Layer System

The layer system is the core architectural pattern of Reactive Agents. Every capability is an independent Effect Layer that can be enabled or disabled.

What is a Layer?

In Effect-TS, a Layer is a recipe for constructing services. Think of it as a factory:

// Layer<AgentService, never, EventBus>
// "I provide AgentService, never fail, and need EventBus"

Layers compose through two operations:

• Layer.merge(a, b) — Provides services from both layers
• Layer.provide(dep) — Satisfies a layer’s requirements

The Runtime Composition

When you call createRuntime(), it composes layers based on your configuration:

const runtime = createRuntime({
  agentId: "my-agent",
  provider: "anthropic",
  enableGuardrails: true,
  enableReasoning: true,
});

Internally, this produces:

CoreServicesLive          → provides EventBus, AgentService, TaskService
  + EventBusLive          → provides EventBus (for optional layers)
  + LLMProviderLayer      → provides LLMService
  + MemoryLayer           → provides MemoryService
  + HookRegistryLive      → provides LifecycleHookRegistry
  + ExecutionEngineLive   → provides ExecutionEngine
  + GuardrailsLayer       → provides GuardrailService
  + ReasoningLayer        → provides ReasoningService, StrategyRegistry

Layer Dependencies

Each layer declares what it provides and what it requires:

Layer	Provides	Requires
Core	EventBus, AgentService, TaskService	Nothing
LLM Provider	LLMService	Nothing
Memory	MemoryService, MemoryDatabase	Nothing
Reasoning	ReasoningService, StrategyRegistry	LLMService
Tools	ToolService	EventBus
Interaction	InteractionManager, ModeSwitcher, …	EventBus
Guardrails	GuardrailService	Nothing
Verification	VerificationService	Nothing
Cost	CostService	Nothing
Identity	IdentityService	Nothing
Observability	ObservabilityService	Nothing
Prompts	PromptService	Nothing
Gateway	GatewayService, SchedulerService, WebhookService, PolicyEngine	EventBus
A2A	A2A server/client helpers	Core (+ tools when serving)
Reactive Intelligence	EntropySensor, ReactiveController, learning hooks	EventBus, reasoning kernel integration
Eval	EvalService, EvalStore	LLMService (for judges)

The runtime automatically satisfies dependencies when composing layers.

Custom Layers

Add your own layers using .withLayers():

import { Layer, Context, Effect } from "effect";

class MyAnalytics extends Context.Tag("MyAnalytics")<
  MyAnalytics,
  { readonly track: (event: string) => Effect.Effect<void> }
>() {}

const MyAnalyticsLive = Layer.succeed(MyAnalytics, {
  track: (event) => Effect.sync(() => console.log(`[analytics] ${event}`)),
});

const agent = await ReactiveAgents.create()
  .withLayers(MyAnalyticsLive)
  .build();

Testing with Layers

Replace any layer with a test implementation:

import { TestLLMServiceLayer } from "@reactive-agents/llm-provider";

// The test provider is a Layer that returns canned responses. Each turn can
// gate on a `match` substring so a specific prompt yields a specific reply.
const testLLM = TestLLMServiceLayer([
  { text: "Paris", match: "capital of France" },
]);

This is the power of the layer system — any service can be swapped at the composition boundary without changing application code.