# ADR 2: DM-First Routing

**Date**: 2026-06-21 (supersedes 2026-04-28 @mention-based routing)

### Decision

Agents are triggered primarily by **Zulip Direct Messages (DMs)**. Users send a DM to any agent's bot user (e.g., `@tanko-bot`) to communicate with that agent. Stream-based `@mention` routing is **deprecated** for agent-owner communication but retained for `@all-bots` broadcasts and multi-agent coordination in `#agent-hub`.

### Context

The original design required users to `@mention` an agent in `#agent-hub` stream topics. This proved unreliable because:

1. Zulip 12.0 SDK may not populate `mentioned_users` on stream message events
2. Every bot subscribed to `#agent-hub` receives every message — wasteful
3. Topic routing (capturing origin stream + topic for reply) adds parsing complexity
4. Distinguishing `type: 'stream'` vs `type: 'private'` required extra event filtering

DMs eliminate all of these concerns:
- Only the recipient bot receives the DM — no broadcast noise
- No @mention parsing needed — the DM itself is the signal
- No topic/stream capture needed — reply in the same DM thread
- `message.type: 'private'` is trivially detectable in the event queue

### DM Event Flow

```
User sends DM to @tanko-bot
  → Zulip pushes event to tanko-bot's event queue
  → Event has: {type: 'message', message: {type: 'private', ...}}
  → Bot checks: message.type === 'private' (trivial)
  → Bot reads message.content, processes, responds
  → Bot sends reply to same DM via /api/v1/messages with
    {type: 'private', to: [user_id], content: 'response'}
```

### @all-bots Exception

The stream-based approach is retained **only** for `@all-bots` broadcasts:
- `@all-bots` in `#agent-hub` → all bots detect the mention and respond
- This remains necessary for cross-platform broadcasts
- Detection can use Zulip SDK `mentioned_users` or content-based `@**All Bots**` matching

### Consequences

- **Positive:** Dramatically simpler routing logic — no @mention parsing, no topic/stream tracking
- **Positive:** Each bot only processes its own DMs — no wasted event processing
- **Positive:** Zulip's native DM threading handles all reply threading automatically
- **Positive:** No dependency on `mentioned_users` field behavior in Zulip 12.0
- **Negative:** Owners must DM their agent directly instead of using stream topics
- **Negative:** Cross-agent visibility in stream topics is reduced (mitigated by `@all-bots`)
- **Neutral:** `@all-bots` in `#agent-hub` still uses the original @mention detection pattern