Klaviyo’s API unlocks powerful opportunities for advanced automation, seamless third-party integrations, and hyper-personalised marketing. Whether you’re syncing CRM data, triggering server-side events, or building custom analytics pipelines, the API gives developers and technically inclined marketers the tools to scale intelligently.

In this guide, we’ll break down how the Klaviyo API works, explore real-world use cases, and walk through 8 smart ways to leverage its full power, backed by official documentation and practical insights.

Understand the Klaviyo API at a Glance

What the Klaviyo API Does

The Klaviyo API serves as a powerful bridge, connecting your Klaviyo account to external platforms and enabling custom integrations, automation, and real-time data synchronisation across your entire tech stack.

As a RESTful API, it supports full CRUD (Create, Read, Update, Delete) operations via HTTP methods and communicates using JSON-formatted requests and responses.

Klaviyo provides two primary API categories:

• v2 (Legacy): Supports basic tasks like list subscriptions, but is deprecated for new integrations.

• v2023‑10‑15: Modern, full-featured Hamilton standard with versioning support via revision: 2023‑10‑15 header

Always check Klaviyo’s API reference for the latest revision, as newer versions may introduce additional features.

Key Capabilities of the Klaviyo API

Supported Content Type

Use application/json to avoid 415 Unsupported Media Type errors for non-JSON payloads. This ensures clean communication across tools, whether you're pushing lead data from a CRM or logging transactions from a custom checkout. 

Use Case Examples

• Syncing Shopify orders with custom loyalty tools

• Sending leads from a Meta lead ad to Klaviyo via Zapier

• Triggering an abandoned quiz flow from a Typeform response

• Pulling metrics into a Google Sheets dashboard for daily reporting

• Updating tags and attributes in real time after a Recharge subscription update

To explore another powerful use case—like triggering a quiz abandonment flow from a form tool, check out our guide on Klaviyo Typeform Integration.

You can also learn how to pull and automate daily performance insights by reading our guide on Klaviyo to Google Sheets: 3 Simple Ways to Sync Your Data.

Choose the Right Authentication Method

Understand API Key Types

Klaviyo offers three primary authentication methods. Choosing the right one is critical for both data security and proper integration functionality. Each technique is designed for a specific context: frontend tracking, backend server communication, or third-party apps.

According to Klaviyo’s authentication documentation, here’s how each method works:

Public API Key

• Found under: Account > Settings > API Keys

• Typically used for:

  • Tracking page views with klaviyo.js

  • Capturing frontend events like “Added to Cart”

• Limited to specific endpoints: /identify, /track, and /subscribe

• Exposed in the browser, so should not be used for sensitive operations

• Rotate Private API Keys regularly (e.g., every 90 days) to minimise security risks, as recommended by Klaviyo’s security guidelines.

Private API Key

• Found under: Account > Settings > API Keys

• Used in secure server environments like AWS, Firebase, or Zapier backends

• Accesses nearly all endpoints, including:

  • /profiles, /events, /lists, /segments, /campaigns, /flows, etc.

• Required for POST/GET/PUT requests when managing customer data at scale

• Should be kept secret, never exposed in client-side code

• Private API Keys can have specific scopes (e.g., read-only or full access), so ensure your key’s permissions match your integration’s needs

OAuth 2.0

• Ideal for developers building multi-tenant apps

• Enables users to connect their Klaviyo account securely via a permissions flow

• More secure and scalable than API keys for external apps

• Authentication flow includes:

  1. User grants access via Klaviyo login

  2. Your app receives an access token

  3. Use the token to authenticate API requests

• Refer to Klaviyo’s OAuth guide for detailed setup steps.

Why This Matters

• Security: Use private/OAuth for backend tasks to keep data safe

• Functionality: Using the wrong key type (e.g. public key for /profiles) will cause 401 errors

• Scalability: OAuth enables secure, delegated access for app marketplaces

If you're exploring OAuth to build B2B lead nurture or platform integrations, our post on Klaviyo LinkedIn Integration explains how to leverage secure authentication flows.

Explore Key API Endpoints

Klaviyo’s API is structured around functional endpoint groups, making it easy to perform precise tasks like creating profiles, tracking custom events, or syncing catalogue data. According to Klaviyo’s API reference, these endpoints are RESTful and support full CRUD operations using JSON payloads.

Understanding which endpoints to use—and when—will help you build clean, scalable integrations.

Common Endpoint Groups

Profiles Endpoint

• Ensure email is unique in /profiles payloads to avoid 409 Conflict errors.

• Create or update a profile with POST /profiles/

• Add profile properties like:

  • first_name, source, location, email_consent, sms_consent

Example payload:

This is especially useful when syncing customer data from CRMs, loyalty tools, or surveys.

Lists & Segments Endpoint

• Use POST /lists/{list_id}/relationships/profiles/ to subscribe a user to a specific list.

• Use GET /segments/{segment_id}/relationships/profiles/ to pull contacts within a segment.

• Use the DELETE method to remove profiles from a list dynamically (great for churn logic).

• For endpoints like /profiles or /segments, use pagination parameters (e.g., page[size], page[cursor]) to handle large datasets efficiently.

Events & Metrics Endpoint

• Track behavioural events with POST /events/

• These can trigger flows, score leads, or power custom analytics

• Example: “Started Free Trial”, “Completed Quiz”, “Watched Demo”

Payload example:

Useful for server-side tracking, especially when bypassing frontend limitations.

Campaigns & Flows Endpoints

• Use GET /campaigns/ or /flows/ to view campaign and automation metadata

• You can:

  • Fetch performance stats

  • Track engagement by user

  • Log when someone entered or exited a flow (indirectly, via event triggers)

• These endpoints don’t send emails directly—they’re useful for monitoring and tagging users based on flow status

• Note: /campaigns/ or /flows/  are primarily read-only, used for fetching metadata or tracking engagement, not for creating new flows or campaigns.

Catalogs Endpoint

• Sync product SKUs and metadata with /catalog-items/ and /variants/

• Important for:

  • Dynamic product blocks in emails

  • Abandoned cart or back-in-stock flows that auto-populate product details

• Ensure catalogue data is clean and updated daily for accuracy

Tags Endpoint

• Assign interest, source, or behavioural tags to profiles for deeper personalisation.

• Use /tags/ to define custom tag groups (e.g., “Interests”, “Acquisition Source”).

• Apply tags /tag-group-values/ to label contacts (e.g., “Self-care”, “Instagram”).

• Ideal for dynamic segmentation, flow triggers, and personalising product blocks.

Top Klaviyo API Use Cases

Klaviyo’s API allows you to connect custom data, trigger personalised messaging, and automate marketing across channels. Below are practical use cases you can implement immediately:

1. CRM or Loyalty Data Sync

• Push membership tiers or loyalty points via /profiles/{id}

• Automate segmented flow access (e.g. VIP flows)

2. Server‑Side Event Tracking

• Bypass tracking blockers by posting events directly to /events

• Use cases: login events, quiz completions, subscription renewals

3. Custom Dashboard Reporting

• Pull metrics using /metrics or /campaign-metrics

• Export data or automate reports with Google Sheets or BI tools

4. Product/Inventory Sync

• POST to /catalog-items to update stock, prices, or variants

• Supports dynamic abandoned cart and restock flows

For guidance on setting up flows that respond to back-in-stock triggers and cart behaviour, see our blog on Klaviyo Cross Sell Flow: 6 Proven Ways to Increase Order Value.

5. Automate SMS Campaigns

• Use POST /sms to trigger SMS flows based on external events

• Example: Send a “Thanks for your purchase!” SMS after a WooCommerce order is confirmed via API

6. Personalise Emails with Dynamic Tags

• Use /tags and /tag-group-values to assign interest or behavioural tags

• Example: Tag users as “Fitness Enthusiast” based on quiz responses to personalise workout product recommendations

7. Sync Subscription Statuses

• Use POST /profiles to update subscription statuses from Recharge, Stripe, or similar tools

• Example: Update a profile’s subscription_status to “active” after a renewal, triggering a loyalty reward email

8. Build Real-Time Abandoned Cart Flows

• Use POST /events to track “Abandoned Cart” from custom checkouts and trigger recovery flows

• Example: Push an “Abandoned Cart” event from a headless commerce platform to trigger a discount email within 30 minutes

These use cases help you create a connected, responsive marketing ecosystem driven by real-time behaviour and data.

Trigger Flows from External Systems

Start automated journeys in Klaviyo based on actions from your app, CRM, or custom backend.

How to Trigger a Flow

• Use POST /profiles to create or update a user profile.

• Use POST /events to push an event like Started Free Trial or Booked Demo.

• Add custom properties like plan type, campaign source, or device used.

This tactic aligns closely with the structure we use for Klaviyo Webhooks, which enable event-based automations across your stack.

Example Use Case

Trigger a welcome flow after a user signs up on your mobile app:

1. POST a new profile to Klaviyo

2. POST a Signed Up via App event

3. Event triggers a tailored welcome flow with a mobile-first design

4. Ensure a flow is pre-configured in Klaviyo to trigger on the ‘Signed Up via App’ event before sending the API call.

   
   

Why This Works

• Works even if the signup doesn’t happen on your e-commerce platform.

• Allows targeting by signup context, not just standard site activity.

• Adds flexibility for apps, SaaS tools, B2B portals, and gated content flows.

Test flow triggers in Klaviyo’s sandbox environment to avoid sending emails or SMS to real users during development.

Integrate With Other Platforms (CRM, CDP, Loyalty)

Connect Klaviyo to your wider stack to create a single source of truth for personalisation.

Common Integration Scenarios

1. CRM Integration (e.g. HubSpot, Salesforce):

• Push lead scores, lifecycle stages, and deal status to Klaviyo profiles.

• Trigger flows based on pipeline movement or lead qualification.

2. CDP Integration (e.g. Segment, RudderStack):

• Stream events and traits to Klaviyo in real time.

• Keep data consistent across email, ads, and site experiences.

3. Loyalty Integration (e.g. LoyaltyLion, Smile.io):

• Sync point balances or VIP tiers via API or webhook.

• Personalise emails with loyalty data (e.g. “You’re 200 points from a reward!”)

4. Paid Media Audiences:

• Export Klaviyo segments into platforms like Meta or Google Ads.

• Target high-value or win-back audiences based on real engagement.

• Example: Export a high-value segment via GET /segments and upload it to Meta Ads for retargeting campaigns.

To go deeper on building high-converting campaign strategies using segment data, check out our playbook on Klaviyo RFM Segments: 6 Strategies to Boost Retention.

Why This Works

• Reduces data silos and manual exports

• Keeps your campaigns reactive to real-time behaviour

• Enables personalised, cross-channel marketing

Monitor Rate Limits and Handle Errors

Stay within safe limits and avoid unexpected integration failures

Klaviyo enforces rate limits to ensure fair and stable use of its API. Understanding these limits and how to handle errors when they occur is key to keeping your automation fast, reliable, and compliant.

Official Klaviyo Rate Limit Tiers

Klaviyo applies rate limits by endpoint group, using a burst and steady rate model. These rates define how many requests you can make in short bursts vs sustained intervals.

These limits apply per API key. You can monitor your usage via the X-Rate-Limit-Remaining and X-Rate-Limit-Reset response headers.

Error Codes & Handling Strategies

When requests exceed limits or fail validation, Klaviyo returns HTTP error codes. Here's how to respond:

Best Practices

• Use exponential backoff when handling 429 or 5XX errors

• Log and track rate headers to monitor usage trends

• Split requests across time intervals for batch updates

• Avoid loops or bulk API calls without pagination or throttling logic

We've outlined the importance of accurate attribution and performance logging in Klaviyo UTM Tracking: 7 Steps to Improve Attribution, GA4 Reporting & ROI.

Why This Matters

• Prevents disruptions in SMS, email, and customer data syncing

• Improves deliverability by reducing API-triggered failures

• Ensures scalable performance as you grow automation

Test, Validate, and Optimise API Integrations

Avoid costly mistakes and ensure your API-powered flows run smoothly.

How to Test Klaviyo API Integrations

Before pushing live, validate every integration with structured testing workflows:

• Use Postman with Klaviyo’s official API collections to simulate and inspect requests

• Validate JSON payloads with tools like JSONLint to catch syntax or formatting issues

• Test against sandbox data or isolated lists to prevent accidental sends to real subscribers

• Use Klaviyo’s dedicated sandbox environment to simulate API calls without affecting live data.

  
  

Source: Klaviyo

Validation Tools to Use

• Klaviyo's Postman Collection – Pre-built for standard endpoints like Profiles, Events, and Lists

• JSON schema validators – Confirm payloads match the required structure

• Console logging – Monitor real-time API responses and flag failures

Optimisation Tips for Long-Term Performance

Once tested, refine performance and reliability:

• Log and benchmark response times to spot lagging endpoints or rate limit issues

• Tag API-triggered flows using UTM parameters or custom properties to isolate and analyse results

• Track success vs failure rates for each endpoint and alert on error spikes

• Check Klaviyo’s API logs under Account > Settings > API Usage to troubleshoot failed requests and identify error patterns.

FAQs

1. Do I need a developer to use the Klaviyo API?

• Yes, most API tasks require basic programming knowledge or developer support, especially for custom integrations and server-side tracking.

2. Can I test API calls without affecting live data?

• Yes. Use tools like Postman and test on dummy profiles or test lists before deploying to your live account.

3. What’s the difference between V2 and V2023-10-15 APIs?

• V2 is older and still used for some legacy endpoints, while V2023-10-15 is the modern, fully RESTful standard recommended for all new builds.

4. How can I secure my API usage?

• Use Private API Keys for backend calls, rotate them regularly, and avoid exposing them in client-side code. OAuth is preferred for third-party apps.

5. Does Klaviyo support webhooks alongside the API?

• Yes. You can use webhooks to send data from Klaviyo to other systems in real time—ideal for profile updates or triggered events.

Conclusion

If you're struggling to connect tools, personalise your emails, or automate based on live customer behaviour, Klaviyo’s API gives you the control you need. It lets your systems talk to each other, so you can send the right message at the right time, without manual work. 

Whether you want to sync loyalty tiers from your CRM, trigger flows from a quiz or app signup, or pull campaign data into your dashboard, this guide shows how to do it the smart way. If you’re also using embedded forms or third-party signup tools, our guide on Klaviyo Embed Form: 7 Steps to Add High-Converting Forms to Your Site can help improve first-touch data capture and sync. With the right setup, the API turns your tech stack into a powerful, connected marketing engine that saves time and boosts revenue.

Key Takeaways

• Unlock Data Sync: Use the API to sync CRMs, loyalty tools, or subscription apps directly with Klaviyo.

• Trigger Smarter Flows: Send server-side events to power real-time automation from any external action.

• Choose the Right Authentication: Understand when to use Public, Private, or OAuth for secure and scalable connections.

• Build Custom Reporting: Pull campaign, flow, or profile data into tools like Google Sheets or BI dashboards.

• Stay Within Rate Limits: Avoid broken calls by managing API usage and handling errors properly.

• Test Before You Launch: Use Postman or a sandbox approach to ensure clean, safe deployments.

Not sure if you’re using the Klaviyo API to its full potential?

Our team will review your current implementation, identify missed opportunities, and build a roadmap for smarter, scalable integrations. Click here to schedule your free Klaviyo API performance audit now.