AI Referral Traffic: Which AI Engines Send utm_source, and Which Hide It

ChatGPT stopped adding utm_source=chatgpt.com to many links on October 9, 2026, and the tag came back on October 15. Adobe's Cedric Huesler said OpenAI confirmed the change was a bug, per Search Engine Journal. We found no public OpenAI statement.

The drop began with GPT-6. The Prompt Insider dates the rollout to October 7 for paid plans and October 8 for free users. Huesler said GPT-6 opens links with noopener,noreferrer, which removes the tag and the referrer at once. His note went out through Search Engine Roundtable on October 14.

Four teams measured the drop, and the numbers agree on direction. Search Engine Land covered the Adobe analysis on October 15. Whether visits were lost or only relabeled is still open. Adobe saw direct traffic rise as ChatGPT traffic fell.

A fix began rolling out early on October 15. Huesler said tagged ChatGPT clicks were back to about 95 to 100% of the same hour a week earlier by 9 a.m. PT, across thousands of domains. Glenn Gabe had not yet seen the change on his accounts, and OpenAI's release notes for October 1 to 15 do not mention link tracking.

Only ChatGPT is widely seen adding a utm_source tag, and Gemini began in September 2026 per vendor and SEJ reports. We found no public evidence that Claude, Perplexity, Copilot, Grok, Mistral or Meta AI add one.

The scorecard below rates 11 engines on two separate things: the referrer the visit carries and whether the engine adds a UTM tag. If the tag is missing, the visit has to be matched by its referrer domain instead.

ChatGPT's tag has limits too. Searchpilot says OpenAI appends utm_source=chatgpt.com to citation links but not utm_medium. SEJ reported that ChatGPT tags only links in answers grounded in web sources. Apps and the Atlas desktop browser often show as direct, per Searchpilot.

Gemini UTM tags are real but undocumented. Google has published no spec, and nobody has confirmed they are permanent.

Search Engine Journal reported on October 1, 2026 that a Reddit user found UTM parameters on Gemini links. John Mueller said he sees them too. He asked whether Gemini passes a referrer that sites can already track, and said it "would be great to have these retained (in addition to the UTM-tagging)."

Vizzybl, an AI monitoring vendor, says tags appeared gradually from September 10. It reports utm_source=gemini on consumer Gemini and utm_source=geminiforbusiness for Workspace and Gemini Enterprise. On September 28, it says 100% of 4,235 captured links carried a tag. It then reports they disappeared after September 29. This is one vendor's own monitoring and no one has verified it.

Mobile is the weak spot. Searchpilot calls the Gemini app the worst mobile performer and says about 91% of its traffic is dark. One Reddit user claimed about 9% referrer pass-through from the Gemini app, per SEJ. Treat that as an anecdote. Gemini links may also end in a text fragment after a # sign. Browsers never send that part to your analytics.

Claude, Perplexity, Copilot, Grok, Mistral and Meta AI traffic usually arrives with a referrer domain and no UTM tag, or as direct traffic.

Claude sends no UTM tag. Searchpilot (May 2026) says "No UTM params from Anthropic" and that claude.ai passes its referrer only inconsistently. Seer Interactive says Claude "still doesn't append UTM parameters by default." Perplexity's web version is the most reliable AI referrer on Searchpilot's list. Its mobile app is about 70% referral and 30% direct.

Public data on Grok, Mistral and Meta AI is thin. Orbit Media lists chat.mistral.ai and meta.ai as referrer domains. Sources disagree on Grok, listing grok.com, grok.x.com and grok.ai. Meta announced Muse Spark in April 2026 to power AI across its apps, per eMarketer. We found no published test of Muse-linked clicks, and none of the pages we read claims a utm_source for these three.

Other LLMs and AI browsers follow the same pattern. Perplexity's Comet browser passes a perplexity.ai referrer, but ChatGPT's Atlas browser often strips it and lands as direct or (not set), per MarTech (Nov 2025). Google's GA4 help page names DeepSeek in its AI assistant list. Semrush's regex covers you.com and search.brave.com. We found no public guide or test that covers Qwen, Kimi, Doubao or Poe, and no page claims a utm_source for any of them.

Their reach is small. SE Ranking found Mistral at 0.22% of EU AI referral traffic only, and Grok near zero after summer 2025. Semrush counted 153 million visits to grok.com in July 2025, so Grok has an audience. It simply sends few clicks to other sites. A missing tag on these engines costs you little today.

AI visits show up as direct traffic when a click carries no UTM tag and no referrer. Either signal can go missing, and the ChatGPT drop removed both at once.

Four things strip a referrer: the link flags (noreferrer), in-app browsers, app-to-browser handoffs, and copy and paste. That is why the tag matters most on mobile. Graphite found 95% of mobile ChatGPT visits to its sites relied on the tag alone.

Adobe's data shows the leak. On mobile, new direct visitors who saw a consent banner rose from 36% to 44%. ChatGPT visitors saw one 55% of the time. The new direct visitors were acting like ChatGPT visitors.

Google AI Mode has the same problem. In May 2025, Google called the stripped referrer a bug, with John Mueller saying "considering this a bug," per PPC Land. Searchpilot says in May 2026 that noreferrer still strips the referrer on most AI Mode clicks.

Since May 13, 2026, GA4 has an AI Assistant default channel. It matches a medium of "ai-assistant" or a referrer on Google's AI assistant list, per Google's help page.

Google names ChatGPT, Gemini, DeepSeek, Copilot and Grok as examples. The full list sits in a spreadsheet Google links but does not show on the page. SEJ named Claude among its examples, so check your own data. Google says the channel excludes AI Overviews and AI Mode, which stay in organic search.

Tags and channels do not always line up. Google's rules key off the medium and the referrer, and ChatGPT adds only a source tag. Test that a tagged click lands in the AI Assistant channel on your property. Visits from before May 13 are not reclassified.

ChatGPT sends most AI referral traffic, but its share depends on who you ask. It was 74.78% in SE Ranking's data and 62.6% in Goodie's B2B-heavy panel.

Claude shows the gap best. It holds 2.62% in SE Ranking's data across 101,574 sites, and 18.5% in Goodie's panel of business-facing brands. If your buyers are technical, Claude may matter more than the all-site numbers suggest.

Track AI traffic in four layers, so a single missing tag cannot erase it: tests, channel rules, a direct-lift check and citation tracking.

Test. Click a cited link from each engine on desktop and mobile, then record the landing URL and the referrer. Repeat after each major model launch. The GPT-6 change went live with no notice.

Segment. Build a custom channel group that matches referrer domains for ChatGPT, Gemini, Claude, Perplexity, Copilot, Grok, Mistral and Meta AI. Add utm_source=chatgpt and utm_source=gemini as sources.

Check direct lift. Compare direct visits to pages that AI engines cite with direct visits to pages they do not. Adobe's method found a rise of about 26,000 direct visits a day while ChatGPT fell. LLM Pulse saw total site traffic stay about flat as ChatGPT sessions fell 46%.

Track citations. Clicks are one signal. Citation tracking shows which of your URLs each engine cites, even when no click or tag arrives. ThriveStack citedby does this across ChatGPT, Gemini, Claude, Perplexity and Google AI. We sell it, so weigh that. See the pricing page, our AI citation sources research and the Google September 2026 spam update guide if your Google traffic moved too.

Put AI visibility on one chart: AI crawler hits, tagged clicks, hidden clicks, citations in answers and revenue. Read it month by month. The chart below uses illustrative data. It is a model, so treat the numbers as an example.

Read the five numbered steps first. Each number matches a mark on the chart below and uses the same color as the line or bar it describes.

1. 1Crawlers arrive first.The dashed line is AI bot hits from server logs. It rises before any human shows up. Server logs show user agents, so they hold up better than UTM tags.
2. 2UTM from AI assistants.Tagged clicks from ChatGPT, Perplexity and Gemini. This is the part a missing tag can erase.
3. 3Hidden in Direct and Organic.Untagged AI clicks land in Direct. AI Overviews and AI Mode sit in Organic. Use the direct-lift check from the plan above.
4. 4Citations in answers.Human-initiated AI citations, found in server logs. They show visibility even when no click arrives.
5. 5Revenue follows.Match signups and deals to the citation in your CRM. Revenue lags traffic by a month or two.

ThriveStack citedby reads this view from your GA4 or ThriveStack tracking. We sell it, so weigh that. See the pricing page.