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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 SDK as UI or SDK listener
  User->>Agent: followup(content)
  Agent-->>SDK: <code>agent/inbox/enqueue</code>
  Agent->>Driver: queued work wakes driver
  Driver-->>SDK: <code>agent/status</code> running
  Note over Agent,Driver: next-step acceptance window opens
  Driver->>Hooks: <code>agent/prompt-submit</code> waterfall
  Hooks-->>Driver: authoritative allow, block, or add context
  alt prompt blocked or admission failed
    Driver-->>Driver: append context-only batch or keep steering boundary pending
  else prompt allowed
  Driver->>Session: <code>turn/start</code>
  Driver->>Session: <code>user/message</code>
  Driver->>Prompt: <code>system-prompt/assemble</code> waterfall
  Driver-->>Driver: <code>agent/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>*
  alt final adapter or terminal in-band request failure
    Driver->>Session: <code>step/end</code>
    Driver->>Hooks: <code>agent/request-error</code> waterfall
    Hooks-->>Driver: return retry action or preserve the original error
  else model request succeeded
  Driver->>Session: <code>assistant/message</code>
  Driver->>Tools: classify pending call by executionMode
  loop barriers and bounded rolling pool, reclassify before start
    opt call starts
      Driver->>Session: <code>tool/call</code>
      Driver->>Tools: ordered pre, concurrent execute
      Tools-->>Session: tool-owned events when applicable
    end
    opt next model-order result ready
      Driver->>Tools: ordered post
      Driver->>Session: <code>tool/result</code>
    end
  end
  Driver->>Session: post-tool context and steering (no prompt-submit)
  Driver->>Session: <code>step/end</code>
  Driver->>Hooks: <code>agent/turn-stopping</code> serial terminal checkpoint
  end
  Note over Agent,Driver: next-step acceptance window closes
  Driver->>Session: <code>turn/end</code>
  end
  Driver-->>SDK: <code>agent/status</code> idle
```

The `assistant/message` edge records every successful provider call, including content-less and `max-tokens` finishes. Empty content stays out of derived history while the durable anchor retains usage and exact chunk provenance, including an explicit empty source set.

`dsh-compact-basic` uses `agent/step` for pressure before request derivation and `agent/request-error` only for canonical context overflow. Once either trigger qualifies, optional tool-result pruning runs before summary selection. Recovery works between the closed failed step and failed turn close, and opens a fresh retry turn only when pruning or summarization advances the surface replacement generation; otherwise the original request error remains authoritative.

The returned `agent/prompt-submit` allow is authoritative; listeners wrapping `next()` preserve downstream content and additional contexts unless replacement is intentional. Steering bypasses that waterfall and joins at its durable checkpoint.

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.
