# 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: agent/queued Agent->>Driver: queued work wakes driver Driver-->>SDK: agent/status running Driver->>Session: turn/start Driver->>Hooks: agent/prompt-submit waterfall Hooks-->>Driver: allow, block, or add context Driver->>Session: user/message or rejected turn/end Driver->>Prompt: system-prompt/assemble waterfall Driver-->>Driver: agent/pre-step serial checkpoint Driver->>Session: step/start Driver->>LLM: agent/request waterfall, then llm/stream waterfall LLM-->>Driver: StreamChunk* Driver->>Session: assistant/chunk* Session-->>SDK: session/event assistant/chunk* Driver->>Hooks: agent/step-result waterfall Driver->>Session: assistant/message Driver->>Tools: classify next call by executionMode loop bounded rolling pool with reclassification before replenishing opt capacity available for an unstarted call Driver->>Session: tool/call pending audit Driver->>Tools: ordered pre / pooled dispatch Tools-->>Session: tool-owned events when applicable end opt next model-order result is ready Driver->>Tools: ordered post Driver->>Session: tool/result end end Driver->>Session: step/end Driver->>Hooks: agent/turn-continuation waterfall Driver->>Hooks: agent/turn-stop serial terminal checkpoint Driver->>Session: turn/end Driver->>Persistence: session/flush parallel checkpoint Driver-->>SDK: agent/status 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.