Engine

Engine is the execution kernel that owns the agent loop: it calls your AgentModule hooks in sequence, executes actions, manages tracing, and enforces stop criteria.

Engine is the single execution kernel for all QitOS agent workflows. It runs the step loop, coordinates your AgentModule hooks, dispatches tool calls to the ToolRegistry for execution, applies critics (post-step evaluators that can approve, stop, or retry the step), checks stop conditions, and writes trace artifacts (persistent records like steps.jsonl and events.jsonl that capture what happened during the run). You interact with it directly only when you need control beyond what agent.run() provides.

QitOS enforces a single-kernel rule: there is exactly one Engine per run. Extensions like parsers, critics, memory adapters, and toolkits attach to this pipeline — they do not introduce a second execution loop.

How the loop works

Each step follows a fixed sequence:

prepare → decide → act → reduce → critics → check_stop → trace

1. prepare — agent.prepare(state) formats state into the model-ready prompt text.
2. decide — agent.decide(state, observation) is checked first; if it returns None, the Engine calls the LLM via the configured parser.
3. act — Tool calls in the decision’s actions list are executed against the ToolRegistry.
4. reduce — agent.reduce(state, observation, decision) updates state with the new observation.
5. critics — Any registered Critic instances (post-step evaluators that can approve, stop, or retry the step) evaluate the step; they can trigger a stop or retry.
6. check_stop — Budget exhaustion, FinalResultCriteria, agent.should_stop(), and any custom StopCriteria are evaluated.
7. trace — The step record and events are written to the TraceWriter.

Constructor

from qitos import Engine

engine = Engine(
    agent=agent,
    budget=RuntimeBudget(max_steps=20),
    parser=ReActTextParser(),
    critics=[my_critic],
    stop_criteria=[FinalResultCriteria()],
    env=host_env,
    trace_writer=trace_writer,
    hooks=[my_hook],
)

Parameter	Type	Description
agent	AgentModule	Required. The policy module to execute.
budget	RuntimeBudget | None	Step/time/token limits. Defaults to RuntimeBudget(max_steps=10).
parser	Parser | None	Parses raw model output into a Decision. Must match your prompt format.
critics	list[Critic] | None	Post-step evaluators that can stop or retry a step.
stop_criteria	list[StopCriteria] | None	Stop conditions. Defaults to [FinalResultCriteria()].
env	Env | None	Environment providing reset/observe/step/is_terminal/close.
trace_writer	TraceWriter | None	Writes manifest.json, events.jsonl, steps.jsonl for this run.
hooks	list[EngineHook] | None	Hook instances called on lifecycle events.
render_hooks	list | None	Render hooks (e.g., terminal UI). Appended to hooks internally.
history_policy	HistoryPolicy | None	Controls how the in-run conversation history is managed.
recovery_policy	RecoveryPolicy | None	Controls how the Engine responds to step failures.

Prefer calling agent.run() for single-run workflows. Use Engine directly when you need to reuse an Engine across multiple runs or configure hooks dynamically between runs.

Engine.run(task)

result = engine.run(task)

Accepts a plain string objective or a structured Task object. Returns an EngineResult.

When you pass a Task, the Engine extracts the budget from task.budget and overrides the Engine’s own budget for that run. It also orchestrates resource staging and environment lifecycle (reset, observe, close) automatically.

EngineResult

@dataclass
class EngineResult(Generic[StateT]):
    state: StateT
    records: List[StepRecord]
    events: List[RuntimeEvent]
    step_count: int
    task_result: Optional[TaskResult]

Field	Description
state	Final typed state after the run. Check state.final_result and state.stop_reason.
records	One StepRecord per step, containing decision, observation, and diffs.
events	All RuntimeEvent objects emitted during the run.
step_count	Number of steps executed (len(records)).
task_result	Structured outcome including success flag and criterion results.

Typical usage:

result = engine.run("summarize the paper")

print(result.state.final_result)   # the agent's final answer
print(result.state.stop_reason)    # why the loop ended
print(result.step_count)           # how many steps ran
print(result.task_result.success)  # whether success criteria passed

Hooks

Hooks observe and react to lifecycle events without modifying Engine internals. They implement EngineHook and are called at on_before_step and on_after_step boundaries.

engine.register_hook(my_hook)
engine.unregister_hook(my_hook)
engine.clear_hooks()

You can also pass hooks at construction time via the hooks parameter, or at run time via agent.run(hooks=[...]).

Budget exhaustion

When the step budget, wall-clock time limit, or token budget is exceeded, the Engine sets state.stop_reason to the appropriate value and emits an END event. The run terminates gracefully and EngineResult is still returned — inspect state.stop_reason to detect this case.

from qitos.engine.states import RuntimeBudget

engine = Engine(
    agent=agent,
    budget=RuntimeBudget(
        max_steps=30,
        max_runtime_seconds=120.0,
        max_tokens=50_000,
    ),
)

Building an Engine from AgentModule

AgentModule.build_engine() is a convenience factory that creates an Engine pre-bound to the agent:

engine = agent.build_engine(
    budget=RuntimeBudget(max_steps=15),
    trace_writer=trace_writer,
)
result = engine.run(task)

This is equivalent to Engine(agent=agent, ...) and is what agent.run() calls internally.