# Can AI detectors catch Claude Opus 5.5?

Source: https://toolbenchmarks.com/can-ai-detectors-catch/claude-opus-5-5/  
Tested: 30 October 2026. Publisher: Tool Benchmarks. Data licence: CC BY 4.0.

> Only about half the time. A majority of the seven free detectors we tested (four or more) flagged 52.4% of Claude Opus 5.5 texts. Fast-DetectGPT, one of the two most reliable detectors, caught 63.5%, and a light humanizer rewrite pushed Desklib's catch rate down from 62.7% to 32.5%.

| Detector | Share flagged as AI |
|---|---|
| Fast-DetectGPT (Qwen2.5-1.5B) | 63.5% (80 of 126) |
| Desklib AI text detector | 62.7% (79 of 126) |
| Fakespot AI text detector | 65.1% (82 of 126) |
| MAGE | 45.2% (57 of 126) |
| E5-small LoRA detector | 64.3% (81 of 126) |
| OpenAI RoBERTa detector | 2.4% (3 of 126) |
| TMR (Target Mining RoBERTa) | 83.3% (105 of 126) |

## How we tested Opus

Claude Opus 5.5 answered the same 150 writing prompts as the other three models, each asking for a text on the topic, in the genre and at about the length of one human text in our set. The prompts asked for plain text the way a person would ask a chatbot, with no instruction to sound human. The middle Opus text was 258 words long. We report on the 126 texts outside the calibration split.

## Results

Three detectors caught Opus text at similar rates: Fakespot at 65.1%, E5-small LoRA at 64.3% and Fast-DetectGPT at 63.5%, with Desklib close behind at 62.7%. The catch rate alone flatters Fakespot and E5-small LoRA, which flagged 34.9% and 50.0% of human texts. Desklib and Fast-DetectGPT are the two whose flags mean something.

TMR caught 83.3% of Opus texts, the highest number here, and it is meaningless: TMR flagged 83.3% of human texts as well. The OpenAI RoBERTa detector caught 2.4%.

Opus was the second hardest model to detect, after Claude Fable 5.1. Our [humanized text results](/humanized-ai-text/) show what a light edit does to the same Opus texts.

## Questions people ask

### Can GPTZero or Turnitin detect Claude Opus 5.5?

We have not tested commercial detectors, so we cannot say. Among the seven open-source detectors we did test, a majority caught 52.4% of Opus texts.

### Is Claude Opus 5.5 harder to detect than Haiku?

Yes. A majority of detectors caught 77.0% of Claude Haiku 5.5 texts but only 52.4% of Opus texts.

### Does rewriting Opus text make it undetectable?

Not completely, but it helps a lot. After a humanizer rewrite, Desklib caught 32.5% of the texts and Fast-DetectGPT 47.6%, against 62.7% and 63.5% for the original versions.