[FEATURE]: Real-time Agent-to-Agent (A2A) & Non-blocking SubAgent Communication #17303

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Describe the enhancement you want to request

Current Problem

OpenCode's SubAgent invocation pattern has the following limitations:

• Blocking execution: Parent Agent is blocked via await on SubAgent completion
• One-way communication: SubAgent returns only a final message, no intermediate updates
• No real-time interaction: Parent Agent cannot respond to user input while waiting for SubAgent
• Limited orchestration: Complex multi-agent workflows difficult to coordinate

Proposed Enhancement

Enable real-time, asynchronous Agent-to-Agent (A2A) communication:

Core Features:

• SubAgent can send intermediate results/status updates to parent Agent (streaming)
• Parent Agent remains responsive—processes new inputs without blocking on SubAgent
• Non-blocking task execution model (fire-and-forget with callback/listener pattern)
• Optional bi-directional messaging during task lifetime
• Parent Agent can cancel, preempt, or modify SubAgent tasks based on new information

Use Cases

1. Complex Workflow Orchestration: Coordinate multiple SubAgents concurrently with dynamic task management
2. Real-time Progress Updates: Stream intermediate results/status to user UI without waiting for full completion
3. Dynamic Task Adaptation: Parent Agent responds to user input or errors while SubAgent is still executing
4. Agent Negotiation: SubAgents can request clarification or escalate decisions back to parent
5. Resource Management: Parent Agent monitors SubAgent resource usage and can preempt if needed

Benefits

• Improved UX: Agents always responsive; users see real-time progress instead of blocking
• Better Observability: Streaming logs/status from SubAgents visible throughout execution
• Advanced Orchestration: Enables agent collaboration, negotiation, and adaptive workflows
• Backward Compatible: Old synchronous mode remains available for simple workflows

Implementation Approach

1. Messaging Layer: Event-based channel or pub-sub system between parent/child sessions
2. Async Await Pattern: Replace blocking await taskTool.execute() with non-blocking handlers
3. Stream-based Results: Use existing session event streams to deliver intermediate messages
4. API Design: Provide both callback-style and promise-style interfaces for flexibility
5. Session Linking: Leverage existing parentID in session creation for parent-child relationship

Technical Considerations

• WebSocket or SSE for real-time delivery (already used in ACP)
• Maintain session isolation while allowing controlled message passing
• Handle error propagation and task cancellation across agent boundaries
• Ensure permission/security model accounts for A2A communication

Related Issues

• #SubAgent-async
• #A2A-agent-interaction
• Task tool in packages/opencode/src/tool/task.ts
• Session management in packages/opencode/src/session/prompt.ts