An independent technical investigation published on September 20 claims that OpenAI's ad-collection system plants a cross-site cookie in users' browsers, and that merchant websites running OpenAI's ad pixel can use it to send a visitor's off-site browsing and purchase activity back to OpenAI — tied to that person's ChatGPT account. The report climbed to the top of Hacker News.
The author reproduced the entire flow on his own phone using two independent packet-capture methods, then cross-checked it against months of accumulated traffic data. OpenAI has not responded to the specific questions raised in the piece.
How the cookie makes its round trip
According to the capture logs, the process runs in three steps. The ChatGPT page first generates a 16-byte identifier and calls /backend-api/bazaar/obi/sync-token, which returns an RS256-signed token, valid for 60 seconds, binding the current account to that identifier. The page then submits the token cross-site to bzr.openai.com, which writes a cookie named __obi on the .openai.com domain with the attributes SameSite=None, Secure and HttpOnly, set to expire in 31,536,000 seconds — exactly one year.
Later, when the same user visits a merchant site that has installed OpenAI's ad pixel, the browser sends __obi along with page data back to OpenAI. The fields the pixel collects include SHA-256-hashed email addresses, phone numbers and names, along with plaintext country, region, city and postal code, plus the page path with query parameters stripped out.
Of the 881 pixels the author observed, 638 had auto-matching enabled. Identity data scraped automatically from the page versus data actively submitted by the merchant broke down 685 to 255. Among the recorded paths were medical-condition pages, debt-relief lead-generation pages, and a litigation sign-up form.
Analytics cookie, or marketing cookie?
The most contested point is classification. OpenAI's cookie policy lists __obi as an analytics cookie, described as helping the company understand how its service is used. Yet every sync token the author decoded carried the field consent_decision: analytics_allowed. His conclusion:
"Someone who allows analytics and refuses marketing gets this."
On one of the author's devices, the same __obi value was sent out by 12 different commercial sites, including Chewy, Wayfair, ThriftBooks, Eventbrite, HelloFresh, Coursera and SeatGeek. Across a wider sample, 12 of 30 distinct cookie values showed up at more than one advertiser, and one value appeared at 10 different sites.
On September 14, the author emailed OpenAI's media and privacy contacts with two questions: why is __obi classified as analytics, and do users who decline marketing consent still receive it? OpenAI support confirmed receipt of the message but answered neither question.
Are Chinese-speaking users at risk?
The mechanism has clear limits. The author confirmed the flow works on Android Chrome; Safari on iPhone blocks third-party cookies by default through Intelligent Tracking Prevention, so the chain breaks there, and he did not test desktop Chrome. Roughly one in five ChatGPT sessions generates a sync token at all, suggesting OpenAI is rolling this out in stages.
ChatGPT is not officially available in mainland China, but plenty of Chinese-speaking users elsewhere — Hong Kong, Macau, Taiwan and overseas — log into ChatGPT on Android phones while also shopping on North American e-commerce sites. All 12 named merchants target the U.S. market, so the actual exposure depends on whether you shop on those sites. To cut the chain, turn on "Block third-party cookies" in Chrome's settings.
The investigation rests on packet captures from a single author's own devices and datasets, and OpenAI has neither confirmed nor denied any of the technical details.
Sources: Buchodi independent investigation (cookie attributes, token fields, pixel and hostname counts), Hacker News discussion, CocoLoop, OpenAI Cookie Policy (__obi classification language).