# core/ — product API spine

The packages every harness build is assembled from: the session log, the system-prompt assembly, the tool registry, the agent vocabulary, and the one concrete loop that drives them. These are **product** packages — the stable surface plugins and consumers build against.

| Package | Role | ctx key |
|---|---|---|
| `session/` | Event-sourced session log + in-memory store | `ctx.sessions` |
| `system-prompt/` | Prompt-section + tool-schema assembly registry | `ctx.systemPrompt` |
| `tools/` | Tool registry + `tools/execute` waterfall | `ctx.tools` |
| `agent/` | Agent interface, registry, `agent/*` event vocabulary | `ctx.agents` |
| `agent-loop/` | The concrete loop plugin: `ReactLoopAgent` + the loop driver | `ctx.agentLoop` |

`agent-loop` is the one concrete implementation of the `agent` seam and lives here because it is the harness's default product loop; everything else in `core/` is interface/vocabulary. Plugins depend on the `agent` vocabulary, never on `agent-loop` directly, so the loop stays swappable.