name: expo-devtools-cli description: Building Expo DevTools Plugins with CLI Interfaces for interacting with running Expo apps using agents.

Building Expo DevTools Plugins with CLI Interfaces

Build CLI tools that communicate with running Expo apps via the DevTools plugin system.

Architecture Overview

┌─────────────────┐     WebSocket      ┌─────────────────┐
│   CLI Client    │◄──────────────────►│  Expo Dev Server │
│  (Bun + Stricli)│                    │   (Metro)        │
└─────────────────┘                    └────────┬────────┘
                                                │
                                       ┌────────▼────────┐
                                       │   React Native  │
                                       │   App + Hook    │
                                       └─────────────────┘

Preferred Tech Stack

Component	Technology	Why
Runtime	Bun	Fast startup, native TypeScript, built-in WebSocket
CLI Framework	@stricli/core	Type-safe, lazy loading, tree-shakeable
App Hook	expo/devtools	`useDevToolsPluginClient` for app-side connection
Protocol	JSON over WebSocket	Simple, debuggable with standard tools

Project Structure

cli/
├── index.ts              # Entry point with shebang
├── app.ts                # Stricli app definition with routes
├── client.ts             # WebSocket client for devtools
├── types.ts              # Shared TypeScript types
├── formatters.ts         # Output formatting (table, JSON)
└── commands/
    ├── query.ts          # Read commands
    ├── write.ts          # Write commands
    └── status.ts         # Status/health commands

src/devtools/
└── useMyPluginDevTools.ts  # App-side message handler hook

Step 1: Configure the Module

Add devtools config to expo-module.config.json:

{
  "name": "MyModule",
  "platforms": ["ios", "android"],
  "devtools": {
    "name": "My Plugin",
    "id": "my-plugin"
  }
}

Step 2: Create the App-Side Hook

// src/devtools/useMyPluginDevTools.ts
import { useEffect } from "react";
import { useDevToolsPluginClient } from "expo/devtools";

interface PluginMessage {
  id: string;
  type: string;
  payload: Record<string, unknown>;
}

export function useMyPluginDevTools() {
  const client = useDevToolsPluginClient("my-plugin"); // Must match devtools.id

  useEffect(() => {
    if (!client) return;

    const handleMessage = (data: PluginMessage) => {
      const { id, type, payload } = data;

      const sendResult = (result: unknown) => {
        client.sendMessage("result", { id, type: "result", data: result });
      };

      const sendError = (error: Error) => {
        client.sendMessage("error", {
          id,
          type: "error",
          error: error.message,
        });
      };

      (async () => {
        try {
          switch (type) {
            case "getData":
              const data = await fetchData(payload.query as string);
              sendResult(data);
              break;
            default:
              sendError(new Error(`Unknown message type: ${type}`));
          }
        } catch (error) {
          sendError(error as Error);
        }
      })();
    };

    const subscription = client.addMessageListener(
      "message",
      (msg: unknown) => {
        handleMessage(msg as PluginMessage);
      }
    );

    return () => {
      subscription?.remove?.();
    };
  }, [client]);
}

Step 3: Create the CLI Client

// cli/client.ts
const DEFAULT_PORT = 8081;
const REQUEST_TIMEOUT = 30000;
const PROTOCOL_VERSION = 1;

export class PluginClient {
  private ws: WebSocket | null = null;
  private pending = new Map<string, { resolve: Function; reject: Function }>();
  private connected = false;
  private browserClientId = Date.now().toString();
  private pluginName = "my-plugin"; // Must match devtools.id

  async connect(port = DEFAULT_PORT): Promise<void> {
    if (this.connected) return;

    return new Promise((resolve, reject) => {
      // IMPORTANT: Use the broadcast endpoint
      const url = `ws://localhost:${port}/expo-dev-plugins/broadcast`;
      this.ws = new WebSocket(url);

      const timeout = setTimeout(() => {
        reject(new Error(`Connection timeout to ${url}`));
      }, 10000);

      this.ws.addEventListener("open", () => {
        clearTimeout(timeout);
        this.connected = true;
        this.sendHandshake();
        resolve();
      });

      this.ws.addEventListener("error", () => {
        clearTimeout(timeout);
        reject(new Error(`Failed to connect to Expo devtools at ${url}`));
      });

      this.ws.addEventListener("close", () => {
        this.connected = false;
      });

      this.ws.addEventListener("message", (event) => {
        this.handleMessage(event.data);
      });
    });
  }

  private sendHandshake(): void {
    // CRITICAL: Must include all these fields
    const handshake = {
      protocolVersion: PROTOCOL_VERSION, // Must be 1
      pluginName: this.pluginName,
      method: "handshake",
      browserClientId: this.browserClientId,
      __isHandshakeMessages: true, // Required flag
    };
    this.ws?.send(JSON.stringify(handshake));
  }

  private handleMessage(data: string | ArrayBuffer): void {
    if (typeof data === "string") {
      try {
        const parsed = JSON.parse(data);
        if (parsed.__isHandshakeMessages) return; // Ignore handshake acks
        if (parsed.messageKey) {
          this.handlePackedMessage(parsed);
        }
      } catch {
        // Not JSON, ignore
      }
    }
  }

  private handlePackedMessage(msg: { messageKey: any; payload: any }): void {
    const { messageKey, payload } = msg;
    if (messageKey.pluginName !== this.pluginName) return;

    if (messageKey.method === "result" || messageKey.method === "error") {
      const response = payload as {
        id: string;
        data?: unknown;
        error?: string;
      };
      const pending = this.pending.get(response.id);
      if (!pending) return;

      this.pending.delete(response.id);
      if (messageKey.method === "error" || response.error) {
        pending.reject(new Error(response.error ?? "Unknown error"));
      } else {
        pending.resolve(response.data);
      }
    }
  }

  async send<T>(type: string, payload: unknown): Promise<T> {
    if (!this.ws || !this.connected) {
      throw new Error("Not connected to Expo devtools");
    }

    const id = crypto.randomUUID();
    return new Promise((resolve, reject) => {
      this.pending.set(id, { resolve, reject });

      // CRITICAL: Send as JSON string, NOT binary ArrayBuffer
      const msg = {
        messageKey: { pluginName: this.pluginName, method: "message" },
        payload: { id, type, payload },
      };
      this.ws!.send(JSON.stringify(msg));

      setTimeout(() => {
        if (this.pending.has(id)) {
          this.pending.delete(id);
          reject(new Error("Request timeout"));
        }
      }, REQUEST_TIMEOUT);
    });
  }

  async disconnect(): Promise<void> {
    this.ws?.close();
    this.ws = null;
    this.connected = false;
  }
}

Step 4: Create the CLI Entry Point

// cli/index.ts
#!/usr/bin/env bun
import { run } from "@stricli/core";
import { app } from "./app";

await run(app, process.argv.slice(2), { process });

// cli/app.ts
import { buildApplication, buildRouteMap } from "@stricli/core";

const routes = buildRouteMap({
  routes: {
    status: () => import("./commands/status").then((m) => m.default),
    query: () => import("./commands/query").then((m) => m.default),
  },
});

export const app = buildApplication(routes, {
  name: "my-cli",
  versionInfo: { currentVersion: "1.0.0" },
});

Step 5: Configure package.json

{
  "bin": {
    "my-cli": "cli/index.ts"
  },
  "scripts": {
    "cli": "bun cli/index.ts"
  },
  "dependencies": {
    "@stricli/core": "^1.1.0"
  }
}

Footguns and Solutions

1. Binary vs JSON Messages

Problem: Messages sent as ArrayBuffer are silently ignored.

// WRONG - Will not work
const encoder = new TextEncoder();
this.ws.send(encoder.encode(JSON.stringify(msg)).buffer);

// CORRECT - Send as JSON string
this.ws.send(JSON.stringify(msg));

Debugging: Use websocat to test the WebSocket:

websocat -v ws://localhost:8081/expo-dev-plugins/broadcast

2. Wrong WebSocket Endpoint

Problem: Using /message or other endpoints won't work.

// WRONG
const url = `ws://localhost:${port}/message`;

// CORRECT - Must use broadcast endpoint
const url = `ws://localhost:${port}/expo-dev-plugins/broadcast`;

Debugging: Use curl to verify WebSocket upgrade:

curl -v -H "Connection: Upgrade" -H "Upgrade: websocket" \
  -H "Sec-WebSocket-Key: test" -H "Sec-WebSocket-Version: 13" \
  http://localhost:8081/expo-dev-plugins/broadcast

3. Missing Handshake Fields

Problem: Connection appears to work but messages aren't routed.

// WRONG - Missing required fields
const handshake = { pluginName: "my-plugin" };

// CORRECT - All fields required
const handshake = {
  protocolVersion: 1, // Must be 1
  pluginName: "my-plugin",
  method: "handshake",
  browserClientId: "unique-id",
  __isHandshakeMessages: true, // Critical flag
};

4. Protocol Version Mismatch

Problem: terminateBrowserClient messages with warning about incompatible clients.

// WRONG
protocolVersion: 2;

// CORRECT - Use version 1
protocolVersion: 1;

5. Plugin Name Mismatch

Problem: Messages sent but never received by app.

The pluginName must match exactly across:

expo-module.config.json → devtools.id
App hook → useDevToolsPluginClient("my-plugin")
CLI client → this.pluginName = "my-plugin"

6. Hook Not Setting Up Listener

Problem: Hook logs "connected" but messages timeout.

Check that useDevToolsPluginClient is imported from the correct package:

// CORRECT
import { useDevToolsPluginClient } from "expo/devtools";

// WRONG - different package
import { useDevToolsPluginClient } from "@expo/devtools-plugin-client";

7. Message Listener Method Name

Problem: App receives connection but not messages.

The addMessageListener method name must match the messageKey.method from CLI:

// CLI sends with method: "message"
const msg = {
  messageKey: { pluginName: "my-plugin", method: "message" },
  payload: { id, type, payload },
};

// App listens for "message"
client.addMessageListener("message", handler);

Debugging Techniques

1. Monitor WebSocket Traffic

# Listen to all broadcasts
websocat --no-close -v ws://localhost:8081/expo-dev-plugins/broadcast

# Send test handshake
echo '{"protocolVersion":1,"pluginName":"my-plugin","method":"handshake","browserClientId":"test","__isHandshakeMessages":true}' | \
  websocat ws://localhost:8081/expo-dev-plugins/broadcast

2. Check App Console Logs

bunx xcobra expo console --json | grep -i "my-plugin\|devtools"

3. Verify Hook is Running

Add temporary logging to the hook:

useEffect(() => {
  console.log("[DevTools] client:", client ? "connected" : "null");
  if (!client) return;
  console.log("[DevTools] Setting up listener");
  // ...
}, [client]);

4. Test Connection Independently

// Minimal test script
const ws = new WebSocket("ws://localhost:8081/expo-dev-plugins/broadcast");
ws.onopen = () => {
  console.log("Connected");
  ws.send(
    JSON.stringify({
      protocolVersion: 1,
      pluginName: "my-plugin",
      method: "handshake",
      browserClientId: "test",
      __isHandshakeMessages: true,
    })
  );
};
ws.onmessage = (e) => console.log("Received:", e.data);

Testing Workflow

Start the app: yarn expo run:ios or have simulator running with Expo Go
Verify Metro is running: Check http://localhost:8081 responds
Test CLI connection: bun cli/index.ts status
Check for errors: Monitor both CLI output and app console

Reference Implementation

See the HealthKit CLI in this repo:

cli/ - Full CLI implementation
src/dev-tools/useHealthKitDevTools.ts - App-side hook
example/App.tsx - Hook usage in app

expo-devtools-cli

$ Installer