Tool and Plugin Boundary

Awaken separates user-facing capabilities (tools) from system-level lifecycle logic (plugins). This page explains the design boundary, when to use each, and how they interact.

Tools

A tool is a capability exposed to the LLM. The LLM decides when to call it based on the tool’s descriptor.

pub trait Tool: Send + Sync {
    fn descriptor(&self) -> ToolDescriptor;
    fn validate_args(&self, args: &Value) -> Result<(), ToolError> { Ok(()) }
    async fn execute(&self, args: Value, ctx: &ToolCallContext) -> Result<ToolOutput, ToolError>;
}

Key properties:

Registered by ID – each tool has a unique string identifier from ToolDescriptor.
LLM-visible – the descriptor (name, description, JSON Schema for parameters) is included in inference requests.
Executed in tool rounds – tools run during the BeforeToolExecute / AfterToolExecute phase window.
Isolated – a tool receives only its arguments and a ToolCallContext. It cannot directly access other tools, the plugin system, or the phase runtime.
User-defined – application code creates tools for domain-specific actions (file operations, API calls, database queries).

Plugins

A plugin is a system-level extension that hooks into the execution lifecycle. Plugins do not appear in the LLM’s tool list.

pub trait Plugin: Send + Sync + 'static {
    fn descriptor(&self) -> PluginDescriptor;
    fn register(&self, registrar: &mut PluginRegistrar) -> Result<(), StateError>;
    fn config_schemas(&self) -> Vec<ConfigSchema> { vec![] }
}

Key properties:

Registered by ID – each plugin has a unique identifier from PluginDescriptor.
LLM-invisible – the LLM does not know plugins exist.
Run in phases – plugin hooks fire at phase boundaries (RunStart, BeforeInference, AfterToolExecute, etc.).
System-wide access – hooks receive PhaseContext with access to the state snapshot, event sink, and run metadata.
Declarative registration – plugins declare their capabilities through PluginRegistrar.

PluginRegistrar

When Plugin::register is called, the plugin uses PluginRegistrar to declare everything it needs:

Registration Method	Purpose
`register_key::<K>()`	Declare a `StateKey` for the plugin’s state
`register_phase_hook()`	Add a hook that runs at a specific phase
`register_tool()`	Inject a tool into the runtime (plugin-provided tools)
`register_effect_handler()`	Handle named effects emitted by hooks
`register_scheduled_action()`	Handle named actions scheduled by hooks
`register_request_transform()`	Transform inference requests before they reach the LLM

Plugins can register tools. This is how delegation tools (AgentTool) and MCP tools enter the runtime – they are tools owned by plugins, not directly registered by user code. The LLM sees these tools identically to user-registered tools.

When to Use a Tool

Use a tool when:

The LLM should be able to invoke the capability by name.
The capability performs a domain-specific action (search, compute, file I/O, API call).
The capability takes structured input and returns a result the LLM can reason about.
The logic does not need access to runtime internals (state, phases, other tools).

When to Use a Plugin

Use a plugin when:

The logic should run automatically at phase boundaries without LLM involvement.
The logic needs to modify inference requests (system prompt injection, tool filtering).
The logic needs to inspect or transform tool results before the LLM sees them.
The logic manages cross-cutting concerns (permissions, observability, reminders, state synchronization).
The logic needs to register state keys, effect handlers, or request transforms.

Interaction Between Tools and Plugins

Plugins can influence tool execution without the tool knowing:

Permission plugin – intercepts tool calls at BeforeToolExecute, blocks or suspends them based on policy rules. The tool itself has no permission logic.
Observability plugin – wraps tool execution with OpenTelemetry spans. The tool does not emit traces.
Reminder plugin – injects context messages after specific tools execute. The tool does not know about reminders.
Interception pipeline – plugins can modify tool arguments, replace tool results, or skip execution entirely through the tool interception mechanism.

This separation means tools remain simple and focused on their domain logic. Cross-cutting concerns are handled uniformly by plugins, applied consistently across all tools without per-tool opt-in.

Plugin-Provided Tools vs User Tools

Aspect	User Tool	Plugin Tool
Registration	`AgentRuntimeBuilder::with_tool()`	`PluginRegistrar::register_tool()`
Lifecycle	Exists for the runtime’s lifetime	Exists while the plugin is active
Configuration	Direct construction	Derived from plugin config or agent spec
Examples	Custom business logic tools	`AgentTool` (delegation), MCP tools, skill tools

Both appear identically to the LLM. The distinction is purely about ownership and lifecycle management.

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