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A 40-line Python detector can flag sentence-length variation and repeated three-word phrases, but its score cannot prove whether a person or an AI wrote a passage. Its author describes using it to triage drafts and reports cases where it misled him; that is useful context, not a controlled accuracy test. Treat the script as a way to spot patterns worth reviewing—not as an authorship verdict.

What the detector measures

The example script combines two hand-selected text features into a heuristic score. It does not identify a writer, inspect a document’s history, or establish where the text came from.

Sentence-length variation (“burstiness”)

The script splits text at a period, exclamation mark, or question mark followed by whitespace, then counts whitespace-separated words in each resulting sentence. It calculates the population coefficient of variation: the standard deviation of sentence word counts divided by their mean. The implementation skips this feature when it finds fewer than three sentences.

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Here, “burstiness” means only that particular coefficient of variation. It is not a universal measure with one standard definition, and it should not be confused with perplexity, which is a different kind of language-model-based measure.

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Repeated three-word sequences

For the second feature, the code lowercases the text, extracts runs of ASCII letters and apostrophes, and counts three-word sequences, or trigrams. It records trigram types that appear at least twice. This is a repetition signal, not a test of whether a phrase is characteristic of an AI system.

How the signals become a score

The final score gives 60% weight to capped inverse burstiness and 40% to a capped count of repeated trigram types. Those fixed weights and caps are choices made by the script’s author; they are not evidence that the score is calibrated or that a particular value corresponds to a reliable probability of AI authorship. Broader descriptions of detectors that examine text properties do not establish that commercial systems use these exact calculations.

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He Harvey He presents the script as a lightweight way to sort drafts. His September 30, 2026, article is the primary source for the implementation and his anecdotes: Burstiness and n-grams: a 40-line Python AI-text detector, and where it breaks.

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Where the approach breaks down

Short passages

Below three sentences, the script does not calculate its sentence-variation feature. He also reports that estimates based on only three or four sentences can swing substantially and says he treated passages under about 40 words as unknown. That roughly 40-word threshold is his practical rule of thumb, not a validated cutoff or a general standard. A score from a short sample should not be read as strong evidence in either direction.

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Formulaic writing

Legal summaries, release notes, and API documentation often follow conventions that produce repeated structures or similar sentence lengths. He reports that the script flagged such human-written material. A feature that marks regularity can therefore mistake the requirements of a genre for evidence about authorship.

Small edits can change the signal

He describes examples in which changing sentence boundaries or order, or replacing a word, removed a repeated-trigram signal without meaningfully changing the prose quality or its author. These examples illustrate sensitivity to editing; they are not a comprehensive robustness audit. They do show why a changed score should not be treated as proof that authorship changed.

Scores do not measure writing quality

A low or high score describes how the chosen measurements behave on a passage. It does not tell you whether the writing is accurate, clear, original, or suitable for its audience. Nor does this script’s score, on its own, establish that a passage is human-written or AI-generated.

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What broader detector evaluations show

Independent studies add an important caution: results depend on what is being evaluated. They do not test this 40-line script, so their findings cannot be used to assign it an accuracy rate.

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  • BUST benchmark: A 2024 Association for Computational Linguistics benchmark assembled 25,000 texts from human writers and seven LLMs, spanning ten tasks and three sources. It evaluated five detectors and found that performance varied substantially across tasks. The study’s coverage underscores why a result on one kind of text cannot automatically be carried over to another. Read the BUST paper.
  • NIST text-to-text pilot: NIST’s 2024 NIST GenAI (Pilot Study): Text-to-Text Evaluation Overview and Results, designated NIST AI 700-1 and published June 25, 2025, reports substantial variation by system and generator. Its abstract says performance varied significantly depending on the systems used; some generators could deceive most discriminators, while some discriminators could detect AI-generated content from almost all generators. This describes the pilot, not the performance of the script discussed here. Read the NIST report.
  • Related metric limitations: A 2023 EMNLP paper discusses why perplexity and burstiness are often unreliable indicators of human-written text, particularly for academic writing and low-resource languages. It also addresses reference-model and out-of-domain limitations. These points support caution about predictability-based inference, but they do not directly measure this script’s error rate. Read the EMNLP paper.

The practical lesson is to evaluate a detector against the intended use: the relevant tasks and genres, the kinds of generators that matter, realistic sample lengths, and both false positives and false negatives. A benchmark that does not resemble the text population in which you plan to use a tool may not tell you how it will behave there.

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How to use the score responsibly

  1. Use it to identify passages for review. Repeated phrasing or unusually uniform sentence lengths may be worth checking, but neither feature identifies a writer.
  2. Review the text itself. Check factual accuracy, clarity, appropriate sourcing, and whether the prose fits the intended audience. These are editorial checks, not conclusions supplied by the detector.
  3. Account for the sample and genre. A short passage gives limited sentence-level information, while technical and formal genres may be patterned by design.
  4. Do not make a consequential authorship decision from this score alone. The script’s author reports no controlled validation, and the broader evaluations above show that detector behavior varies with the evaluation conditions.

What the evidence does—and does not—establish

The code’s two signals and fixed weighting are clear enough to explain and inspect. He’s reported failure cases make plausible, concrete cautions about short samples, formulaic prose, and sensitivity to small edits. But the article does not report a false-positive rate or an accuracy figure for this script, and none of the cited external evaluations tests it. No numerical claim about its reliability is therefore established here.

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