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Every event Upstack captures starts with whatever identity signals the visitor provides — often just an IP address and browser cookie. Upstack ID enriches these events with resolved identity data, connecting anonymous sessions to known customers and dramatically improving the quality of data sent to your destinations.
This page covers technical implementation — the identity fields on events, how session stitching works, and the impact on destination match quality. For a conceptual overview of identity resolution and how the identity graph works, see Identity Resolution.
Events that pass through identity resolution carry 3–5x more match keys on average. This directly translates to higher EMQ scores on Meta, better audience match rates on TikTok, and more accurate attribution.

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Identity Fields on Events

After identity resolution, events carry these identity-related fields:

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How Identity Attaches to Events

Identity resolution happens at the Resolve stage of the pipeline, between event capture and destination forwarding. When a visitor lands on your store, the Upstack Pixel sets a first-party cookie. Every event in that browsing session shares the same session_id. If the visitor has been seen before (returning visitor), the cookie links to their existing identity profile.

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2. Deterministic Identification

When a visitor provides a hard identifier — logging in, entering an email at checkout, clicking a Klaviyo email link — Upstack ID creates a deterministic match:
  • The session_id is linked to the customer’s identity_id
  • em (hashed email) and ph (hashed phone) are attached to the event
  • external_id is set from the Shopify customer ID

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3. Session Stitching

This is where the real value emerges. When identification happens mid-session (e.g., at checkout), Upstack ID retroactively enriches all prior anonymous events in that session with the resolved identity data. A typical session flow: Without stitching, the PageView, ViewContent, and AddToCart events would be forwarded to destinations with weak identity signals. With stitching, every event in the session carries the full set of match keys.
Session stitching is retroactive within the session. Events are held briefly in the pipeline to allow late-arriving identity data to propagate. The hold window is short enough to maintain near-real-time forwarding.

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4. Cross-Device Linking

When the same email or phone appears across sessions on different devices, Upstack ID merges those sessions under a single identity_id. A shopper who browses on mobile and purchases on desktop is recognized as one customer. Cross-device linking enables:
  • Accurate attribution across devices
  • Larger retargeting audiences (one profile instead of two)
  • Complete customer journey visibility in analytics

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Impact on Destination Match Quality

Identity resolution directly drives the match keys available for each destination: The difference is especially pronounced for upper-funnel events (PageView, ViewContent) that typically fire before the visitor has identified themselves. Session stitching ensures these events reach destinations with full identity data.
Identity resolution depends on the visitor eventually identifying themselves within the session. Fully anonymous sessions (no login, no checkout, no email link click) cannot be resolved. The identification rate varies by store — typically 30–60% of sessions are resolved.

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How This Connects to the Broader Pipeline

Identity resolution sits between event capture and destination forwarding:
  1. Capture — Upstack Pixel fires raw events with whatever signals are available.
  2. Resolve — Upstack ID attaches identity, stitches sessions, and links devices. (You are here.)
  3. Enrich — Product catalog data and UTM parameters are merged.
  4. Activate — Identity-enriched events are forwarded to destinations with maximum match keys.
For a deeper dive into how identity resolution works at the infrastructure level, see the Identity Resolution concept page.

Identity Resolution

Conceptual overview: identity graph, deterministic matching, cross-device linking.

JavaScript SDK

How to call identify() to associate user identity with sessions.

Properties & Context

Full reference for all identity fields, custom data, and context on events.

Standard Events

Events that carry identity data after resolution.