Pluggable Architecture Guide

CopilotKit is built around extension points. Almost everything is optional and replaceable. This guide catalogs every pluggable part, where it's configured, and what happens when you don't provide it.

Overview: All Extension Points

graph TB
    subgraph "Frontend (React / Angular / Vanilla)"
        FT["Frontend Tools<br/><i>Functions agents can call</i>"]
        CTX["Agent Context<br/><i>Data agents can read</i>"]
        RTC["Tool Call Renderers<br/><i>Custom UI for tool calls</i>"]
        HIL["Human-in-the-Loop<br/><i>Approval before execution</i>"]
        RAM["Activity Renderers<br/><i>Custom activity messages</i>"]
        RCM["Custom Message Renderers<br/><i>Inject UI before/after messages</i>"]
        SUG["Suggestions Config<br/><i>AI or static suggestions</i>"]
        SUBS["Event Subscribers<br/><i>React to lifecycle events</i>"]
    end

    subgraph "Backend (Runtime)"
        BM["Before Middleware<br/><i>Auth, logging, transforms</i>"]
        AM["After Middleware<br/><i>Post-processing</i>"]
        RUNNER["Agent Runner<br/><i>How agents execute</i>"]
        TS["Transcription Service<br/><i>Audio → text</i>"]
    end

    subgraph "Agent Level"
        MW["AG-UI Middleware<br/><i>Intercept agent pipeline</i>"]
    end

Frontend Extension Points

1. Frontend Tools

What: Functions in your app that agents can call during a conversation.

Where configured:

React: useFrontendTool() hook or frontendTools provider prop
Angular: copilotKit.addTool() or tools in config
Vanilla: copilotKit.addTool()

Default when not provided: No tools — agent can only send text messages.

// Type signature
type FrontendTool<T> = {
  name: string;
  description?: string;
  parameters?: z.ZodType<T>;
  handler?: (args: T, context: FrontendToolHandlerContext) => Promise<unknown>;
  followUp?: boolean; // Re-run agent after tool completes
  agentId?: string; // Scope to specific agent
};

sequenceDiagram
    participant Agent
    participant Core as CopilotKitCore
    participant Tool as Your Tool Handler

    Agent->>Core: TOOL_CALL_START { name: "myTool" }
    Agent->>Core: TOOL_CALL_ARGS { ... }
    Core->>Tool: handler(args)
    Tool-->>Core: result
    Core->>Agent: TOOL_CALL_RESULT
    opt followUp = true
        Core->>Agent: Re-run agent with result
    end

2. Agent Context

What: JSON data that gets sent to agents as context (like "the user is on the settings page").

Where configured:

React: useAgentContext() hook
Angular / Vanilla: copilotKit.addContext() / removeContext()

Default when not provided: No extra context — agent only sees messages and tool definitions.

type AgentContextInput = {
  description: string; // Human-readable label
  value: JsonSerializable; // Any JSON value
};

3. Tool Call Renderers

What: Custom React components that render while a tool is being called — showing progress, args, and results.

Where configured:

React: useRenderToolCall() hook or renderToolCalls provider prop
Angular: renderToolCalls in config

Default when not provided: Generic built-in rendering.

type ReactToolCallRenderer<T> = {
  name: string; // Tool name to render
  args: z.ZodSchema<T>; // Schema for type-safe args
  agentId?: string; // Scope to specific agent
  render: React.ComponentType<
    | { status: "in-progress"; args: Partial<T>; result: undefined }
    | { status: "executing"; args: T; result: undefined }
    | { status: "complete"; args: T; result: string }
  >;
};

graph LR
    IP["in-progress<br/><i>Args streaming in<br/>Partial&lt;T&gt; available</i>"]
    EX["executing<br/><i>Handler running<br/>Full args available</i>"]
    CO["complete<br/><i>Result available</i>"]
    IP --> EX --> CO

4. Human-in-the-Loop

What: Tools that pause and wait for user input before continuing. The user sees a custom UI with approve/deny buttons.

Where configured:

React: useHumanInTheLoop() hook or humanInTheLoop provider prop
Angular: humanInTheLoop in config

Default when not provided: No approval required — tools execute immediately.

type ReactHumanInTheLoop<T> = Omit<FrontendTool<T>, "handler"> & {
  render: React.ComponentType<{
    args: T;
    status: "in-progress" | "executing" | "complete";
    respond: (result: unknown) => Promise<void>; // Call this to approve/deny
  }>;
};

sequenceDiagram
    participant Agent
    participant Core as CopilotKitCore
    participant UI as Your Approval UI
    participant User

    Agent->>Core: TOOL_CALL { name: "deleteUser" }
    Core->>UI: Render with status: "executing"
    UI->>User: "Delete user X?"
    User->>UI: Clicks "Approve"
    UI->>Core: respond("approved")
    Core->>Agent: TOOL_CALL_RESULT
    Agent->>Agent: Continues

5. Activity Message Renderers

What: Custom UI for structured activity messages (non-chat messages like progress indicators or MCP app outputs).

Where configured:

React: useRenderActivityMessage() hook or renderActivityMessages provider prop

Default when not provided: Built-in MCP Apps renderer is included. Other activity types show generic display.

type ReactActivityMessageRenderer<T> = {
  activityType: string; // Use "*" for wildcard
  agentId?: string;
  content: z.ZodSchema<T>;
  render: React.ComponentType<{
    activityType: string;
    content: T;
    message: ActivityMessage;
    agent: AbstractAgent | undefined;
  }>;
};

6. Custom Message Renderers

What: Inject custom UI before or after specific messages (e.g., add a "copy" button, show state snapshots).

Where configured:

React: useRenderCustomMessages() hook or renderCustomMessages provider prop

Default when not provided: No custom rendering — standard message display.

type ReactCustomMessageRenderer = {
  agentId?: string;
  render: React.ComponentType<{
    message: Message;
    position: "before" | "after";
    runId: string;
    messageIndex: number;
    agentId: string;
    stateSnapshot: any;
  }> | null;
};

7. Suggestions Configuration

What: Configure AI-generated or static prompt suggestions shown to users.

Where configured:

React: useConfigureSuggestions() hook
Core: suggestionsConfig in config

Default when not provided: No suggestions.

// AI-generated suggestions
type DynamicSuggestionsConfig = {
  instructions: string; // What to suggest
  minSuggestions?: number; // Default: 1
  maxSuggestions?: number; // Default: 3
  available?: SuggestionAvailability; // When to show
  providerAgentId?: string; // Which agent generates them
  consumerAgentId?: string; // Which agent receives them ("*" = all)
};

// Static suggestions
type StaticSuggestionsConfig = {
  suggestions: Array<{ title: string; message: string }>;
  available?: SuggestionAvailability;
  consumerAgentId?: string;
};

type SuggestionAvailability =
  | "before-first-message" // Default for static
  | "after-first-message" // Default for dynamic
  | "always"
  | "disabled";

graph TB
    subgraph "Suggestion Types"
        DYN["Dynamic<br/><i>AI generates suggestions<br/>from instructions</i>"]
        STA["Static<br/><i>You provide fixed<br/>suggestion list</i>"]
    end

    subgraph "Availability"
        BFM["before-first-message"]
        AFM["after-first-message"]
        ALW["always"]
        DIS["disabled"]
    end

    DYN -.->|default| AFM
    STA -.->|default| BFM

8. Event Subscribers

What: Listen to lifecycle events — connection status, tool execution, agent changes, errors.

Where configured:

Any: copilotKit.subscribe(subscriber)
Returns: { unsubscribe() } for cleanup

Default when not provided: No listeners — events still fire internally.

type CopilotKitCoreSubscriber = {
  onRuntimeConnectionStatusChanged?: (event) => void;
  onToolExecutionStart?: (event) => void;
  onToolExecutionEnd?: (event) => void;
  onAgentsChanged?: (event) => void;
  onContextChanged?: (event) => void;
  onSuggestionsChanged?: (event) => void;
  onSuggestionsStartedLoading?: (event) => void;
  onSuggestionsFinishedLoading?: (event) => void;
  onPropertiesChanged?: (event) => void;
  onHeadersChanged?: (event) => void;
  onError?: (event) => void;
};

Backend Extension Points

9. Before Request Middleware

What: Intercept HTTP requests before they reach the handler. Use for auth, logging, request transformation.

Where configured: CopilotRuntime constructor — beforeRequestMiddleware

Default when not provided: Requests pass through unchanged.

type BeforeRequestMiddleware = (params: {
  runtime: CopilotRuntime;
  request: Request;
  path: string;
}) => MaybePromise<Request | void>;
// Return modified Request, or void to pass through
// Return a Response to short-circuit (e.g., 401)

graph LR
    REQ["Incoming Request"]
    BM["beforeRequestMiddleware"]
    HANDLER["Route Handler"]
    REJECT["401 / Error Response"]

    REQ --> BM
    BM -->|pass through| HANDLER
    BM -->|reject| REJECT

10. After Request Middleware

What: Run code after the response is prepared. Use for logging, metrics, cleanup.

Where configured: CopilotRuntime constructor — afterRequestMiddleware

Default when not provided: No post-processing.

type AfterRequestMiddleware = (params: {
  runtime: CopilotRuntime;
  response: Response;
  path: string;
}) => MaybePromise<void>;

11. Agent Runner

What: Controls how agents are executed and how thread state is managed.

Where configured: CopilotRuntime constructor — runner

Default when not provided: InMemoryAgentRunner — in-process, ephemeral (threads lost on restart).

abstract class AgentRunner {
  abstract run(request: AgentRunnerRunRequest): Observable<BaseEvent>;
  abstract connect(request: AgentRunnerConnectRequest): Observable<BaseEvent>;
  abstract isRunning(request: AgentRunnerIsRunningRequest): Promise<boolean>;
  abstract stop(request: AgentRunnerStopRequest): Promise<boolean | undefined>;
}

Implementation	Storage	Persistence	Use case
`InMemoryAgentRunner`	RAM	No	Development, stateless apps
`SQLiteAgentRunner`	Disk	Yes	Production, long-running threads
Custom	Your choice	Your choice	Redis, PostgreSQL, etc.

graph TB
    RT["CopilotRuntime"]
    RUNNER["runner (AgentRunner)"]

    RT --> RUNNER

    subgraph Implementations
        IM["InMemoryAgentRunner<br/><i>Default — in-process</i>"]
        SQ["SQLiteAgentRunner<br/><i>Persistent on disk</i>"]
        CU["YourCustomRunner<br/><i>Redis, Postgres, etc.</i>"]
    end

    RUNNER -.-> IM
    RUNNER -.-> SQ
    RUNNER -.-> CU

12. Transcription Service

What: Convert audio files to text. Enables the /transcribe endpoint.

Where configured: CopilotRuntime constructor — transcriptionService

Default when not provided: /transcribe endpoint returns 404.

abstract class TranscriptionService {
  abstract transcribeFile(options: {
    audioFile: File;
    mimeType?: string;
    size?: number;
  }): Promise<string>;
}

Agent-Level Extension Points

13. AG-UI Middleware

What: Intercept and transform the agent execution pipeline. Cross-cutting concerns like logging, filtering, and backward compatibility.

Where configured: At the agent level (outside CopilotKit core).

Default when not provided: Direct agent execution.

abstract class Middleware {
  abstract run(
    input: RunAgentInput,
    next: AbstractAgent,
  ): Observable<BaseEvent>;
}

// Built-in implementations:
// - FunctionMiddleware — wrap a function as middleware
// - FilterToolCallsMiddleware — filter which tools are sent

graph LR
    INPUT["RunAgentInput"]
    MW1["Middleware 1<br/><i>e.g., logging</i>"]
    MW2["Middleware 2<br/><i>e.g., tool filtering</i>"]
    AGENT["Agent.run()"]

    INPUT --> MW1 --> MW2 --> AGENT

Complete Map: Where Each Extension Plugs In

graph TB
    subgraph "Provider / Config"
        P["CopilotKitProvider<br/>or provideCopilotKit()"]
        P --> FT_P["frontendTools"]
        P --> RTC_P["renderToolCalls"]
        P --> RAM_P["renderActivityMessages"]
        P --> RCM_P["renderCustomMessages"]
        P --> HIL_P["humanInTheLoop"]
        P --> HDR["headers"]
        P --> CRD["credentials"]
        P --> PRP["properties"]
        P --> DC["showDevConsole"]
    end

    subgraph "Hooks / Service Methods"
        UFT["useFrontendTool()"]
        UAC["useAgentContext()"]
        URT["useRenderToolCall()"]
        UHL["useHumanInTheLoop()"]
        UCS["useConfigureSuggestions()"]
        URA["useRenderActivityMessage()"]
        URC["useRenderCustomMessages()"]
    end

    subgraph "CopilotRuntime"
        RT["new CopilotRuntime()"]
        RT --> AGENTS["agents (required)"]
        RT --> RUNNER["runner"]
        RT --> BM["beforeRequestMiddleware"]
        RT --> AM["afterRequestMiddleware"]
        RT --> TS["transcriptionService"]
    end

    subgraph "Core API"
        SUB["copilotKit.subscribe()"]
        AT["copilotKit.addTool()"]
        AC["copilotKit.addContext()"]
    end

Summary Table

Extension Point	Location	Config Method	Default	Optional
Frontend Tools	Frontend	Hook / Provider / `addTool()`	None	Yes
Agent Context	Frontend	Hook / `addContext()`	None	Yes
Tool Call Renderers	Frontend	Hook / Provider	Generic rendering	Yes
Human-in-the-Loop	Frontend	Hook / Provider	Immediate execution	Yes
Activity Renderers	Frontend	Hook / Provider	MCP Apps included	Yes
Custom Message Renderers	Frontend	Hook / Provider	None	Yes
Suggestions Config	Frontend	Hook / Config	None	Yes
Event Subscribers	Frontend	`subscribe()`	None	Yes
Before Middleware	Backend	Runtime constructor	Pass-through	Yes
After Middleware	Backend	Runtime constructor	None	Yes
Agent Runner	Backend	Runtime constructor	InMemoryAgentRunner	Yes
Transcription Service	Backend	Runtime constructor	None (404)	Yes
AG-UI Middleware	Agent	Agent-level config	Direct execution	Yes

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Pluggable Architecture Guide

Overview: All Extension Points

Frontend Extension Points

1. Frontend Tools

2. Agent Context

3. Tool Call Renderers

4. Human-in-the-Loop

5. Activity Message Renderers

6. Custom Message Renderers

7. Suggestions Configuration

8. Event Subscribers

Backend Extension Points

9. Before Request Middleware

10. After Request Middleware

11. Agent Runner

12. Transcription Service

Agent-Level Extension Points

13. AG-UI Middleware

Complete Map: Where Each Extension Plugs In

Summary Table

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Pluggable Architecture Guide

Overview: All Extension Points

Frontend Extension Points

1. Frontend Tools

2. Agent Context

3. Tool Call Renderers

4. Human-in-the-Loop

5. Activity Message Renderers

6. Custom Message Renderers

7. Suggestions Configuration

8. Event Subscribers

Backend Extension Points

9. Before Request Middleware

10. After Request Middleware

11. Agent Runner

12. Transcription Service

Agent-Level Extension Points

13. AG-UI Middleware

Complete Map: Where Each Extension Plugs In

Summary Table