SMS agents

Build AI agents that converse over text message — async, threaded, with the same brain as your voice and chat agents.

SMS agents handle two-way text conversations. They use the same LLM, the same knowledge bases, the same tools as your voice and chat agents — but respond via SMS instead of speech or web chat.

When you need one

High-intent inbound — when customers text the same number they call (e.g., "txt us for a quote").
Long-running threads — SMS doesn't have to resolve in 60 seconds; a customer can text back hours later and pick up where they left off.
Offline / low-friction channels — anyone with a phone can text. No app install, no signup.
Appointment confirmations and reminders — customers can reply "yes" or "no" instead of clicking a link.

Create an SMS agent

Dashboard → Agents → New agent → SMS.

Required fields:

Name — internal label.
System prompt — the personality and behavior. Same as voice/chat, except optimized for short text replies: avoid long monologues, ask one question at a time.
First message — sent automatically when a contact texts in for the first time. Can be a brief greeting + offer to help.
LLM provider + model — see Models. Chat and SMS use the same LLM lineup.

Optional but recommended:

Knowledge base — let the agent answer FAQ-style questions accurately.
CRM integration — write each conversation to your CRM as a thread.
Custom tools — webhook tools your agent can call mid-thread (e.g., "look up my order").

Session model

A session is a single text thread between a contact and your number:

Contact sends their first text to your number.
Nerva creates a new session and routes the message to your agent.
Agent replies via SMS.
The thread continues until one of:
- 24 hours of inactivity — session is closed automatically. The next text from the same contact starts a fresh session.
- Explicit close by the agent (e.g., "anything else? No? Great, have a good day!" followed by a tool call to end).

The 24-hour window matches how customers actually use SMS — they reply hours later, sometimes the next morning. A 30-minute timeout would close too many real conversations prematurely.

Phone numbers

SMS agents need an SMS-capable phone number:

Pick one with SMS capability when you provision it from Dashboard → Numbers.
Or import an existing Twilio/Plivo number that has SMS enabled.

In the US, SMS-capable long codes require A2P 10DLC registration — you'll provide your business name, brand, and use case to your provider (Twilio handles this in their console). Without registration, your SMS sends will be filtered by US carriers.

Toll-free numbers and short codes have different rules — check with your carrier.

Crafting the agent prompt for SMS

SMS has different rhythm than voice or chat. A few prompt notes:

Keep replies under 160 characters when possible. Longer texts get split into multiple segments and feel spammy.
One question at a time. "What's your name, email, and date of birth?" gets ignored. Ask sequentially.
No formatting. SMS doesn't render markdown, bullets, or bold. Plain text only.
Use emoji sparingly. Tasteful but not gratuitous.
Always offer "STOP". Customers expect to be able to opt out at any time. Train your agent to acknowledge "STOP" and "UNSUBSCRIBE" cleanly.

Example system prompt addition:

Keep every reply under 160 characters. Ask one question at a time. Never use markdown. If the contact says "STOP" or "UNSUBSCRIBE", reply once with "Understood — you won't hear from us again." and end the conversation.

Outbound SMS

Two ways to send the first message:

Customer texts you first. The agent replies via the configured first-message and the conversation begins.
You send the first message programmatically (e.g., from a webhook handler, your CRM, or your backend). Use the send_sms tool from within another agent, or call the SMS send API directly.

For volume outbound (e.g., reminder texts to 500 patients), use Campaigns configured for SMS — same UX, just text instead of voice.

Webhooks

Subscribe to real-time events:

sms.started — first inbound message of a new session
sms.ended — 24 hours after the last message
sms.analyzed — after post-processing completes
sms.failed — send/receive errors

See Webhook events.

Compliance

SMS is heavily regulated in many countries:

US / Canada (TCPA) — written consent required before texting. Honor STOP / opt-out instantly. A2P 10DLC registration required.
UK / EU (PECR / GDPR) — clear consent + ability to opt out + record of consent.
India (TRAI) — DLT registration, header IDs, template approval.
Other regions — check your local SMS regulator. Your messaging provider (Twilio/Plivo) will warn you on common pitfalls.

We provide the agent platform. Consent collection, opt-out handling, and regulatory registrations are your responsibility.

Tips for SMB owners

Reply speed matters. Set short LLM models (GPT-4o-mini, Claude Haiku) so replies arrive in 2–3 seconds, not 15.
Save the agent's voice/tone. Customers can tell when SMS feels robotic. A casual, conversational tone outperforms a corporate one.
Don't overshare data. Don't include order details, account numbers, or anything sensitive in an SMS — they're not encrypted at rest on most phones.
Watch deliverability. Carriers filter spammy patterns. If your agent suggests links, use short, branded URLs (not generic shortened ones) to avoid being marked as spam.