How to sound formal without sounding like AI: keep the polish, lose the tells

Why formal writing gets flagged as AI

Most detectors score text on predictability, often using a metric called perplexity. Lower perplexity means the next word is easy to guess. Formal writing is naturally predictable: standard sentence shapes, expected vocabulary, few surprises. That is exactly what detector models were trained to flag.

The bias is documented. In the Stanford study by Liang and colleagues, detectors classified 61.22% of TOEFL essays written by non-native English students as AI-generated, and 97% of those essays were flagged by at least one detector. The reason: non-native writers score lower on lexical diversity and complexity, which makes their text look like machine text to a perplexity-based detector. We explain more in how accurate AI detectors actually are.

Turnitin is open about the limits. Its published false positive rate is under 1% at document level for text with 20% or more AI writing, but about 4% at sentence level. That means one sentence in twenty can be highlighted as AI-written when a person wrote it. Formal, precise sentences are the ones most often flagged.

The difference between formal and robotic

Formal is a register. Robotic is a failure of rhythm and voice. The two get confused because AI defaults to formal language, so people assume formality is the problem. It is not. The problem is the mechanical patterns that come with it.

Think of two documents. One is a well-written legal memo: precise, structured, a little distant, but with clear reasoning and the occasional sharp sentence. The other is a list of balanced, equally long sentences that each make one polite point and never take a side. Both are formal. Only the second one reads like AI.

The fix is to keep the register and change the mechanics. You do not need slang, jokes, or contractions to sound human. You need variety, specificity, and a position. If you are trying the opposite direction, making formal AI text less stiff, that guide covers the casual side.

The formal AI tells that give you away

Uniform sentence length is the biggest tell. AI drafts tend to land every sentence at a similar length, usually ten to fifteen words, with the same rhythm. Human formal writing moves: a long sentence that builds an argument, then a short one that lands it.

Smoothing connectors are second. Words whose only job is to make the text look organized. Real formal writing uses logical connectors that carry meaning: because, however, therefore, although. It skips the filler that exists purely for flow.

The list of three is third. AI loves triads: three adjectives, three reasons, three steps. Used constantly, the pattern becomes visible. Human writers use pairs, single points, and occasional threes, not a steady drumbeat of them.

The polite middle distance is fourth. AI text rarely commits. It says one could argue, it is important to note, in many cases. A human with authority says what they think and defends it. You can be formal and still take a position.

Finally, the overpolished symmetry. Every paragraph has a topic sentence, a middle, and a neat wrap. Formal writing does not need to be that tidy. Let some paragraphs end on a concrete fact instead of a summary sentence.

How to keep formal structure without the AI patterns

Vary sentence length on purpose. After a long, careful sentence, follow with a short one. You keep the formal tone, but the rhythm becomes readable and human. Aim for a range, not a rule.

Replace smoothing words with real information. Instead of a connector that signals next point, use a phrase that references what came before. That argument fails for three reasons is formal, specific, and human.

Take a position, then defend it. Formal does not mean neutral. State your view clearly, support it with evidence, and let the reader disagree. This single change removes most of the AI feel from professional writing.

Add concrete details. Names, numbers, dates, references. The audit found issues is AI-safe but weak. The audit found 14 issues across three departments is formal, specific, and clearly written by someone who knows the subject.

Use contractions sparingly, or not at all. Contractions are not required to sound human. Many formal registers avoid them completely. What matters is that the text does not feel stiff and uniform, not whether you use do not or don't.

Formal email and proposal examples

Here is a before and after. The before is a typical AI-generated follow-up email: balanced sentences, polite filler, no specific reason for writing. The after keeps the formal register but reads like a person. For the full workflow, see our guide to professional email writing with AI.

Before: I hope this email finds you well. I am writing to follow up on the proposal we discussed last week. I believe it would be mutually beneficial to schedule a call to discuss next steps. Please let me know your availability at your earliest convenience.

After: Thanks for the time last week. I have two questions about the proposal before we move forward. First, is the timeline workable if we start in September? Second, who should own the handoff on your side? A short call this week would settle both.

The second version is shorter, formal, and specific. It names the questions, the timeline, and the handoff. No filler, no vague politeness, no list of three. That is what formal human writing looks like.

A proposal example. Before: Our solution is designed to streamline operations and drive efficiency across the organization. After: We cut invoice processing time from 6 days to 2 days for three clients in the last year. Here is how we would do it for your team.

A prompt that keeps the register and drops the tells

You can ask AI to write the first draft with the register you need. The trick is telling it what to avoid, not just what to produce. A prompt that says keep it formal produces the robotic version. A prompt that says formal, but vary the rhythm and take a position, produces something closer to a person.

Try this: Draft a formal [email / proposal / report section] for [audience]. Keep a professional register: no slang, no casual tone. Vary sentence length. Take a clear position. Use concrete details where you have them. Do not use smoothing filler. End with a specific next step.

Then edit. The prompt gets you 70% of the way. The last 30% is your judgment: which details to include, which sentences to cut, where to push back. That part is the voice. It is also the part no tool can do for you. For more on tone instructions, see how to tell AI what tone to write in.

When AI is the wrong tool for formal writing

Humanizer tools are not the answer. Independent testing covered by George Kao looked at 16 popular humanizer tools and found only 2 that actually worked. The rest mangled meaning, dropped up to 20% of the original content, and still got flagged. They also run against the spirit of Google's spam policy, which names automated synonymizing and paraphrasing as violations.

For high-stakes formal writing, write the core yourself. Apologies, performance reviews, legal letters, anything where the reader will hold you accountable for every word. AI can gather material and give you a structure, but the final text should be yours.

For routine formal writing, AI with editing is fine. Follow-up emails, status reports, meeting notes. Draft with AI, then apply the checklist below. The combination is faster than writing from scratch and better than sending raw AI output. If tone is the problem, start with our guide to fixing robotic email tone.

The formal writing checklist before you send

Read it out loud. If a sentence makes you stumble, rewrite it. Formal writing should still be speakable, just with more distance than casual speech.

Check sentence variety. Look at five consecutive sentences. If they all start the same way or run the same length, vary them.

Remove filler connectors. If you can delete a word or phrase without losing meaning, delete it.

Confirm you took a position. Find the main claim in the text. If a reader cannot tell what you think, the writing is too safe. Formal writing can be clear and committed at the same time.

Check for triads. If you see three parallel items in a row, decide whether the third one earns its place. Often it exists for rhythm, and cutting it makes the text more human.

Send it. Formal writing that sounds human is not about being less professional. It is about being precise, varied, and present. Keep the polish, lose the tells, and your reader will hear a person, not a model. If the problem started with an AI draft, here is how to fix AI writing that sounds too perfect.

Frequently asked questions