Add MCP-based delivery ack spec — replaces brittle echo detection

[khaliqgant]

Summary

This spec proposes replacing the current brittle PTY echo-based delivery acknowledgment with an MCP-native approach. Since relaycast MCP is now the only communication channel for agents (no file-based outbox, no inline ->relay: protocol), we can leverage MCP tool calls themselves as implicit delivery confirmation.

Key insight: When an agent calls relay_send after receiving a message, that MCP call is the ack — no pattern matching needed.

How it works

• Broker injects message → relaycast tracks a pending delivery_id
• Agent responds via relay_send MCP call → relaycast correlates and marks delivery as confirmed
• No MCP activity within timeout → delivery marked as timeout
• Simple state machine: injected → confirmed | timeout

What this replaces

• Echo detection pattern matching in inject.rs
• Optimistic delivery assumptions in relay-pty
• All output-based ack heuristics

Implementation phases

1. Delivery tracking table in relaycast
2. MCP call correlation with pending deliveries
3. WebSocket delivery events + SDK sendAndWaitForDelivery() integration

Est. effort: 2-3 days

🤖 Generated with Claude Code

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

[github-actions]

Preview deployed!

Environment	URL
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Health	https://pr43-api.relaycast.dev/health
Observer	https://pr43-observer.relaycast.dev

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