# Agent Turn And Step Lifecycle This sequence is the visual companion to [architecture.md](architecture.md#default-loop-lifecycle). 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: agent/inbox/spliced Agent-->>SDK: agent/inbox/inserted { message } Agent->>Driver: queued work wakes driver Driver-->>SDK: agent/status running Note over Agent,Driver: claim pending next-step input plus one queued prompt Driver-->>SDK: agent/inbox/spliced pure deletion Driver-->>SDK: agent/inbox/claimed { message, turn } per message Driver->>Hooks: agent/pre-step waterfall Hooks-->>Driver: authoritative reject or enter(messages) alt proposed step rejected or pre-step failed Driver-->>Driver: claimed batch stays removed, no turn opens else enter proposed step Driver->>Session: turn/start Driver->>Session: step/start Driver->>Session: user/message per entered message Driver->>Prompt: system-prompt/assemble waterfall Driver->>LLM: agent/request waterfall, then llm/stream waterfall LLM-->>Driver: StreamChunk* Driver->>Session: assistant/chunk* Session-->>SDK: session/event assistant/chunk* alt final adapter or terminal in-band request failure Driver->>Session: step/end Driver->>Hooks: agent/request-error waterfall Hooks-->>Driver: return retry action or preserve the original error else model request succeeded Driver->>Session: assistant/message Driver->>Tools: classify pending call by executionMode loop barriers and bounded rolling pool, reclassify before start opt call starts Driver->>Session: tool/call 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: tool/result end end Driver->>Session: step/end opt natural stop and next-step inbox empty Driver->>Hooks: agent/turn-stopping serial terminal checkpoint end opt next-step input is pending Driver-->>Driver: claim pending next-step input Driver-->>SDK: agent/inbox/claimed { message, turn } per message Driver->>Hooks: agent/pre-step waterfall Hooks-->>Driver: authoritative reject or enter(messages) end end Driver->>Session: turn/end end Driver-->>SDK: agent/status idle ``` The `assistant/message` event records every successful provider call, including content-less and `max-tokens` finishes. Empty content stays out of derived history, while the durable event keeps usage and `sourceEventSeqs` listing the exact `assistant/chunk` events, including an explicit empty list. `dsh-compact-basic` uses `agent/pre-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/pre-step` decision is authoritative; listeners wrapping `next()` preserve downstream messages unless replacement is intentional. Steering and injected context pass through the same waterfall after a later boundary claims their next-step batch. 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.