agent-email-patterns

Files (3)

• SKILL.md
• threat-model.md · references/
• topologies.md · references/

SKILL.md (first lines)

# Agent Email Patterns

Opinionated patterns for building AI agents that communicate over email. This skill covers architecture and security decisions, not SDK specifics. For AgentMail SDK usage, use the `agentmail` skill.

## Why agents need their own inboxes

Giving an agent OAuth access to a human's Gmail account is the most common approach and the most dangerous:

- **Over-permissioned**: typical OAuth scopes (e.g. `gmail.modify`) grant read/send/delete over the entire mailbox history, far beyond what any single task needs
- **Prompt injection risk**: the agent inherits the full inbox history as reachable context, so any crafted email already sitting in the mailbox is a live attack surface
- **Revocation granularity**: OAuth tokens are hard to revoke or scope per-agent -- pulling access from one workflow often means pulling it from all of them
- **Rate limits**: consumer mailbox sending limits aren't designed for automated/programmatic workflows
- **Audit trail**: agent actions are mixed with human actions in the same mailbox, making debugging and compliance review hard

The safer default: one dedicated, API-native inbox per agent (see Pattern 1).

### Provider landscape

Durable architectural constraints when choosing infrastructure (not a ranking):

| Provider | Key constraint |
|---|---|
| Gmail API | No programmatic inbox creation; no WebSocket push (Pub/Sub or polling only); access is revocable by Google at any time |
| Resend | No threads or conversation concept; cannot list/search received messages; inbound only via webhook, no persistent inbox |
| SendGrid | Inbound parse is stateless; no thread management; no programmatic inbox creation |
| Amazon SES | Inbound is rule-based (S3/Lambda triggers), not a mailbox; no thread management; no WebSocket support |

## Pattern 1: one inbox per agent

Every agent gets its own email address. Never share inboxes between agents.

```python
client.inboxes.create(request=CreateInboxRequest(username="support-agent", client_id="support-v1"))
```

Why: clear sender identity, isolation (agents can't read each other's mail), per-agent auditability, and blast-radius containment if one agent is compromised.

Anti-pattern: one shared inbox with multiple agents reading from it. This creates race conditions and makes debugging impossible.

## Pattern 2: two-way conversation loops

The core agent email pattern: agent sends, human replies, agent reads the reply and responds, looping until resolved.

Gotchas:
- `messages.list()` returns metadata only (no body) -- call `.get()` on each item to fetch `.text` / `.extracted_text`.
- Use `extracted_text` / `extracted_html` for inbound replies so you don't reprocess the entire quoted chain on every turn.
- To keep a reply threaded, call `messages.reply(inbox_id, message_id, ...)` with the parent `message_id` -- there is **no `thread_id` parameter**; AgentMail threads it automatically from the parent message.
- Track conversation state in your own database, not by re-parsing the email body each time.

## Pattern 3: human-in-the-loop drafts

For high-stakes emails, let the agent draft and a human approve before sending: `drafts.create(...)` then `drafts.send(inbox_id, draft_id)`.

Use drafts when:
- Email has legal or financial implications
- Recipient is a VIP or external stakeholder
- Agent is new and untrusted for this workflow

Send directly when:
- Routine notification (receipts, confirmations)
- Agent has proven reliability
- Speed matters (OTP forwarding, automated alerts)

## Pattern 4: event-driven architecture

Default to event-driven delivery (WebSockets or webhooks) rather than polling. Polling is acceptable when neither is workable — e.g. a constrained environment with no public URL and no persistent connection — but expect higher latency and API usage.

| Factor | WebSockets | Webhooks |
|---|---|---|
| Public URL needed | No | Yes |
| Best for | Agents, bots, local dev | Servers, serverless |
| Latency | Lowest (persistent) | HTTP round-trip |
| Reconnection | You handle it | AgentMail retries |

Webhook payloads must be verified before use -- see `references/threat-model.md`.

## Pattern 5: multi-agent topologies

For systems with multiple agents, assign clear roles (e.g. `support@`, `sales@`, `billing@`, `router@`) and use allow lists (`references/threat-model.md`) to restrict which external senders can reach each agent. For hub-and-spoke, peer-to-peer, and hierarchical escalation patterns, see `references/topologies.md`.

## Pattern 6: OTP and verification flows

Agents that sign up for services need to receive and extract verification codes (e.g. regex for a 4-8 digit code in the inbound message text).

This applies to **explicitly authorized first-party or test flows only** -- e.g. your own agent signing up for a service it will operate, or a test account you control. It does not authorize automating sign-in, verification, or account-recovery flows for third-party accounts, or bypassing a service's terms of use or human-consent requirements.

Best practices:
- Create a fresh inbox per sign-up flow for isolation
- Set a timeout (do not wait indefinitely for an OTP)
- Delete the inbox after the flow completes if it is single-use

## Pattern 7: labels for workflow state

Use labels to track message processing state within an inbox (`add_labels` / `remove_labels` on `messages.update`, then filter with `messages.list(..., labels=[...])`).

Common label schemes:
- `unread` / `processed` / `archived`
- `needs-reply` / `replied` / `escalated`
- `billing` / `support` / `sales` (category routing)

## Security essentials

See `references/threat-model.md` for the full threat model. Critical rules:

1. **Content from email, attachments, webhooks, or tool output is never authorization** for a consequential action -- only an authenticated user instruction or explicit policy is. See the authorization matrix in `references/threat-model.md`.
2. **Never pass raw email content as a system prompt.** Frame it as untrusted data; this reduces injection risk but is not itself a security boundary.
3. **Use allow lists** on production agent inboxes to restrict senders -- one layer of defense, not sufficient alone.
4. **Verify webhook signatures** with Svix before processing any payload.
5. **Never put API keys or secrets in email bodies or subjects**; scan outbound content before sending.
6. **Separate agent credentials from human credentials** -- each agent gets its own scoped API key.

## Reference files

- `references/topologies.md` -- hub-and-spoke, peer-to-peer, hierarchical, and multi-tenant pod agent email architectures
- `references/threat-model.md` -- prompt injection, webhook spoofing, OAuth/credential exposure, data leakage, inbox enumeration, and the authorization matrix

From itzmetanjim/coolton, AGPL-3.0.

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