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Documentation Index

Fetch the complete documentation index at: https://docs.nixflex.com/llms.txt

Use this file to discover all available pages before exploring further.

Beyond mid-call and post-call SMS, you can send standalone messages via the API. Useful for appointment reminders, status updates, follow-ups — anything that doesn’t need a phone call.

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Send one message

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": "Reminder: your appointment is tomorrow at 2pm. Reply YES to confirm or NO to reschedule."
  }'
FieldRequiredNotes
agent_idYesThe agent this SMS belongs to (determines reply behaviour)
to_numberYesRecipient in E.164 format
from_numberYesOne of your imported Twilio numbers
bodyYesThe message text (max 1600 chars; longer is split)
context_promptNoCustom instruction for the agent if the caller replies
Response: 
{
  "message_id": "SM1234567890",
  "status": "queued",
  "to_number": "+447386172392",
  "from_number": "+447446466847"
}
Status updates flow through Twilio’s status callbacks. The final status (delivered, failed, etc.) will be reflected in the message record within ~30 seconds.

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Reply handling

If the recipient replies to your SMS within 24 hours, the agent can automatically respond using the conversation context. The reply window:
  • Starts when you send the SMS
  • Counted per recipient number
  • Resets if you send another SMS to the same number
  • Default 24 hours; configurable per agent up to 30 days
When a reply comes in:
  1. Nixflex receives it via Twilio’s incoming SMS webhook
  2. Looks up the recent SMS to that number
  3. Uses the agent’s system prompt + the recent SMS context to compose a reply
  4. Sends the reply automatically
The conversation thread appears in the dashboard and via GET /v1/sms.

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Custom reply context

For more control over how the agent replies, pass context_prompt: 
{
  "agent_id": "agent_xxx",
  "to_number": "+447386172392",
  "from_number": "+447446466847",
  "body": "Hi Sarah, this is Acme Dental. Your appointment is tomorrow at 2pm. Reply YES to confirm.",
  "context_prompt": "The caller is Sarah Johnson, booked for a check-up on Tuesday 8th March at 2pm. If she confirms (YES), respond warmly. If she wants to reschedule, offer Wednesday 9th 10am or 2pm. Anything else, escalate by texting: For other requests, please call us back on 020 8760 1234."
}
The agent has tighter, more relevant guidance to handle replies.

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Disabling replies

If you want the SMS to be one-way (no auto-replies), set the agent’s enable_sms_reply to false. Inbound replies are still received and logged but no automatic response is sent.

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Tracking

Every SMS gets a message_id you can use to fetch status: 
curl https://api.nixflex.com/v1/sms/SM1234567890 \
  -H "Authorization: Bearer KEY_ID:KEY_SECRET"
Response includes status (queued, sent, delivered, failed), error code if any, and the conversation thread if replies happened.

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Compliance

UK only at launch. US SMS requires A2P 10DLC registration on your Twilio account. Without it, US messages are rejected by carriers.You’re responsible for:
  • Opt-in records (you have permission to text these numbers)
  • Handling STOP / UNSUBSCRIBE keywords (Twilio handles this automatically by default)
  • Respecting time-of-day rules (no messages 9pm-9am in the recipient’s timezone)
Nixflex provides infrastructure; compliance stays with you.

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Pricing

SMS uses your Twilio account directly. Twilio charges per message at standard rates:
  • UK domestic: ~£0.04 per message
  • US (after A2P): ~$0.0075 per segment
  • International: varies by country
Nixflex doesn’t mark up SMS pricing.