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# Agent Turn And Step Lifecycle

This sequence is the visual companion to [architecture.md](architecture.md#loop-lifecycle-session--turn--step). It keeps durable replay facts on `session/event` and live control/status on `agent/*`.

```mermaid
sequenceDiagram
  participant User
  participant Agent
  participant Driver
  participant Hooks as hook listeners
  participant Prompt as ctx.systemPrompt
  participant LLM as ctx.llm
  participant Tools as ctx.tools
  participant Session
  participant Persistence
  participant SDK as UI or SDK listener
  User->>Agent: send(content)
  Agent-->>SDK: <code>agent/queued</code>
  Agent->>Driver: queued work wakes driver
  Driver-->>SDK: <code>agent/status</code> running
  Driver->>Session: <code>turn/start</code>
  Driver->>Hooks: <code>agent/prompt-submit</code> waterfall
  Hooks-->>Driver: allow, block, or add context
  Driver->>Session: <code>user/message</code> or rejected <code>turn/end</code>
  Driver->>Prompt: <code>system-prompt/assemble</code> waterfall
  Driver-->>Driver: <code>agent/pre-step</code> serial checkpoint
  Driver->>Session: <code>step/start</code>
  Driver->>LLM: <code>agent/request</code> waterfall, then <code>llm/stream</code> waterfall
  LLM-->>Driver: StreamChunk*
  Driver->>Session: <code>assistant/chunk</code>*
  Session-->>SDK: <code>session/event</code> <code>assistant/chunk</code>*
  Driver->>Hooks: <code>agent/step-result</code> waterfall
  Driver->>Session: <code>assistant/message</code>
  Driver->>Session: <code>tool/call</code>
  Driver->>Tools: execute through pre and post waterfalls
  Tools-->>Session: tool-owned events when applicable
  Driver->>Session: <code>tool/result</code> and <code>step/end</code>
  Driver->>Hooks: <code>agent/turn-continuation</code> waterfall
  Driver->>Session: <code>turn/end</code>
  Driver->>Persistence: <code>session/flush</code> parallel checkpoint
  Driver-->>SDK: <code>agent/status</code> idle
```

SDK users that need replayable transcript data should consume `session/event`; `agent/*` is the live coordination surface for queue/status, prompt interception, request shaping, steering, continuation, and errors.

Maintenance mode: curated Mermaid sequence; exact event signatures live in the generated Cordis catalog.
