Expose OpenCode capabilities through Agent2Agent (A2A) Protocol

[Copilot]

What does this PR do?

Implements A2A Protocol v0.3.0 to expose all OpenCode capabilities for agent-to-agent communication. OpenCode now operates as an A2A-compliant server, allowing discovery and task delegation from other agents.

Core Implementation (packages/opencode/src/a2a/)

• executor.ts - Executes A2A tasks through OpenCode sessions, handling message conversion and response streaming
• card.ts - Generates agent card from ToolRegistry, advertising all available tools as A2A skills
• Protocol types defined inline (no external dependencies)

HTTP Endpoints (packages/opencode/src/server/routes/a2a.ts)

• GET /a2a/.well-known/agent.json - Agent discovery card
• POST /a2a/jsonrpc - JSON-RPC 2.0 interface for standard agent communication
• POST /a2a/rest/execute - HTTP+JSON alternative

Key Design Decisions

• Protocol-compatible implementation without @a2a-js/sdk dependency (avoiding Express requirement, using Hono)
• Automatic tool exposure: all ToolRegistry entries map to A2A skills
• Instance.directory for execution context (contextId is conversation identifier only)
• Full TypeScript types for JSON-RPC requests/responses

Example usage:

# Discover capabilities
curl http://localhost:4096/a2a/.well-known/agent.json

# Execute via JSON-RPC
curl -X POST http://localhost:4096/a2a/jsonrpc -H "Content-Type: application/json" -d '{
  "jsonrpc": "2.0",
  "method": "agent/execute",
  "params": {
    "messages": [{
      "kind": "message",
      "messageId": "msg-001",
      "role": "user",
      "parts": [{"kind": "text", "text": "List TypeScript files"}]
    }]
  },
  "id": 1
}'

How did you verify your code works?

• Unit tests for agent card generation and tool exposure (test/a2a/card.test.ts)
• Manual verification of endpoint responses (agent card returns valid JSON with all tools)
• Verified URL path manipulation handles edge cases (split/join vs regex)
• Type checking passes (proper JsonRpcRequest/Response types throughout)

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• modelcontextprotocol.io
  • Triggering command: /usr/bin/curl curl -s REDACTED (dns block)
• unpkg.com
  • Triggering command: /usr/bin/curl curl -s REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

I want to expose all opencode capabilities throught Agent2Agent (A2A) Protocol

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.