---

name: fullstory-identify-users version: v2 description: Comprehensive guide for implementing Fullstory's User Identification API (setIdentity) across web applications. Teaches proper uid handling, property passing, re-identification behavior, and session management. Includes detailed good/bad examples for login flows, multi-account scenarios, and SPA applications to help developers correctly identify users for analytics and session replay. related_skills:

• fullstory-anonymize-users
• fullstory-user-properties
• fullstory-user-consent
• fullstory-async-methods
• fullstory-privacy-strategy
• fullstory-banking
• fullstory-healthcare
• fullstory-saas

---

Fullstory Identify Users API

Overview

Fullstory's User Identification API allows developers to associate session data with your own unique customer identifiers. By calling FS('setIdentity'), you link a user's Fullstory session to their identity in your system, enabling you to:

• Search for sessions by customer ID
• View all sessions for a specific user across devices
• Connect Fullstory data with your internal analytics and CRM systems
• Attribute behavior patterns to known users

This skill covers implementation patterns, best practices, and common pitfalls for browser/web applications using the v2 Browser API.

Core Concepts

When Identification Happens

• On Login: Call setIdentity immediately after a successful authentication
• On Page Load (Already Authenticated): Call setIdentity on every page load if the user is logged in
• After Authentication Redirects: Ensure identification persists across OAuth/SSO redirects

User Identity vs Anonymous Sessions

• Anonymous Session: Default state before setIdentity is called. User is tracked via the fs_uid first-party cookie but not linked to your system.
• Identified Session: After setIdentity, the session is permanently linked to the provided uid.

How Identification Works (Cookie Behavior)

Fullstory uses a first-party cookie (fs_uid) to track users across sessions.

Why First-Party Cookies Matter

Key Benefit: Because Fullstory uses first-party cookies on YOUR domain, user identity cannot be connected between multiple sites using Fullstory. Each site has its own separate fs_uid cookie - your customers' data is isolated to your site only.

Cookie-Based Session Linking

1. Before identification: The fs_uid cookie links all sessions from the same browser/device together (persists for 1 year unless deleted)
2. When setIdentity is called: ALL previous anonymous sessions with that same fs_uid cookie are retroactively merged into the identified user
3. Cross-device linking: If the same uid is used on different devices, all sessions across all devices are linked

Copy┌─────────────────────────────────────────────────────────────────────────┐
│                    Session Merging on Identification                     │
├─────────────────────────────────────────────────────────────────────────┤
│  Day 1: Anonymous visit          fs_uid: abc123  → "Anonymous User"     │
│  Day 3: Anonymous visit          fs_uid: abc123  → Same anonymous user  │
│  Day 7: User logs in, setIdentity(uid: "user_456")                      │
│         ↓                                                               │
│  Result: ALL sessions (Day 1, 3, 7) now linked to "user_456"           │
└─────────────────────────────────────────────────────────────────────────┘

Important: This means a user's entire journey from first visit through conversion can be tracked, even if they only identify on their 5th session.

Reference: Why Fullstory uses First-Party Cookies

Re-identification Behavior

• CRITICAL: You cannot change a user's identity once assigned within a session
• If you call setIdentity with a different uid, Fullstory automatically splits the session into a new session
• This is by design to maintain data integrity and prevent identity pollution

Key Principles

1. Use a stable, unique identifier (database ID, UUID) - never use PII like email as the uid
2. Call setIdentity as early as possible after authentication
3. Include meaningful properties for searchability (displayName, email)
4. Handle logout properly by calling anonymize

setIdentity vs setProperties (User)

Important: The properties object in setIdentity is a convenience - you can include initial properties when identifying. However, for updating properties after identification or for anonymous users, use setProperties with type: 'user' instead. See the fullstory-user-properties skill for details.

Copy// At login - identify with initial properties
FS('setIdentity', {
  uid: user.id,
  properties: {displayName: user.name, email: user.email, plan: user.plan},
})

// Later - user upgrades plan (DON'T re-identify!)
FS('setProperties', {
  type: 'user',
  properties: {plan: 'enterprise', upgraded_at: new Date().toISOString()},
})

---

API Reference

Basic Syntax

CopyFS('setIdentity', {
  uid: string,           // Required: Your unique user identifier
  properties?: object,   // Optional: Additional user properties
  schema?: object        // Optional: Type inference for properties
});

Parameters

Special Property Fields

Supported Property Types

Rate Limits

• Sustained: 30 calls per page per minute
• Burst: 10 calls per second

---

✅ GOOD IMPLEMENTATION EXAMPLES

Example 1: Basic Login Identification

Copy// GOOD: Call setIdentity immediately after successful login
async function handleLogin(credentials) {
  try {
    const response = await authenticateUser(credentials)
    const user = response.user

    // Identify user in Fullstory right after successful auth
    FS('setIdentity', {
      uid: user.id, // Use stable database ID, not email
      properties: {
        displayName: user.fullName,
        email: user.email,
        accountType: user.plan,
        signupDate: user.createdAt, // ISO8601 format
      },
    })

    // Continue with login flow
    redirectToDashboard()
  } catch (error) {
    handleLoginError(error)
  }
}

Why this is good:

• ✅ Uses stable database ID as uid, not PII
• ✅ Called immediately after successful authentication
• ✅ Includes displayName for easy identification in Fullstory
• ✅ Includes email for searchability
• ✅ Adds business-relevant properties (accountType, signupDate)

Example 2: Page Load with Existing Session (SPA/SSR)

Copy// GOOD: Check authentication state and identify on every page load
function initializeFullstory() {
  const currentUser = getCurrentAuthenticatedUser()

  if (currentUser && currentUser.id) {
    // User is logged in - identify them
    FS('setIdentity', {
      uid: currentUser.id,
      properties: {
        displayName: currentUser.name,
        email: currentUser.email,
        role: currentUser.role,
        companyId: currentUser.organizationId,
        plan: currentUser.subscription.plan,
        trialEndsAt: currentUser.subscription.trialEnd,
      },
    })
  }
  // If not logged in, user remains anonymous (default state)
}

// Call on app initialization
document.addEventListener('DOMContentLoaded', initializeFullstory)

Why this is good:

• ✅ Handles both authenticated and anonymous states
• ✅ Calls on every page load to ensure identification survives navigation
• ✅ Includes organization context for B2B analytics
• ✅ Captures subscription data for segment analysis

Example 3: React/Next.js Integration

Copy// GOOD: React hook for Fullstory identification
import {useEffect} from 'react'
import {useAuth} from './auth-context'

function useFullstoryIdentity() {
  const {user, isAuthenticated, isLoading} = useAuth()

  useEffect(() => {
    // Wait for auth state to be determined
    if (isLoading) return

    if (isAuthenticated && user) {
      FS('setIdentity', {
        uid: user.id,
        properties: {
          displayName: `${user.firstName} ${user.lastName}`,
          email: user.email,
          role: user.role,
          teamSize: user.team?.memberCount,
          features: user.enabledFeatures, // Array of strings
          lastLoginAt: new Date().toISOString(),
        },
      })
    }
  }, [user, isAuthenticated, isLoading])
}

// Usage in App component
function App() {
  useFullstoryIdentity()

  return <AppContent />
}

Why this is good:

• ✅ Waits for auth state to stabilize before identifying
• ✅ Re-runs when user state changes
• ✅ Handles loading states properly
• ✅ Captures feature flags for segmentation
• ✅ Updates lastLoginAt for recency tracking

Example 4: OAuth/SSO Callback Handling

Copy// GOOD: Identify user after OAuth callback
async function handleOAuthCallback() {
  const urlParams = new URLSearchParams(window.location.search)
  const code = urlParams.get('code')

  if (!code) {
    redirectToLogin()
    return
  }

  try {
    // Exchange code for tokens and user info
    const {user, tokens} = await exchangeOAuthCode(code)

    // Store tokens
    setAuthTokens(tokens)

    // Identify in Fullstory BEFORE redirecting away
    FS('setIdentity', {
      uid: user.id,
      properties: {
        displayName: user.name,
        email: user.email,
        authProvider: 'google', // Track SSO provider
        ssoOrganization: user.hostedDomain,
      },
    })

    // Now safe to redirect
    redirectToDashboard()
  } catch (error) {
    handleOAuthError(error)
  }
}

Why this is good:

• ✅ Identifies user before any redirects
• ✅ Captures authentication provider for analytics
• ✅ Handles SSO organization context
• ✅ Proper error handling flow

Example 5: With Explicit Schema Types

Copy// GOOD: Using schema for explicit type control
FS('setIdentity', {
  uid: 'usr_a1b2c3d4e5',
  properties: {
    displayName: 'Sarah Johnson',
    email: 'sarah.johnson@company.com',
    accountBalance: 1500.5,
    loginCount: 42,
    isPremium: true,
    signupDate: '2023-06-15T00:00:00Z',
    permissions: ['read', 'write', 'admin'],
  },
  schema: {
    accountBalance: 'real',
    loginCount: 'int',
    isPremium: 'bool',
    signupDate: 'date',
    permissions: 'strs',
  },
})

Why this is good:

• ✅ Explicit schema ensures proper type handling
• ✅ Enables numeric comparisons in Fullstory search
• ✅ Boolean enables is/is-not filtering
• ✅ Date enables time-based queries
• ✅ String array enables "contains" searches

---

❌ BAD IMPLEMENTATION EXAMPLES

Example 1: Using Email as UID

Copy// BAD: Using email as the unique identifier
FS('setIdentity', {
  uid: user.email, // BAD: PII as uid
  properties: {
    displayName: user.name,
  },
})

Why this is bad:

• ❌ Email is PII - exposes it in Fullstory URLs and exports
• ❌ Email can change when user updates their email address
• ❌ May cause session linking issues if email is reused
• ❌ Violates privacy best practices

CORRECTED VERSION:

Copy// GOOD: Use stable database ID
FS('setIdentity', {
  uid: user.id, // Stable, non-PII identifier
  properties: {
    displayName: user.name,
    email: user.email, // Email as a searchable property, not the uid
  },
})

Example 2: Identifying Before Authentication Completes

Copy// BAD: Calling setIdentity before auth is confirmed
function handleLoginClick() {
  const credentials = getFormCredentials()

  // BAD: Identifying with form data before server confirms identity
  FS('setIdentity', {
    uid: credentials.username,
    properties: {
      email: credentials.email,
    },
  })

  // This might fail - user isn't actually authenticated yet!
  authenticateUser(credentials)
}

Why this is bad:

• ❌ Identifies user before authentication succeeds
• ❌ If login fails, wrong identity is associated with session
• ❌ Username from form may not match actual user ID
• ❌ Creates data integrity issues

CORRECTED VERSION:

Copy// GOOD: Only identify after successful authentication
async function handleLoginClick() {
  const credentials = getFormCredentials()

  try {
    const response = await authenticateUser(credentials)

    // Only identify AFTER server confirms auth
    FS('setIdentity', {
      uid: response.user.id,
      properties: {
        displayName: response.user.name,
        email: response.user.email,
      },
    })

    redirectToDashboard()
  } catch (error) {
    // Login failed - user remains anonymous
    showLoginError(error)
  }
}

Example 3: Attempting to Change Identity

Copy// BAD: Trying to switch users without proper anonymization
function switchUserAccount(newUser) {
  // This will cause a session split, not update the identity!
  FS('setIdentity', {
    uid: newUser.id,
    properties: {
      displayName: newUser.name,
    },
  })
}

Why this is bad:

• ❌ Cannot change identity of an already-identified user
• ❌ Causes automatic session split (may be unexpected)
• ❌ Creates confusing user journey in Fullstory
• ❌ May fragment analytics data

CORRECTED VERSION:

Copy// GOOD: Properly handle account switching
async function switchUserAccount(newUser) {
  // First, anonymize the current session
  FS('setIdentity', {anonymous: true})

  // Then identify as the new user (starts fresh session)
  FS('setIdentity', {
    uid: newUser.id,
    properties: {
      displayName: newUser.name,
      email: newUser.email,
    },
  })
}

Example 4: Calling setIdentity on Every Interaction

Copy// BAD: Over-calling setIdentity
function handleButtonClick(buttonId) {
  // BAD: Don't call setIdentity on every interaction!
  FS('setIdentity', {
    uid: currentUser.id,
    properties: {
      displayName: currentUser.name,
      lastAction: buttonId, // Trying to track actions via identity
    },
  })

  performAction(buttonId)
}

Why this is bad:

• ❌ Wastes rate limit quota (30 calls/minute max)
• ❌ May hit burst limit (10 calls/second)
• ❌ Properties on identity shouldn't track transient actions
• ❌ Misuse of API - should use trackEvent for actions

CORRECTED VERSION:

Copy// GOOD: Identify once, use events for actions
// On page load / auth
FS('setIdentity', {
  uid: currentUser.id,
  properties: {
    displayName: currentUser.name,
    email: currentUser.email,
  },
})

// For tracking actions - use trackEvent instead
function handleButtonClick(buttonId) {
  FS('trackEvent', {
    name: 'Button Clicked',
    properties: {
      buttonId: buttonId,
      buttonSection: getButtonSection(buttonId),
    },
  })

  performAction(buttonId)
}

Example 5: Missing displayName and email

Copy// BAD: Minimal identification with no useful properties
FS('setIdentity', {
  uid: user.id,
  // No properties at all!
})

Why this is bad:

• ❌ No displayName - sessions show cryptic ID in Fullstory UI
• ❌ No email - can't search users via email
• ❌ No business context - can't segment users effectively
• ❌ Wastes the opportunity to enrich user data

CORRECTED VERSION:

Copy// GOOD: Rich user context
FS('setIdentity', {
  uid: user.id,
  properties: {
    displayName: user.fullName || user.username,
    email: user.email,
    role: user.role,
    plan: user.subscriptionPlan,
    companyName: user.organization?.name,
    signupDate: user.createdAt,
  },
})

Example 6: Type Mismatches in Properties

Copy// BAD: Wrong value formats for intended types
FS('setIdentity', {
  uid: 'usr_123',
  properties: {
    displayName: 'John Doe',
    loginCount: '42', // BAD: String instead of number
    accountBalance: '$150.00', // BAD: Currency symbol in number
    isPremium: 'yes', // BAD: Should be boolean
    signupDate: 'June 15 2023', // BAD: Not ISO8601 format
  },
  schema: {
    loginCount: 'int',
    accountBalance: 'real',
    isPremium: 'bool',
    signupDate: 'date',
  },
})

Why this is bad:

• ❌ loginCount '42' won't parse correctly as int
• ❌ '$150.00' won't parse as real due to $ symbol
• ❌ 'yes' is not a valid boolean value
• ❌ Date format won't be recognized

CORRECTED VERSION:

Copy// GOOD: Properly formatted values
FS('setIdentity', {
  uid: 'usr_123',
  properties: {
    displayName: 'John Doe',
    loginCount: 42, // Number type
    accountBalance: 150.0, // Clean number
    currency: 'USD', // Currency as separate field
    isPremium: true, // Boolean type
    signupDate: '2023-06-15T00:00:00Z', // ISO8601 format
  },
  schema: {
    loginCount: 'int',
    accountBalance: 'real',
    isPremium: 'bool',
    signupDate: 'date',
  },
})

---

COMMON IMPLEMENTATION PATTERNS

Pattern 1: Authentication State Machine

Copy┌─────────────────┐
│  Anonymous      │ ← Initial state (no setIdentity called)
│  Session        │
└────────┬────────┘
         │ User logs in
         ▼
┌─────────────────┐
│  Identified     │ ← setIdentity({ uid: 'xxx' }) called
│  Session        │
└────────┬────────┘
         │ User logs out
         ▼
┌─────────────────┐
│  New Anonymous  │ ← setIdentity({ anonymous: true }) called
│  Session        │
└─────────────────┘

Pattern 2: Multi-Account Application

Copy// For apps where users can switch between accounts
class FullstoryIdentityManager {
  currentUserId = null

  identify(user) {
    if (this.currentUserId && this.currentUserId !== user.id) {
      // Different user - must anonymize first
      this.anonymize()
    }

    FS('setIdentity', {
      uid: user.id,
      properties: {
        displayName: user.name,
        email: user.email,
      },
    })

    this.currentUserId = user.id
  }

  anonymize() {
    FS('setIdentity', {anonymous: true})
    this.currentUserId = null
  }

  updateProperties(properties) {
    // Use setProperties for property-only updates
    FS('setProperties', {
      type: 'user',
      properties: properties,
    })
  }
}

Pattern 3: Progressive Identification

Copy// For apps with guest checkout → account creation flow
class ProgressiveIdentification {
  // During guest checkout - don't identify yet
  handleGuestCheckout(cart) {
    // User remains anonymous
    // Track the checkout event instead
    FS('trackEvent', {
      name: 'Guest Checkout Started',
      properties: {
        cartValue: cart.total,
        itemCount: cart.items.length,
      },
    })
  }

  // When guest creates account after checkout
  handleAccountCreation(user) {
    // NOW identify them - this links all prior anonymous activity
    FS('setIdentity', {
      uid: user.id,
      properties: {
        displayName: user.name,
        email: user.email,
        accountCreatedDuringCheckout: true,
      },
    })
  }
}

---

INTEGRATION WITH OTHER FS APIS

Identification + User Properties

Copy// setIdentity can include properties
FS('setIdentity', {
  uid: user.id,
  properties: {
    displayName: user.name,
    email: user.email,
    plan: 'starter', // Initial plan at signup
  },
})

// Later, update properties without re-identifying
// (user upgraded their plan)
FS('setProperties', {
  type: 'user',
  properties: {
    plan: 'professional',
    upgradedAt: new Date().toISOString(),
  },
})

Identification + Events

Copy// Identify user
FS('setIdentity', {
  uid: user.id,
  properties: {displayName: user.name},
})

// Events are automatically linked to identified user
FS('trackEvent', {
  name: 'Feature Used',
  properties: {
    featureName: 'Advanced Export',
    exportFormat: 'CSV',
  },
})

Identification + Page Properties

Copy// User identity
FS('setIdentity', {
  uid: user.id,
  properties: {displayName: user.name},
})

// Page context (separate concern)
FS('setProperties', {
  type: 'page',
  properties: {
    pageName: 'Dashboard',
    dashboardView: 'analytics',
  },
})

---

TROUBLESHOOTING

Sessions Not Linking to User

Symptom: User appears anonymous despite calling setIdentity

Common Causes:

1. ❌ setIdentity called after Fullstory script fully loaded
2. ❌ uid is undefined, null, or empty string
3. ❌ Fullstory blocked by ad blocker
4. ❌ Browser privacy mode preventing storage

Solutions:

• ✅ Verify uid is a non-empty string before calling
• ✅ Check browser console for Fullstory errors
• ✅ Use async API: await FS('setIdentityAsync', {...})
• ✅ Verify Fullstory snippet is loading correctly

Unexpected Session Splits

Symptom: User has many fragmented sessions

Common Causes:

1. ❌ Calling setIdentity with different uid
2. ❌ Calling setIdentity on every page without checking current identity
3. ❌ Identity state not persisting across page navigations

Solutions:

• ✅ Track current identity state in your app
• ✅ Only call setIdentity when actually identifying/changing users
• ✅ Use proper logout flow with anonymize

Properties Not Appearing

Symptom: User properties don't show in Fullstory

Common Causes:

1. ❌ Property values have wrong types
2. ❌ Property names have invalid characters
3. ❌ Hitting property limits

Solutions:

• ✅ Use schema to specify types explicitly
• ✅ Use camelCase or snake_case for property names
• ✅ Check property limits (varies by plan)

---

LIMITS AND CONSTRAINTS

UID Requirements

• Maximum 256 characters
• Must be a non-empty string
• Should be stable and unique per user
• Should NOT be PII (don't use email, phone, etc.)

Property Limits

• Check your Fullstory plan for specific limits
• Property names: alphanumeric, underscores, hyphens
• High cardinality properties may be limited

Call Frequency

• Sustained: 30 calls per page per minute
• Burst: 10 calls per second
• Exceeding limits may result in dropped calls

---

KEY TAKEAWAYS FOR AGENT

When helping developers implement User Identification:

1. Always emphasize:

   • Use stable database IDs as uid, never PII
   • Call setIdentity AFTER successful authentication
   • Include displayName and email as properties
   • You cannot change identity without anonymizing first

2. Common mistakes to watch for:

   • Using email as uid
   • Identifying before auth completes
   • Calling setIdentity excessively
   • Trying to update identity instead of using setProperties
   • Missing displayName/email properties

3. Questions to ask developers:

   • What's your unique user identifier? (database ID, UUID)
   • When does authentication complete in your flow?
   • Do users switch between accounts in your app?
   • What user attributes are important for segmentation?

4. Integration considerations:

   • SPA: Ensure identification survives client-side navigation
   • SSR: Call on both server hydration and client-side auth changes
   • OAuth: Identify in the callback, before redirects
   • Mobile web: Handle app-to-browser identity handoffs

---

REFERENCE LINKS

• Identify Users: https://developer.fullstory.com/browser/identification/identify-users/
• Anonymize Users: https://developer.fullstory.com/browser/identification/anonymize-users/
• Set User Properties: https://developer.fullstory.com/browser/identification/set-user-properties/
• Custom Properties: https://developer.fullstory.com/browser/custom-properties/
• Help Center - Identifying Users: https://help.fullstory.com/hc/en-us/articles/360020623294-Identifying-users

---

This skill document was created to help Agent understand and guide developers in implementing Fullstory's User Identification API correctly for web applications.