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Nixflex agents perform actions during calls — transfers, SMS, ending the call — by emitting special tags in the LLM’s response. The engine watches for these tags, strips them from what the caller hears, and runs the action. You don’t have to wire up function calling or webhooks for built-in actions. Just tell the agent to use the tag when appropriate.

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How tags work

The LLM generates a normal reply. If the reply contains a tag, the engine intercepts it:
The LLM says: “Sure, I’ll transfer you to our manager now. [TRANSFER:+442087601234]” Caller hears: “Sure, I’ll transfer you to our manager now.” Engine action: Cold transfer to +442087601234.
The caller never hears the tag itself. It exists only as a signal between the LLM and the engine.

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Built-in tags

End call

[END_CALL] — gracefully end the conversation

Transfer

[TRANSFER:+number] — hand off to a human

Send SMS

[SEND_SMS: text] — text the caller mid-call

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Telling the agent when to use them

Add an Actions section to your system prompt with one-line guidance for each tag you want the agent to use: 
ACTIONS
- End the call after a successful booking or when the caller says goodbye: [END_CALL]
- Transfer to the manager if the caller asks for a refund: [TRANSFER:+442087601234]
- Text confirmation after booking: [SEND_SMS: Your appointment is confirmed for {date} at {time}. Reply YES to confirm.]
The agent will use the tags when the conversation calls for them — without you having to script every branch.

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Multiple tags in one response

The agent can use more than one tag in a single reply. Common pattern after a booking:
“Perfect, you’re booked in for Tuesday at 2pm. I’ve just sent you a text with the details. [SEND_SMS: Your appointment with Acme Dental is confirmed for Tuesday 5th March at 2pm. Call us if anything changes.] Have a great day. [END_CALL]”
The engine processes them in order: send the SMS, then end the call.

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Beyond built-in actions

For anything more complex (looking up customer records, booking appointments, processing payments), the agent can call your own APIs during the conversation. See Cal.com for live booking, or the integrations section for connecting your own tools.