# Phase 1: Session Engine

**Purpose:** Days-long autonomous session management with checkpoint/resume.
**TS Module:** `src/tier1-foundation/session-engine.ts`

## What It Does

Manages the session lifecycle: `idle → running → checkpointing → resumed → completed → archived`
Saves checkpoints at every iteration. Resumes from latest checkpoint on restart.

## Checkpoint/Resume

```bash
# CLI
fable-agent session start "Your goal here"
fable-agent session resume <session-id>
fable-agent session list
fable-agent session inspect <session-id>
```

## Signals

- **Heartbeat**: written every 30s. If no heartbeat for >60s, session is stalled.
- **Checkpoint**: saved after each loop iteration. Latest and numbered copies.
- **Stall detection**: `session inspect` shows stall warning if heartbeat expired.

## Key Rules

- Start a session before running any task
- Checkpoint before dreaming
- Resume from checkpoint if interrupted
- Complete sessions explicitly (avoids dangling state)