We published a new paper: Agents in the Commons. It measures how much of Reddit is now written by language models, where, since when, and by what kind of accounts. The study took a few days, between nearly a dozen Claude Opus 5.5 agents, and analyzes millions of Reddit posts since before and after the advent of ChatGPT.
What the study is
Reddit sells access to its archive on the basis that it's real people. Google and OpenAI both licensed it in 2024, after the release of LLMs and agent harnesses. Published estimates of how much Reddit text is AI-written disagree by about ten times (from 2.5% to 15%), and none of them correct for how often detectors get it wrong.
We sampled 158 subreddits every month from January 2019 to September 2026 and scored the text locally with an open AI-text detector. Then we calibrated the detector on Reddit itself. False positives were measured on comments written before ChatGPT existed. Detection rates were measured on Reddit-style replies we generated, from plain to deliberately disguised. Every estimate in the paper corrects for both.
Most interesting findings
About 2.3% of substantive comments and 7.9% of substantive posts are now model-written. That's roughly one comment in forty and one post in thirteen, over the last twelve months, counting items of 50 words or more.
Before ChatGPT, the share was zero. It jumped at ChatGPT's release and rose again from mid-2024.
It's concentrated where text makes money. Business and marketing communities are at 23%. Sports and gaming are at 0.1%. The median subreddit is at 0.7%. The highest individual subreddits: r/AppBusiness (45%), r/juststart (37%) and r/passive_income (29%).
The accounts look disposable. 34% of sampled accounts with a flagged item write mostly model text. They start posting a median of 5 days after first appearing, 77% stop posting within the study window, and their posts get removed more often (3.4% vs 0.6% for matched human accounts).
They keep human hours. Unlike agents on agent-only sites, these accounts don't post on a fixed schedule. Their activity follows a normal daily pattern, about seven hours earlier than human accounts.
People notice. 15.4% of threads started by these accounts get at least one reply accusing the poster of being AI, against 1.3% for human-started threads.
Limitations
The exact level depends on how well real agents avoid detection, which nobody can measure directly. If the detector caught every model-written comment, the share would be 1.4%; if it caught only a quarter, 5.8%. The rise since ChatGPT holds up under every check, including a second detector. The most recent quarter is lower than the one before it; one quarter is not enough to call a trend.
Read the paper
The full paper is free to read and download at somewhere.systems/research/reddit-agents under CC BY 4.0. It includes the calibration method, all ten pre-registered hypotheses and their results, and a 2.5-minute narrated video. If you train on or analyze Reddit data, the practical takeaway is to treat post-2023 data from marketing and AI communities as partly synthetic.