feat: A2A (Agent-to-Agent) protocol support - Phase 1

[roomote-v0]

Summary

This PR attempts to address Issue #12007 by implementing Phase 1 of A2A (Agent-to-Agent) protocol support, following the guidance from @farazoman.

What's Included (Phase 1)

New A2A Types (packages/types/src/a2a.ts)

• Agent configuration schema (URL, headers, timeout, disabled state)
• A2A protocol types: Task, Message, Part (text/file/data), Artifact, TaskStatus
• JSON-RPC request/response types per the A2A spec
• Agent Card type for optional capability discovery
• Runtime agent state for UI display

A2aHub Service (src/services/a2a/A2aHub.ts)

• Manages A2A agent connections (mirrors MCP Hub pattern)
• Settings file management: a2a_settings.json (global) and .roo/a2a.json (project-level)
• File watching for auto-reload on config changes
• JSON-RPC 2.0 over HTTP communication (tasks/send, tasks/get, tasks/cancel)
• Configurable headers for authentication (JSON key-value pairs)
• Optional Agent Card fetching from /.well-known/agent.json

delegate_to_agent Tool (src/core/tools/DelegateToAgentTool.ts)

• New tool for delegating tasks to external A2A agents
• Parameters: agent_name (required), message (required)
• Validates agent existence, gets user approval, formats results
• Handles task states including input-required for continued conversation

Integration

• New a2a tool group added to tool system and code mode
• System prompt capabilities section lists available A2A agents
• Native tool definition registered for model consumption
• NativeToolCallParser handles delegate_to_agent args
• ClineProvider initializes and manages A2aHub lifecycle

Design Decisions per Issue Discussion

1. Latest A2A spec - JSON-RPC 2.0 over HTTP
2. Phased approach - Phase 1 (client delegation) only; Phase 3 out of scope
3. Manual configuration only - No auto-discovery
4. Background sub-tasks - Prompts aware of A2A capabilities
5. Simple auth - Configurable headers as JSON key-value pairs
6. Custom agent use case - Seamless alternative to wrapping agents in MCP

Configuration Example

{
  "a2aAgents": {
    "my-agent": {
      "url": "https://my-agent.example.com/a2a",
      "headers": {
        "Authorization": "Bearer sk-..."
      },
      "description": "My specialized agent",
      "timeout": 300
    }
  }
}

Tests

• A2aHub service tests (10 tests passing)
• All existing tests continue to pass (validateToolUse, mcp_server, sections)
• Full lint and type-check pass across all 14 packages

Feedback and guidance are welcome!

Interactively review PR in Roo Code Cloud