Module plugin 

Plugin extension points.

All cross-cutting concerns plug into the loop through these traits. No inline if special_case_X branches inside the loop; keep hook discipline in explicit extension points.

Two families:

1. Capability traits (this module) — BeforeToolCall, AfterToolCall, ContextTransform, EventObserver, SteeringSource, FollowUpSource. Each is narrow: a hook that needs the assistant message gets the assistant message, never a fat &mut LoopState. New capabilities add a new trait; they do not widen an existing one.

2. Plugin marker — a single registry entry that may implement one or more capability traits. AgentBuilder holds plugins as Arc<dyn Plugin> and dispatches to whichever capabilities the plugin declares via Plugin::capabilities.

Structs

AfterToolCallContext

Read-only context handed to an AfterToolCall hook.

AfterToolDecision

Override returned by an AfterToolCall hook. Each field is opt-in: omitted fields keep the original tool result. No deep merge.

BeforeToolCallContext

Read-only context handed to a BeforeToolCall hook.

BeforeToolDecision

Decision returned by a BeforeToolCall hook.

ChannelSteering

tokio::sync::mpsc-backed steering source. Producer side (SteeringHandle) lets external code call .steer(message) from anywhere; consumer side implements SteeringSource and drains the channel each batch.

PluginCapabilities

Bitset of which extension points a plugin participates in.

SteeringHandle

ToolGateContext

Read-only loop state handed to a ToolGate so its decision is a pure function of observables, not of internal flag bookkeeping. New fields are additive — gates that don’t care can ignore them.

TransformContext

Read-only context handed to a ContextTransform hook.

Enums

ToolGateClass

How a tool gate should compose with explicit recovery owners.

Traits

AfterToolCall

Hook that runs after tool execution, before the result is appended to history. May override the result, flip the error flag, or vote to terminate.

BeforeToolCall

Hook that runs after argument validation, before tool execution.

ContextTransform

Hook that transforms the message slice before it’s converted to the LLM provider format.

EventObserver

Pure observation hook. Logs, telemetry, replay writers. Cannot change loop state — the event sink (crate::event::EventSink) is the formal channel; this trait exists so plugins can subscribe declaratively alongside their other hooks instead of wiring a separate sink.

FollowUpSource

Source of “follow-up messages” — extra messages the loop should process after the agent would otherwise stop.

Plugin

A registered extension. Each plugin declares which capability traits it implements via PluginCapabilities.

SteeringSource

Source of “steering messages” — extra messages the user / harness wants to inject mid-run.

ToolGate

Per-turn allowlist of tool names the model may invoke.