OpenAI’s ad collector links your ChatGPT account to activity on other websites via a cookie

How the __obi Cookie Is Generated and Stored

When a user visits chatgpt.com, the client creates a 16‑byte random value and sends it to the endpoint /backend‑api/bazaar/obi/sync-token or /backend‑anon/ if the user is signed out.

The backend replies with an RS256 signed JWT that includes the account identifier (sub) and a 22‑character identifier (obi) that is valid for 60 seconds.

The client then posts the JWT to bzr.openai.com/v1/obi/sync, which responds with a Set‑Cookie header that creates the __obi cookie on the .openai.com domain.

The cookie is marked HttpOnly, Secure, SameSite=none, and has a Max‑Age of one year, allowing it to be sent on cross‑site requests.

How Advertiser Sites Forward the Identifier

Any company that purchases ads on ChatGPT embeds a small OpenAI script on its own pages, similar to existing Meta and Google tracking pixels.

When a browser loads that script, it automatically includes the __obi cookie in the request to bzrcdn.openai.com/sdk/oaiq.min.js, exposing the identifier before any OpenAI code runs.

Subsequent POST requests to bzr.openai.com/v1/sdk/events carry the identifier as an obref parameter, linking page events to the same __obi value.

Three request patterns were observed: a script load, a conversion event POST, and a bare‑body POST, all transmitting the cookie.

What Information Is Collected Through the SDK

The SDK categorises data into four sources: explicit advertiser values (in) and three scraped sources labelled fm, ht, and js.

In traffic captured over several months, scraped data (685 events) far exceeded advertiser‑supplied data (255 events).

The tag‑manager bus provides the majority of email addresses by hijacking window.dataLayer.push and reading adobeDataLayer or renamed GTM layers.

Email, phone, first name, and last name are hashed with SHA‑256 before transmission, while country, region, city, and postal code are sent in clear text.

Postal code appears most frequently, recorded in 100 events across 28 sites.

URLs are stripped to origin and path only; none of the 23,929 observed URLs included query strings.

The author reproduced the entire mechanism on a personal phone, verified it with two independent capture methods, and cross‑checked it against traffic from 936 distinct advertiser pixels on 1,029 hostnames.

“The bottom line is that OpenAI can connect what you do on those sites to your ChatGPT account,” the researcher concluded.

This capability effectively extends OpenAI’s visibility beyond the ChatGPT interface to the broader web browsing behavior of its users.

Because the cookie persists for a year, the link between account and external activity can be maintained across multiple sessions and devices.

OpenAI’s internal name for the ads platform, bazaar, and the issuing service, wadi, are reflected in the JWT fields iss and aud.

The mechanism relies on standard web tracking practices, yet the cross‑site identifier is uniquely tied to a logged‑in ChatGPT user.

Users who are signed out receive a token from /backend‑anon/ that still generates a __obi cookie, albeit without an account subject.

The presence of the cookie on third‑party sites means that any advertiser employing OpenAI’s pixel can potentially infer the identity of a ChatGPT user.

OpenAI’s approach mirrors the data‑sharing models used by other major ad networks, but the direct association with a user account is a distinctive feature.

Regulators and privacy advocates may scrutinise this practice, given the potential for profiling and targeted advertising based on ChatGPT usage.

For developers and advertisers, the SDK’s ability to harvest form fields, page text, and tag‑manager data expands the granularity of the collected profile.

OpenAI’s documentation does not publicly disclose the full scope of this tracking, leaving users to discover it through independent analysis.

Understanding the __obi cookie’s lifecycle helps users assess the privacy implications of interacting with ChatGPT‑enabled ads.

Why This Matters: OpenAI can now associate a user’s ChatGPT account with browsing activity on any site that runs its ad pixel, enabling cross‑site profiling. Why This Matters