and Breaks APIs

**Verdict:** request-changes
**Reviewer's pace:** thorough

## Findings

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

**Quote:**
```python
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 that corrupts the reader's understanding.

**Request:** Declare the fields as 'datetime' to match what they actually contain after validation. Change lines 35-37 (sessions.py) and 29-31 (breaks.py) to:
```python
from datetime import datetime
start_time: datetime = Field(description="ISO 8601 datetime (UTC)")
end_time: datetime = Field(description="ISO 8601 datetime (UTC)")
```
The validators can stay as-is. This makes the contract honest: the annotation matches the actual type that the field contains after validation.

---

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

**Quote:**
```python
from typing import Optional
```

**Read:** Imported but never referenced in the module.

**Concern:** Dead imports obscure actual dependencies and increase noise when scanning a module's imports to understand what it uses.

**Request:** Remove line 13: `from typing import Optional`.

---

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

**Quote:**
```python
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. They are syntactically identical and serve the same purpose.

**Concern:** Code duplication creates maintenance risk. If the settings schema changes in the future (e.g., to track additional configuration like session_intensity), both classes must be updated. Forgetting one creates a divergence where sessions and breaks record different snapshot structures, violating the invariant that both should reflect identical settings at the time of creation.

**Request:** Move SettingsSnapshot to a shared module (e.g., `src/backend/api/schemas.py`) and import it in both sessions.py and breaks.py:

```python
# src/backend/api/schemas.py
from pydantic import BaseModel, Field

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")
```

Then in both sessions.py and breaks.py:
```python
from src.backend.api.schemas import SettingsSnapshot
```

This establishes a single source of truth and prevents drift.

---

## Approvals

- **REST structure is clean:** POST to create (/api/sessions and /api/breaks), GET by ID to retrieve one, GET without ID to list all. Sensible ordering (by start_time) on list endpoints makes history queries efficient.
  
- **Immutability enforced:** The API has no PUT, PATCH, or DELETE endpoints. Records are write-once. This matches the contract exactly.

- **Settings snapshot captured at creation:** The snapshot is stored as JSON in the database and reconstructed to SettingsSnapshot on retrieval. This preserves the immutable snapshot of settings at the time of session/break creation.

- **Error handling is explicit:** HTTPException with 400 for validation failures (IntegrityError caught and reported), 404 for not found. Error details include context from the database.

- **Database invariants enforced:** CheckConstraints (duration > 0, end >= start) enforce invariants at the data layer, independent of API request validation. This provides a second layer of defense.

- **ISO 8601 timezone handling correct:** The 'Z' suffix is correctly replaced with '+00:00' for compatibility with datetime.fromisoformat(). Handles UTC timezone-aware datetime parsing.

---

## Cross-domain references

- Once type annotations are fixed, verify frontend (Tweedledee) contract alignment — does the frontend send ISO strings or datetime objects? The backend is currently set up to accept ISO strings and convert them, so the frontend should send strings.
