Prompting Guide

Write prompts that produce reliable Finn voice agents.

4 min read

Prompting Guide

A Finn voice agent's behavior is 90% the prompt. This guide covers what to write, what to avoid, and how to debug.

The 4-part structure

Every reliable Finn prompt has four sections. In order:

Identity — who the agent is, who they work for, what their job is.
Goal — the single outcome of the call. One sentence.
Rules — what to say, what NOT to say, what to fall back on.
Examples — 2–4 short snippets of ideal turns.

Skip any part and the agent gets brittle.

Identity (1–3 lines)

You are Aria, a friendly appointment coordinator at Smile Dental Clinic.
You speak with patients calling to confirm, reschedule, or cancel an upcoming visit.
You are warm, brief, and never push extra services.

Anchor a name, a role, an employer, and a tone in 3 lines. Skip backstory — it eats latency and confuses the agent.

Goal (1 line)

Goal: confirm, reschedule, or cancel the patient's upcoming appointment within 90 seconds.

One concrete outcome. If the goal is multi-step (qualify + book + warm-transfer), break the agent into a workflow with separate nodes — don't cram it into one prompt.

Rules

State both positives and negatives.

Rules:
- Always ask "Am I speaking with {customer_name}?" before sharing any appointment details.
- If the caller says "remove me" or "stop calling", honor immediately and end the call politely.
- If asked if you're a real person, say honestly: "I'm Aria, a voice assistant for Smile Dental."
- Never quote a price — transfer to the front desk if asked.
- Never schedule a Sunday appointment — the clinic is closed.
- If the caller's intent is unclear after two clarifying questions, offer human transfer.

The "never" rules are as important as the "always" rules.

Examples

Show 2–4 micro-dialogues. They anchor tone better than any adjective.

Example 1 — Smooth confirm
Aria: "Hi, is this Priya? It's Aria from Smile Dental, calling about your visit on Thursday."
Caller: "Yes."
Aria: "Great. We have you booked for 4 PM Thursday with Dr. Mehta for a cleaning. Still works?"
Caller: "Yes, that works."
Aria: "Confirmed. See you Thursday. Have a great day."

Example 2 — Reschedule
Caller: "I need to move it."
Aria: "No problem. We have openings Friday at 11 AM or 3 PM, or Monday at 10. What works best?"

Variables

Use {variable} syntax. Finn auto-injects audience columns and live call data.

Built-in variable	What it resolves to
`{customer_name}`	The audience row's `name` column
`{customer_phone}`	The dialed number
`{current_time}`	Caller's local time, formatted
`{call_duration}`	Seconds since pickup

Any column in your uploaded audience CSV becomes a variable (e.g. {appointment_date}, {loan_amount}).

Common mistakes

Mistake	Why it breaks	Fix
Multi-paragraph identity	Bloats first token, slow response	3 lines max
Vague tone ("be helpful")	Means nothing	Pick 3 tone adjectives: "warm, brief, never pushy"
Goal stated as "do whatever the user wants"	Agent meanders, calls run long	One outcome per agent
Negative-only rules	Agent has no positive frame	Mix positive + negative
No examples	Tone drifts under pressure	Always include 2–4
Examples that mirror the rules	Adds nothing	Show edge cases — angry caller, ambiguous request

Debugging a prompt

When calls go off-script:

Listen to the recording. Don't guess.
Find the first weird turn. Usually a missing rule, not a model failure.
Add a rule for that exact scenario with one example.
Re-test on the same number. If fixed, move on. If not, the issue is upstream (workflow, audience, knowledge base).

Most "the model is dumb" complaints are missing-rule bugs.

Prompt length

Length	Latency	Use case
< 300 words	Lowest	Simple confirm / reminder / survey
300–800 words	Normal	Most agents
800–1500 words	Acceptable	Complex sales / triage
> 1500 words	Slow + erratic	Split into a workflow

Tools that help

AI Copilot — review and rewrite prompts in place.
Playbooks — 46 ready-to-deploy voice agent templates.
Knowledge Base — for facts the prompt shouldn't carry inline.

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