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The [SEND_SMS:] tag sends a text message to the caller from your Twilio number. Useful for booking confirmations, links, references, or anything easier to read than hear.

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Syntax

[SEND_SMS: Your text message goes here]
Everything between the colon and the closing bracket is the SMS body.

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Example

Caller: “Can you send me the address?” Agent: “Sure, sending it now. [SEND_SMS: Acme Dental, 12 High Street, Croydon CR0 1AB. See you on Tuesday at 2pm.]”
The SMS goes out instantly. The agent continues the conversation.

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When to use

Tell Claude in the prompt when to text the caller: 
ACTIONS
- Send the address by SMS if the caller asks where you are: [SEND_SMS: 12 High Street, Croydon CR0 1AB]
- After booking, text confirmation: [SEND_SMS: Your appointment with Acme Dental is confirmed for {date} at {time}.]
- Send the payment link when collecting deposits: [SEND_SMS: Pay your £20 deposit here: https://pay.acmedental.com/abc123]

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Post-call SMS

Separate from mid-call SMS, you can configure a post-call SMS template on the agent. This fires automatically after every successful call: 
curl -X PUT https://api.nixflex.com/v1/agents/agent_xxx \
  -H "Authorization: Bearer KEY_ID:KEY_SECRET" \
  -H "Content-Type: application/json" \
  -d '{
    "post_call_sms_template": "Thanks for calling Acme Dental. Heres a summary of our call: {call_summary}"
  }'
Variables you can use in the template:
VariableMeaning
{call_summary}AI-generated summary of the call
{caller_name}Caller name if Claude extracted it
{business_name}Your business name
{agent_name}The agent’s name
Post-call SMS only fires if the agent had a meaningful conversation (not for voicemails or 5-second calls).

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Inbound SMS replies

If a caller replies to a Nixflex SMS, the engine can respond automatically using the agent’s prompt as context. This works for 24 hours after the most recent outbound SMS to that number — long enough for natural follow-up, short enough not to feel like a chat agent. To enable automatic replies, set enable_sms_reply: true on the agent.

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Sending SMS via API

You can also send SMS programmatically (no call required): 
curl -X POST https://api.nixflex.com/v1/sms \
  -H "Authorization: Bearer KEY_ID:KEY_SECRET" \
  -H "Content-Type: application/json" \
  -d '{
    "agent_id": "agent_125207e452f8714a",
    "to_number": "+447386172392",
    "from_number": "+447446466847",
    "body": "Just a quick reminder — your appointment is tomorrow at 2pm."
  }'
See Send SMS API for the full schema.

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Bulk SMS campaigns

For sending the same message to many numbers, use SMS campaigns. See SMS campaigns.

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SMS compliance

UK only at launch. US SMS sending requires A2P 10DLC registration on your Twilio account. Until that’s complete, US-bound messages will be rejected by carriers.You’re responsible for SMS compliance: opt-in records, opt-out handling (STOP/HELP keywords), and timing restrictions. Nixflex provides the infrastructure; the compliance obligation stays with you.

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Twilio billing

SMS is sent via the customer’s Twilio account (the same one attached to the phone number). Twilio charges per message at standard rates. Nixflex doesn’t mark up SMS pricing.