The Client ID (cid) is a 14-character alphanumeric string that anonymously identifies a visitor. It answers who — as opposed to the Click ID, which answers which action.
Where the Client ID comes from
Section titled “Where the Client ID comes from”It is set as a first-party cookie by the Tracking Tag when a visitor lands, or minted server-side via the collect endpoint when the Tag can’t run (Facebook Lead Ads, offline conversions, headless checkouts, or a contact created directly inside a CRM).
How the Client ID is used
Section titled “How the Client ID is used”To raise Event Match Quality, AnyTrack sends the Client ID back to Facebook Ads and TikTok as external_id through the Conversion API. ChatGPT Ads receives it as external_ids_sha256, and Microsoft Ads as anonymousId. On Google Ads, the Tracking Tag sets the Client ID as user_id on the Google tag in the browser. The server-side connection to Google Ads sends the Google client ID. In server-side setups, you can send the Property ID joined to the Client ID ({propertyid}{cid}, 26 characters). AnyTrack matches it to the latest event of the visitor before the conversion.
Client ID vs External ID
Section titled “Client ID vs External ID”When the Click ID can’t reach a destination (such as a CRM), you alias the visitor with an External ID (refId) — an email, phone, or your own customer ID. The External ID is something you control; the Client ID is the anonymous cookie AnyTrack sets.
Frequently asked questions
FAQ was last reviewed on 2026-11-01