Nixflex - AI Voice Platform for Building AI Phone Agents

Create agent

curl --request POST \
  --url https://api.nixflex.com/v1/agents

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://api.nixflex.com/v1/agents",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
]);

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  echo "cURL Error #:" . $err;
} else {
  echo $response;
}

package main

import (
	"fmt"
	"net/http"
	"io"
)

func main() {

	url := "https://api.nixflex.com/v1/agents"

	req, _ := http.NewRequest("POST", url, nil)

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := io.ReadAll(res.Body)

	fmt.Println(string(body))

}

require 'uri'
require 'net/http'

url = URI("https://api.nixflex.com/v1/agents")

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true

request = Net::HTTP::Post.new(url)

response = http.request(request)
puts response.read_body

POST

agents

Create agent

curl --request POST \
  --url https://api.nixflex.com/v1/agents

<?php

$curl = curl_init();

curl_setopt_array($curl, [
  CURLOPT_URL => "https://api.nixflex.com/v1/agents",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 30,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
]);

$response = curl_exec($curl);
$err = curl_error($curl);

curl_close($curl);

if ($err) {
  echo "cURL Error #:" . $err;
} else {
  echo $response;
}

package main

import (
	"fmt"
	"net/http"
	"io"
)

func main() {

	url := "https://api.nixflex.com/v1/agents"

	req, _ := http.NewRequest("POST", url, nil)

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := io.ReadAll(res.Body)

	fmt.Println(string(body))

}

require 'uri'
require 'net/http'

url = URI("https://api.nixflex.com/v1/agents")

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true

request = Net::HTTP::Post.new(url)

response = http.request(request)
puts response.read_body

Creates a new AI agent. The agent immediately becomes available to attach to phone numbers and use for outbound calls.

Request

curl -X POST https://api.nixflex.com/v1/agents \
  -H "Authorization: Bearer KEY_ID:KEY_SECRET" \
  -H "Content-Type: application/json" \
  -d '{
    "name": "Acme Dental Receptionist",
    "system_prompt": "You are the friendly front-desk assistant at Acme Dental in Croydon...",
    "welcome_message": "Hello, Acme Dental, how can I help?",
    "voice_id": "Ashley",
    "language": "multi",
    "transfer_number": "+442087601234",
    "webhook_url": "https://api.acmedental.com/nixflex/calls"
  }'

Body parameters

Only name is recommended in practice — every other field has a sensible default. You can create a working agent with a one-line payload.

Core

Field	Type	Required	Default	Notes
`name`	string	No	`"Untitled Agent"`	Display name (max 100 chars)
`system_prompt`	string	No	friendly assistant	The AI’s instructions (max 16000 chars, about 4000 tokens)
`welcome_message`	string	No	`"Hello! How can I help you today?"`	First line spoken on inbound calls
`voice_id`	string	No	`"Ashley"`	Voice name. Browse voices in the dashboard
`language`	string	No	`"multi"`	Default language. `multi` follows the caller across English, Spanish, French, German, Italian, Portuguese, Dutch, Hindi, Russian and Japanese. Arabic, Hebrew, Korean, Chinese and Polish need their own code. See Languages
`is_active`	bool	No	`true`	Set false to disable without deleting

Behaviour

Field	Type	Required	Default	Notes
`temperature`	number	No	`0.3`	AI temperature, `0`-`1`
`response_length`	enum	No	`"short"`	`short`, `medium`, `long`
`ai_speaks_first`	bool	No	`true`	Agent greets caller immediately
`dynamic_greeting`	bool	No	`false`	AI improvises greeting per call instead of static welcome
`interruption_sensitivity`	int	No	`50`	`0`-`100`. Lower = harder to interrupt
`response_eagerness`	int	No	`200`	How long the agent waits after the caller stops speaking, in milliseconds. Range `0`-`1000`. Lower is snappier; `0` replies the moment the words are final. Shown as Response Delay in the dashboard
`boosted_keywords`	string	No	`null`	Comma-separated words your callers say that speech recognition might mishear - staff names, treatments, products, place names. Up to 50. Example: `Dr Patel, hygienist, root canal, Meadow Lane`
`pickup_delay`	int	No	`10`	Seconds the agent waits after pickup before speaking. Range 5–30.

Call controls

Field	Type	Required	Default	Notes
`max_call_duration_seconds`	int	No	`600`	Hard cap on call length. Range 60–1800 (1–30 min). See End call.
`silence_hangup_seconds`	int	No	`30`	Hang up if caller silent this long. Range 5–45. See End call.
`record_call`	bool	No	`true`	Save call audio to Supabase Storage. Recordings are deleted after 90 days, and a number can override this. See Call recording.
`recording_source`	string	No	`"provider"`	Who records when `record_call` is on. `"provider"` = your carrier records (whole call, carrier recording charges apply). `"nixflex"` = the voice engine records the conversation itself at no extra cost (a transfer to a person is not included). Anything else is rejected with `invalid_recording_source`. See Call recording.
`amd_enabled`	bool	No	`false`	Ask the carrier to detect an answering machine and hang up about 4 seconds in. Costs extra per answered call on your own carrier bill. With it OFF the agent detects voicemail from the first thing it hears and can leave a message, so ON means a fast hangup and NO message. See Voicemail detection.
`dtmf_enabled`	bool or null	No	`null`	Let callers type digits on their phone keypad instead of saying them. `null` means not set, and a number can override it. See Keypad input.
`require_booking_confirmation`	bool	No	`false`	The engine reads the appointment back and waits for a clear yes before any calendar create, reschedule or cancel. Numbers can override it. See Booking confirmation.
`webhook_url`	string	No	`null`	Where to POST post-call data

Functions (tools the agent can call mid-conversation)

Field	Type	Required	Default	Notes
`func_end_call`	bool	No	`true`	Agent can end the call cleanly
`transfer_whisper`	string	No	`null`	Warm transfer briefing text. If empty, the AI auto-generates from the conversation.
`func_send_sms`	bool	No	`false`	Not enforced. Accepted and returned, but the engine does not read it. Use `incall_sms_enabled` to control texting during a call.
`incall_sms_enabled`	bool	No	`false`	Lets the agent text the caller during a call, and sends booking confirmations by text. With it off, no text is sent and the agent confirms details out loud. Agents created before this setting existed were switched on. See In-call SMS.
`func_cold_transfer`	bool	No	`false`	Not enforced. Same as above - accepted, returned, and not read. Transfer is controlled by your prompt and by `transfer_number`.
`transfer_type`	enum	No	`"cold"`	`cold` or `warm`. Applies to inbound, outbound, and batch calls made by this agent. See Transfer.
`transfer_number`	string	No	`null`	Default destination for transfers

How to stop the agent texting or transferring. Texting is controlled by incall_sms_enabled: with it off no text is sent, whatever the prompt says. Transfers are controlled by your prompt - the engine tells the model to transfer only if the developer prompt asks it to, so a prompt that never mentions transferring produces an agent that never transfers. func_send_sms and func_cold_transfer are kept because removing a field developers may already send would break them silently, but they have no effect.

A prompt instruction is reliable but not absolute - a language model can still ignore one. If you need a hard guarantee that a number never texts or never transfers, do not rely on the prompt alone. Get in touch: a per-number switch is on the roadmap and is worth building against a real requirement rather than a guess.

Other

Field	Type	Required	Default	Notes
`voice_speed`	number	No	`0.85`	Ignored. Accepted and returned for backwards compatibility, but nothing in the engine reads it. Use `speaking_rate` below.
`speaking_rate`	number or null	No	`null`	How fast the agent talks. `1` is normal, `0.5` is half speed, `1.5` is half again as fast. A value outside that range is clamped to the nearest limit rather than rejected. `null` means normal speed, which is not the same as sending `1`. A number can override this. See Voices.
`greet_by_name`	bool	No	`true`	When a returning caller is recognised, greet them by the name on file (for example, Hi John). Set `false` for a neutral greeting.
`fallback_message`	string	No	”I am sorry, could you say that again?”	Said when transcription fails
`phone_number`	string	No	`null`	Legacy. Numbers are now managed via `/phone-numbers`

Response

201 Created:

{
  "id": "1083c0db-699e-49ee-aa6c-aab29e2d87cf",
  "agent_id": "agent_125207e452f8714a",
  "api_key_id": "nxf_a1b2c3d4...",
  "name": "Acme Dental Receptionist",
  "system_prompt": "You are the friendly front-desk assistant...",
  "welcome_message": "Hello! How can I help you today?",
  "voice_id": "Ashley",
  "language": "multi",
  "temperature": 0.3,
  "response_length": "short",
  "ai_speaks_first": true,
  "dynamic_greeting": false,
  "interruption_sensitivity": 50,
  "pickup_delay": 10,
  "max_call_duration_seconds": 300,
  "silence_hangup_seconds": 10,
  "record_call": true,
  "recording_source": "provider",
  "func_end_call": true,
  "transfer_whisper": null,
  "func_send_sms": false,
  "incall_sms_enabled": false,
  "transfer_type": "cold",
  "transfer_number": null,
  "webhook_url": null,
  "phone_number": null,
  "voice_speed": 0.85,
  "speaking_rate": null,
  "dtmf_enabled": null,
  "require_booking_confirmation": false,
  "amd_enabled": false,
  "greet_by_name": true,
  "transfer_whisper": null,
  "fallback_message": "I am sorry, could you say that again?",
  "is_active": true,
  "created_at": "2026-05-24T01:48:10Z",
  "updated_at": "2026-05-24T01:48:10Z"
}

Save the agent_id — you’ll use it for every subsequent operation.

Next step

Attach a phone number, on Twilio or Telnyx, so the agent can make and receive calls. See Import phone number.

Errors

Code	Cause
`prompt_too_long`	`system_prompt` exceeds 16000 characters
`invalid_voice_id`	Voice not in supported list
`invalid_language`	Language code not supported
`invalid_request`	Missing required field or bad format
`deprecated_field`	`twilio_account_sid` or `twilio_auth_token` was sent. Agents do not hold carrier credentials - import the number with `POST /v1/phone-numbers` instead.

The response never includes twilio_auth_token. See Errors for handling patterns.

API

Platform API

Create agent

Request

Body parameters

Core

Behaviour

Call controls

Functions (tools the agent can call mid-conversation)

Other

Response

Next step

Errors

​Request

​Body parameters

​Core

​Behaviour

​Call controls

​Functions (tools the agent can call mid-conversation)

​Other

​Response

​Next step

​Errors

Request

Body parameters

Core

Behaviour

Call controls

Functions (tools the agent can call mid-conversation)

Other

Response

Next step

Errors