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When a customer texts your business number, the engine routes the message to your agent and the agent replies — using the same system_prompt that powers your voice calls. No separate setup, no special “SMS mode”. One agent, two channels.
SMS replies are automatic for every inbound text. There’s no opt-in window, no campaign linkage required. If a phone number is wired to an agent, that agent answers texts to that number.

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What the agent sees on every reply

Before calling the agent, the engine builds a system prompt with four layers:
1

Real-time context

Date, time, day of week, timezone, customer phone, channel — all auto-included. The time is formatted in your agent’s timezone, not server time.
2

Caller history (if any)

If this phone number has called your agent before, the most recent call summary is injected automatically. First-time customers get no history block — clean and short.
3

Your agent's system prompt

Used as-is, with any {{variables}} substituted. Same content you wrote for voice.
4

Per-number business profile (if set)

If the dialled number has a custom_prompt on its phone_numbers row, it is added as a BUSINESS PROFILE section - the specific business identity for that number (name, address, hours), overriding any conflicting general details in the agent prompt. Numbers without one fall back to the agent prompt alone. See Phone numbers.
5

SMS reply rules

Short replies (max ~2 sentences), no emoji, no invented facts, match the customer’s language.

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Voice and SMS share memory

This is one of the most useful properties of Nixflex: a customer who called you yesterday can text you today, and the agent remembers them. The engine looks up the most recent voice call from the customer’s number and includes the call summaries plus usage instructions for the agent — the same way it does on inbound voice. Across channels, your AI feels continuous. Example exchange after a prior call:
Customer texts: “Hey, did I book yet?” Agent replies: “Yes — table for 4 on Friday at 7pm. Want me to change the time?”
The customer didn’t have to re-introduce themselves. The agent referenced the past call naturally, only because the customer asked something it could answer from history.
Caller history is opt-out for the customer in the sense that you control what gets summarised. The summary comes from your existing post-call analysis. See Post-call analysis.

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Variables work the same as voice

Everything in Variables applies to SMS replies. Use {{current_date}}, {{caller_phone}}, {{caller_history}}, etc. in your agent prompt — the engine substitutes them per message. 
Your prompt:    "It's {{current_day_of_week}}. We open at 9am sharp."
Customer sees:  "It's Tuesday. We open at 9am sharp."
You can also just write naturally — the engine puts a context block at the top of every prompt, so the agent already knows the date and time without you needing to reference variables explicitly. Variables are for when you want to phrase something precisely.

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Promotions, campaigns, special events

If you’re running a promotion (SMS campaign, seasonal offer, anything time-bound), mention it in your agent’s system_prompt so the agent can handle replies intelligently. 
You are the receptionist at Acme Dental.

We are running a March promotion: 20% off cleanings, valid weekdays
9am-5pm next week. If a customer replies YES, confirm the offer and
ask which day suits them. If they ask about the offer, explain it warmly.
Now when someone replies to your campaign — or texts asking about the offer — the agent already knows what to say. The agent prompt is the source of truth. This is also why we don’t need a separate “campaign reply window” — the agent is always context-aware as long as you put the context in the prompt.

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Conversation history

The engine loads the last 10 SMS messages between this customer and this agent into the prompt as chat history. The agent has full conversational context, so replies feel continuous over time. If a customer has texted you 50 times over a year, only the most recent 10 are loaded. This keeps prompts small and predictable.

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Channel awareness

The prompt explicitly tells the agent: Channel: SMS. This means the AI knows to write for text:
  • Shorter sentences than voice
  • No emoji or special characters (carriers can mangle them)
  • No “let me check that for you” pauses
  • Plain text only
You don’t need to repeat any of this in your system_prompt — it’s added automatically by the engine.

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What replies cost

CostSource
Per-message Twilio feeCharged directly to your Twilio account (BYO Twilio)
LLM token usageCharged to your Nixflex usage, same rate as voice
A typical SMS reply uses ~150–500 input tokens (the prompt) and ~30–80 output tokens. That’s well under a penny per reply.

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When replies don’t happen

The engine returns silently (no error to the customer) in these cases:
ReasonWhat happened
Phone number not linked to an agentThe dialled number isn’t in phone_numbers and has no legacy mapping
Agent has no Twilio credentialsThe agent is configured but its twilio_account_sid / twilio_auth_token are missing
The agent returned an empty responseRare — usually a transient API issue, no message is sent
Twilio rejected the sendCarrier issue, invalid number format, etc. Logged for review.
All four show up in your Logs page in the dashboard.

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Customer privacy

Caller history pulls only from calls made by the same phone number to your agent. It never crosses agents, never crosses developers, never crosses Nixflex tenants. The data lives in your Supabase tenancy. If a customer asks you to forget them, deleting their rows from calls and sms_messages removes them from future replies.

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SMS replies are conversational only

SMS replies are a text conversation - the agent answers questions, gives information, and holds context across messages. They do not run booking tools. Unlike a voice call, the agent on SMS cannot check live availability, book, reschedule, cancel, or look up an appointment on a connected calendar. If a customer texts asking to book or change an appointment, have the agent guide them to call (or to your booking link) in your system_prompt - for example: If a customer asks to book or change an appointment over text, ask them to call us on the number so we can confirm it properly. Booking actions are reliable on calls, where the agent confirms details live; keeping them off SMS avoids mis-bookings from a missed or ambiguous text.

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See also