AI Detector Comparison

Supported AI Detectors

Last updated: 2026-04-25

Five primary detectors plus optional extras. Each entry covers: API endpoint, auth, known accuracy issues, and the citation that documents the issue.

Contents

• 1. GPTZero
• 2. Originality.ai
• 3. ZeroGPT
• 4. Sapling
• 5. Copyleaks
• Optional / extended detectors
• Why the spread matters
• Quick stats to drop in a reply

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1. GPTZero

• Web: https://gptzero.me
• API docs: https://api.gptzero.me/v2/predict/text
• Auth: x-api-key header. Free tier: 10k words/month. Paid from $9.99/mo.
• Returns: documents[0].class_probabilities.ai (0.0-1.0) plus per-sentence breakdown.

Known issues:

• Stanford study (Liang et al. 2023) included GPTZero in the cohort that flagged 61.3% of TOEFL essays from non-native English writers as AI. ESL bias is documented and reproducible.
• Inflates scores on technical / dense prose regardless of authorship.
• Will not run on text under 250 characters; gives unstable scores under 100 words.

Citation: Liang, W., Yuksekgonul, M., Mao, Y., Wu, E., & Zou, J. (2023). "GPT detectors are biased against non-native English writers." Patterns, 4(7). https://doi.org/10.1016/j.patter.2023.100779

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2. Originality.ai

• Web: https://originality.ai
• API docs: https://docs.originality.ai/
• Auth: X-OAI-API-KEY header. No free tier — $0.01 per 100 words minimum.
• Returns: score.ai (0.0-1.0), score.original (0.0-1.0).

Known issues:

• Marketed as "99% accurate" but multiple independent tests put real-world accuracy in the 60-80% range.
• Aggressively flags any text that has been edited by Grammarly or similar tools, since editing patterns mimic LLM patterns.
• Sergey's team meeting test (2026): scored a hand-written article 100% AI while GPTZero scored the same article 82% and ZeroGPT scored 50%. 50-point spread on identical text.

Citation: Internal CCC team test, March 2026 meeting transcript (projects/coactor/transcripts/); also referenced in Sergey Bulaev's April 2026 LinkedIn post on detector unreliability.

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3. ZeroGPT

• Web: https://www.zerogpt.com
• API docs: https://api.zerogpt.com/api/detect/detectText
• Auth: ApiKey header. Free tier: 5 requests/min. Paid plans available.
• Returns: data.fakePercentage (0-100 integer), data.isHuman boolean.

Known issues:

• Famously unstable — the same input pasted twice 30 seconds apart can return scores 20+ points apart.
• Flags US Constitution, Bible verses, and Declaration of Independence at 90%+ AI when pasted as plain text.
• Susceptible to trivial paraphrasing — adding two typos drops a 95% score to 30%.

Citation: Multiple replicated demos on Twitter/X 2023-2024; Vanderbilt University communication on disabling Turnitin (Aug 2023) cited similar instability across the detector category. https://www.vanderbilt.edu/brightspace/2023/08/16/guidance-on-ai-detection-and-why-were-disabling-turnitins-ai-detector/

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4. Sapling

• Web: https://sapling.ai/ai-content-detector
• API docs: https://sapling.ai/docs/api/aidetect
• Auth: key field in JSON body. Free tier: 50 requests/day.
• Returns: score (0.0-1.0), per-sentence sentence_scores.

Known issues:

• Tends to score lower than GPTZero/Originality on the same text — useful as a contrarian signal in the parallel test.
• Worse on creative writing than on technical prose.
• Does not handle markdown — strip formatting before sending.

Citation: Sapling's own published benchmarks (https://sapling.ai/ai-content-detector/benchmark) acknowledge ~3-5% false positive rate even in their best-case dataset.

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5. Copyleaks

• Web: https://copyleaks.com/ai-content-detector
• API docs: https://api.copyleaks.com/documentation/v3/writer-detector/submit
• Auth: 2-step. POST to /v3/account/login with email + key, get bearer token, then POST to /v2/writer-detector/{scanId}/check.
• Returns: summary.ai, which Copyleaks has documented both as a 0-1 probability and as a 0-100 percentage. test_detectors.py normalises either shape and rejects anything that still falls outside 0-100. Per-paragraph breakdown alongside it.

Known issues:

• Adelphi University used Copyleaks-style detector output as the sole evidence in the case that became Newby v. Adelphi University (Oct 2025). Federal court ordered the violation expunged.
• Heavily penalizes formal academic writing regardless of authorship.
• Unstable across re-submissions of the same text.

Citation: Newby v. Adelphi University, U.S. District Court (E.D.N.Y.), October 2025. Coverage: Inside Higher Ed, "Court Orders University to Drop AI-Cheating Charge" (Oct 2025).

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Optional / extended detectors

None of these have a free API, so the script cannot call them. Run them by hand and enter the scores with --manual; there is no --extra flag.

• Turnitin AI Writing — disabled by Vanderbilt, Cambridge, others. No public API; institutional only.
• Winston AI — https://gowinston.ai. Paid only.
• Crossplag AI — https://crossplag.com. Paid only.
• Writer.com AI Content Detector — free web UI, no API. Use --manual mode.
• Scribbr AI Detector — free web UI, no API. Use --manual mode.

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Why the spread matters

OpenAI shut down its own AI Text Classifier in July 2023 with this public statement: "low rate of accuracy" — internally measured at 26%. If the company that ships the model cannot reliably detect its own output, no third-party detector built on weaker signals can be trusted as ground truth.

Reference: OpenAI blog, "New AI classifier for indicating AI-written text" (Jan 31, 2023), updated July 2023 with discontinuation notice.

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Quick stats to drop in a reply

• 61.3% — TOEFL essays by ESL writers misclassified as AI by 7 detectors (Stanford 2023)
• 5.1% — same detectors' false positive rate on US 8th-grade essays (Stanford 2023)
• 26% — OpenAI's own classifier accuracy before shutdown (July 2023)
• 50 points — spread observed on a single article in CCC team testing (2026)
• 0 — number of US courts that have upheld a "detector said so" finding without corroborating evidence as of April 2026