AI agents should be able to call browser-side tools with the same discoverability as server-side tools - but with direct access to browser context and client-defined flexibility.

• Client-defined tools: Tool interfaces are defined on the client, not the server - enabling flexible, context-aware tool providers
• AI-aware schemas: Tools expose schemas for introspection, just like MCP - AI agents can discover available tools and their parameters
• No round-trip tax: Multi-step workflows execute in a single request instead of one round-trip per tool call
• Browser-native: Tools have direct access to browser state, DOM, and web application context

• MCP: Server-side operations, database access, external API calls
• BTCP: Browser-side tools, web app automation, client-state operations

They work together: Use MCP for server tools and BTCP for browser tools in the same agent.

BTCP tools are defined by the client (browser) and exposed to AI agents with discoverable schemas:

interface BTCPToolProvider {
  namespace: string;              // e.g., 'gmail', 'sheets'
  getInterface(): ToolInterface;  // Schema for AI discovery
  getAPI(): Record<string, Function>;
}

interface ToolInterface {
  name: string;
  version: string;
  methods: MethodSchema[];        // AI-readable method definitions
}

AI agents can introspect available tools at runtime:

__interfaces  // Returns all tool schemas
__getToolInterface('gmail')  // Returns specific tool schema

Unlike server-defined tools, BTCP tool providers can be:

• Dynamically registered based on the current page/application
• Customized per user or session
• Extended with application-specific capabilities

BTCP requires a secure sandbox environment that provides:

• Global isolation: No access to window, document, globalThis, or browser globals
• Controlled APIs: Only registered tool providers are accessible
• Frozen interfaces: Tool methods cannot be modified at runtime
• Network isolation: No direct access to fetch, XMLHttpRequest, or WebSocket

The protocol is agnostic to the specific sandbox implementation. Compliant implementations may use:

• SES (Secure ECMAScript) compartments
• Web Workers with restricted APIs
• iframe sandboxes
• WebAssembly-based isolates
• Custom JavaScript interpreters

BTCP is transport-agnostic and supports multiple communication channels:

• SSE (Server-Sent Events) - Primary transport for real-time browser communication
• HTTP Polling - Short and long polling for environments without SSE
• WebSocket - Bidirectional streaming for high-frequency interactions
• Direct - Function calls for testing and embedded scenarios