Nixflex - AI Voice Platform for Building AI Phone Agents

A variable is a placeholder you write in your instructions, like {{current_date}}, that the engine swaps for a real value at the start of every call. It lets one set of instructions say something different on every call - today’s date, the number that dialled, the customer’s name from your campaign.

Most agents never need them. The engine already tells the agent the date, time, day of the week, timezone and who is calling, before the first word. Use a placeholder only when you want the exact wording under your control.

Do I need variables at all?

Probably not for the basics. At the start of every call the engine adds a short block of live facts to your instructions - the date, tomorrow’s date, the next seven days, the time, the timezone, the caller’s number, your number and the call direction. The agent reads it automatically. Nothing to set up, nothing to write. Write a placeholder when the phrasing matters:

Your instructions:   "Today is {{current_date}}. We close at 11pm."
The agent sees:      "Today is Monday, May 25, 2026. We close at 11pm."

Both work. The block is enough for “what day is it on Thursday?”; the placeholder is better when you want the agent to say a specific sentence.

Every variable

Write the name in double curly braces, exactly as shown. Names are case-sensitive - {{Current_date}} will not match. Date and time

Variable	Example value	Use it when
`{{current_date}}`	Monday, May 25, 2026	You want today’s date spoken in full
`{{current_time}}`	14:32	Opening-hours logic, “we close in an hour”
`{{current_day_of_week}}`	Monday	”We are closed on Sundays” style rules
`{{tomorrow_date}}`	Tuesday, May 26, 2026	Offering tomorrow without the agent working it out
`{{upcoming_days}}`	Next 7 days: Monday 14 September (2026-09-14), …	Already included automatically. Use it only if you move it somewhere specific in your instructions
`{{current_timezone}}`	Europe/London	Confirming the timezone to a caller in another country
`{{current_iso_date}}`	2026-05-25	Passing a date to your own system in `YYYY-MM-DD`
`{{current_iso_time}}`	14:32:07	Timestamps for your own records
`{{tomorrow_iso_date}}`	2026-05-26	Same, for tomorrow

All of these follow the agent’s timezone setting, and daylight saving is handled for you. This call

Variable	Example value	Use it when
`{{caller_phone}}`	+447446466847	Texting the caller, or passing their number to your system. On outbound this is the number being called
`{{business_phone}}`	+442087601234	Your number - the one that was dialled, or that you called from
`{{call_direction}}`	inbound	One set of instructions used for both directions
`{{call_id}}`	CA1b5c…	Giving the caller a reference, or matching the call in your own records
`{{agent_id}}`	agent_125207e452f8714a	Rarely needed - the agent’s own ID

The caller

Variable	What it holds	Use it when
`{{caller_history}}`	A returning caller’s name and one line on what they wanted last time	You want to control exactly how a returning caller is greeted

How does the agent know who is calling?

When a number that has called before dials again, the engine looks up the most recent call that captured a name and passes a short note to the agent:

PREVIOUS CALL FROM THIS NUMBER
Name: John. Last time they called about: wanted to book a check-up.

That is the whole thing - a name and what they wanted. Never what happened: no bookings, outcomes, dates or summaries are ever put into a live call. That is deliberate. When booking details were injected, agents confirmed bookings they had never made. The name is picked up automatically from the conversation; there is nothing to configure. A first-time caller adds nothing to the prompt. Turning off Greet by name stops the agent opening with the name, but the note itself is still available to the agent. See Caller context.

Can I pass my own data into a call?

Yes, on outbound calls. One call - send dynamic_vars with the call:

curl -X POST https://api.nixflex.com/v1/calls \
  -H "Authorization: Bearer KEY_ID:KEY_SECRET" \
  -H "Content-Type: application/json" \
  -d '{
    "agent_id": "agent_125207e452f8714a",
    "to_number": "+447446466847",
    "dynamic_vars": {
      "first_name": "Jane",
      "appointment_time": "Tuesday at 3pm"
    }
  }'

Then write {{first_name}} and {{appointment_time}} in your instructions. A campaign - every column in your recipient list becomes a variable with the column’s name, automatically. You do not have to write placeholders at all: the agent is handed each recipient’s details as context for that call. Write naturally (“remind them about their appointment”) and the agent has the values. See Outbound calls.

Reserved names. The variable names in the tables above belong to the engine. If your campaign file has a column called current_date or caller_phone, the engine’s value wins and yours is ignored. Give your columns their own names - appointment_date, customer_phone.

Inbound calls do not take your own values yet - an inbound caller is a stranger until they say who they are, and the engine already supplies everything known about the call.

Why is being said out loud on my calls?

Because that name does not exist. When the engine does not recognise a placeholder it leaves the text exactly as written, so it ends up in what the agent reads - and the agent may say it. This is deliberate: a visible mistake is easier to find than a silently deleted one. Check the spelling against the tables above, and remember names are case-sensitive and need both pairs of braces: {{current_date}}, not {current_date} or {{Current_Date}}.

Which braces go where

Everything uses double braces except one: the message a caller gets after the call.

You are writing	Syntax	Example
Agent instructions (inbound and outbound)	`{{double}}`	`Remind {{first_name}} about {{appointment_time}}.`
SMS campaign message	`{{double}}`	`Hi {{first_name}}, your check-up is due.`
Custom function fields	`{{double}}`	`{"ref": "{{call_id}}"}`
Post-call SMS template	`{single}`	`Thanks for calling {business_name}.`

The post-call template is the odd one out because it came first and changing it would break every template already saved. If a placeholder is not being replaced, check the brace count before anything else.

When are variables filled in?

Stage	What happens
Call starts	The engine loads the agent, the live facts and any caller history, all at the same time
Before the greeting	Placeholders are replaced and the live-facts block is added
During the call	Values stay as they were. They are not recalculated per reply
Call ends	Everything is discarded

Values are read once, so there is no added delay on the call. If something changes minute by minute, do not put it in a variable - put it in your instructions, or fetch it during the call with a custom function.

What we do not have, and why

We are deliberate about this. Every variable is a promise - once it exists, agents depend on it.

Often asked for	Why not
`{{agent_name}}`	You already write the agent’s name in your own instructions. A variable just adds a step
`{{business_hours}}`	Hours rarely fit one line. Put them in your instructions so the agent can handle “closed for lunch” and bank holidays
`{{caller_name}}` on inbound	An inbound caller is a stranger until they say who they are. Once they have given a name on a previous call, `{{caller_history}}` has it
Your own values on inbound	Coming later. Available on outbound today with `dynamic_vars`

Good to know

On inbound calls an unknown name is left in place, so you can see the mistake. On outbound calls and in SMS it is replaced with nothing, so a wrong name leaves a gap in the sentence instead.
Values are turned into text. true and false appear as the words “true” and “false”.
Names must be letters, numbers or underscores - {{first_name}} works, {{first name}} does not.
Both directions get the live-facts block. Only outbound gets your own values.

Agents - where the timezone and other settings live
Caller context - what the engine remembers about a returning caller
Outbound calls - campaigns and recipient columns
Custom functions - fetch live data during the call instead
Writing a good agent prompt - where variables fit in your instructions

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Variables

Do I need variables at all?

Every variable

How does the agent know who is calling?

Can I pass my own data into a call?

Why is being said out loud on my calls?

Which braces go where

When are variables filled in?

What we do not have, and why

Good to know

​Do I need variables at all?

​Every variable

​How does the agent know who is calling?

​Can I pass my own data into a call?

​Why is being said out loud on my calls?

​Which braces go where

​When are variables filled in?

​What we do not have, and why

​Good to know

​Related

Do I need variables at all?

Every variable

How does the agent know who is calling?

Can I pass my own data into a call?

Why is being said out loud on my calls?

Which braces go where

When are variables filled in?

What we do not have, and why

Good to know

Related