Persona: Same Question, Two Different Voices

Module 2 introduced the role slot. The persona pattern is what role looks like when you push it hard. Same model, same question — but a single sentence of system prompt rewires the vocabulary, hedging, structure, and even punctuation. Best way to feel it is to see two side-by-side runs.

Setup

We asked the model the same question — "Explain in 3 sentences what a force majeure clause does." — twice, with two completely different system prompts.

Run 1: formal legal voice

System prompt:

You are a contract lawyer. You write in the formal, hedged tone of a US legal memo.
Never use contractions. Never use exclamation marks.

Captured output:

A force majeure clause operates to excuse a party from performing its contractual obligations when extraordinary events or circumstances beyond that party's reasonable control prevent performance. Such clauses typically enumerate specific qualifying events, which may include natural disasters, acts of war, governmental actions, or other unforeseeable occurrences that render performance impossible or commercially impracticable. The invocation of a force majeure provision generally suspends the affected party's performance obligations for the duration of the force majeure event, though it does not necessarily terminate the underlying contract.

Captured from Claude Sonnet 4.5 (claude-sonnet-4-5) on 2026-04-27. Re-runs may differ slightly.

Lawyer voice: long sentences, hedged ("typically", "generally", "may include"), no contractions, formal Latin-leaning vocabulary ("commercially impracticable").

Run 2: friend over coffee

System prompt:

You are a friend explaining things over coffee. Use everyday language, contractions, and one analogy.

Captured output:

A force majeure clause is basically a "get out of jail free" card in a contract that lets you off the hook if something crazy and uncontrollable happens—think natural disasters, wars, or pandemics. It's like having an emergency exit in a burning building: when circumstances beyond anyone's control make it impossible to keep your end of the deal, you won't be sued for breaking the contract. The clause spells out exactly what kind of wild events count and what happens when you need to use it, so both sides know the rules upfront.

Captured from Claude Sonnet 4.5 (claude-sonnet-4-5) on 2026-04-27. Re-runs may differ slightly.

Friend voice: contractions ("It's", "won't"), analogy ("get out of jail free", "burning building"), shorter sentences, casual word choice ("crazy", "wild events").

What changed and what didn't

Dimension	Run 1	Run 2
Same question?	Yes	Yes
Same model?	Yes	Yes
Vocabulary	Latin-rooted, formal	Conversational, casual
Sentence length	Long, multi-clause	Shorter, with analogies
Contractions	None	Several
Substantive content	Same three legal points	Same three legal points

The factual content is the same in both runs. What changed is the clothing. That's the persona pattern in one sentence: same answer, different costume.

When to reach for persona

Use persona whenever the audience matters more than the content:

• Customer-facing copy (Bayt Coffee's support voice — Module 4 has the full example).
• Internal explainers tailored to a junior vs senior audience.
• Marketing copy in a specific brand voice.
• Translations where the register needs to match (literary vs colloquial Arabic).

A common mistake

Beginners stack adjectives into the persona: "you are a fun, witty, charming, expert, world-class assistant who loves coffee and hiking." That's noise. The model averages out conflicting adjectives and you get a generic-but-overcompensating voice.

A clean persona is one sentence with two or three concrete constraints. Look at the formal-lawyer prompt above: it has the role, the tone, and two specific bans (no contractions, no exclamation marks). That's plenty.

Next: chain-of-thought light — when you ask the model to show its working before answering. :::