GPT-6.1 Terra caught 79 of 90 planted mistakes, 4th of 16 models tested. It was strongest on grammar and spelling (42 of 45) and missed most on redundancies (11 of 15). The 4 checks took 29s and cost $0.07; asked for all of them in one prompt, it caught 76.
Every comment GPT-6.1 Terra left on our 6,000-word test story, where it left it. Click a missed mistake to find it in the text. The checks come from icanwrite, a markdown editor for writers.