AI bot traffic on websites jumps from 8% to 13% of sessions, 51Degrees finds
51Degrees analyzed more than 3 billion website visits over the 12 months to May 2026 and found AI harvesting bots represented 13% of all web sessions, up from 8% a year earlier, a 62.5% increase. AmazonBot was the most active crawler at 4.1% of sessions, ahead of Meta-ExternalAgent at 2.9% and Microsoft BingBot at 1.35%. Anthropic's bot accounted for 0.19%, OpenAI 0.09%, Perplexity 0.04% and Google Gemini 0.0003%, figures the UK firm said reflect how much of the web those companies have already indexed. Rosewell said bots impose costs on publishers twice: through the bandwidth their crawling consumes, and again when they pull traffic toward AI-generated answers instead of the original site.
Traffic and engagement numbers pulled from publisher analytics increasingly include non-human sessions that land in the same logs media planners review. None of this bot traffic qualifies as valid advertising traffic, according to 51Degrees, though the firm stops short of accusing publishers of passing the sessions off as ad impressions. Agencies buying against publisher-reported reach risk overstating audience size unless bot filtering is applied before the numbers reach a media plan. Cloudflare's separate estimate that agentic AI bots generate 57.4% of global web requests suggests the share of contaminated traffic will keep growing, pushing platforms like Google and Cloudflare toward token-based systems such as Private Access Control Tokens to separate human sessions from bot and AI agent activity.