Klaviyo’s BFCM Scale in 2025

Every Black Friday and Cyber Monday (BFCM), millions of shoppers make decisions driven by marketing messages, real-time data, and personalized experiences powered by Klaviyo. For our customers, BFCM isn’t just a revenue event, it’s the most important four days of the year. For Klaviyo Engineering, it’s our annual reliability championship.

Thanks to months of preparation, a strong engineering culture, and systems we’ve spent years evolving, this year’s BFCM was smooth, predictable, and record-breaking.

Here’s a rundown of the weekend and a behind the scenes look at how it all came together.

First, The Numbers

• Our customers drove over $3.8 billion in total Klaviyo Attributed Value, a 27% YoY increase
• Nearly 20,000 customers hit their best sales day ever
• 22.7B total messages sent over BFCM 2025, a 25% YoY increase
• Our event pipeline ingested over 900 million events per hour during our peak hour
• Our flows engine processed over 2.8 billion steps
• Over 4.2B inbound webhooks processed
• Our segmentation system handled 125 billion updates to segment memberships. This is actually lower than our total in 2024, but that’s because we made significant investments in the pipeline to reduce overall load and only update segments when necessary

Why BFCM Matters at Klaviyo

Our success is defined by our customers’ success.

Every metric we track, from event throughput to email send rates, rolls up to one goal: helping brands grow. That’s why our north star is Klaviyo Attributed Value (KAV), a metric that captures the revenue customers generate directly from Klaviyo.

In 2024, we publicly shared that Klaviyo customers drove over $3 billion in KAV. This year, they topped that record with more than $3.8B in total Klaviyo-attributed revenue for BFCM 2025, a 27% YoY increase!

BFCM is a make-or-break moment for brands

During BFCM, our customers send campaigns at their highest volumes, rely on live segmentation and AI to react instantly to shopper behavior, and trust Klaviyo to deliver messages quickly, accurately, and reliably.

If we fail, they fail. That’s why we treat BFCM as a full-fledged product: planned collaboratively, tested thoroughly, and executed with precision.

How We Prepared for BFCM 2025

BFCM prep is a team sport, we give guidance and support as a leadership group and have people tasked with the larger perspective, in the end each team owns their success inside of that framework. This year we had a few people who were asked to focus on BFCM prep, so before we continue, it’s worth shouting out John Meichle, Patty de Castro, and Chris Chiodo.

For 2025, we modified the approach we’ve taken in past years, while keeping many aspects that have historically worked for us. Let’s take a look at how these two approaches work together.

Bottom-Up: Every Team Owns Its Scale

Our BFCM readiness starts at the ground level. Each team validates not only the capacity of its own services, but also the cross-service data flows that matter most during peak traffic. Things like campaign sending, on-site data syncing, checkout events, segmentation, flows, and more.

In 2025, we introduced a quarterly BFCM readiness scorecard to bring further consistency, transparency, and accountability to this process. Teams measure and report on both service-level and team-level metrics across a wide range of categories. And while measurement and reporting generally are considered table-stakes for an engineering organization, we view it as critical in reinforcing Klaviyo’s culture of ownership, continuous learning, and data backed process improvements.

Top-Down: Central Coordination and Standards

To complement the bottom-up preparation, Engineering leadership sets the structure, clarity, and communication needed to keep the entire organization aligned. This direction ensures teams stay focused on the highest-priority work required to win the weekend. It includes a set of rituals and systems such as:

• Guidance in defining scale targets
• Creating, enhancing, and socializing the BFCM readiness scorecard process
• A cross-engineering review process for scorecards and enumerated risks
• A centralized risk registry capturing all BFCM risks
• Tracking readiness across engineering pillars, teams, and services
• A support forum for any BFCM-related preparation, technical or process
• A clear timeline for code and infrastructure chill, freeze, and thaw
• Adoption of a revamped incident-response system
• All hands meetings to celebrate wins and highlight areas of risk

This structure establishes consistency and removes ambiguity. Teams know exactly what “ready for BFCM” means, and how readiness is measured.

Always Be BFCM-ing

The most notable changes in 2025 centered on improving the efficiency and precision of our preparation process. We kept the core components that have historically worked well, but refined how we executed them to reduce unnecessary overhead and ensure all work directly supported BFCM-scale readiness.

One of the largest shifts was in our approach to scale-out testing. In previous years, these tests ran periodically throughout the year and were treated as coordinated, engineering-wide maintenance windows. For 2025, we streamlined the process: we ran fewer — more targeted — tests, aligned them much closer to BFCM, and integrated them directly with team-owned load tests. This ensured that scale-out work validated the things that matter most: connection limits across datastores, system stability at projected BFCM scale, and the accuracy of our capacity assumptions. All without consuming cycles earlier in the year when load patterns were less representative.

We also significantly expanded the use of combined load tests across engineering. With Klaviyo composed of many independently deployed applications, each generating distinct downstream workload characteristics, combined load testing provided a realistic picture of platform-wide behavior. In 2025, we coordinated the largest set of teams to date to generate production load simultaneously. This indirect load surfaced system interactions and bottlenecks that would not appear in isolated tests. By pairing these efforts with scale-out tests, we ensured infrastructure was already sized to BFCM targets, allowing teams to focus on application-level and architectural constraints rather than basic capacity validation.

This year also introduced Cortex.io as the foundation of an internal service graph. We began using Cortex to centralize service metadata, readiness scorecards, ownership, and dependency mapping. This complimented the manual, ad-hoc mechanisms we previously relied on and gave teams a single source of truth for operational context. BFCM served as the first major test of this system and demonstrated its value in accelerating decision-making and cross-team coordination.

Finally, we formalized a model that combines bottom-up system ownership with structured top-down program coordination. Engineering pillars became directly accountable for their own readiness, with senior technical leaders driving verification within their domains and escalating cross-pillar needs to the core BFCM team. Given the scale and complexity of Klaviyo’s architecture, now spanning thousands of services, this distributed ownership model has become essential for maintaining depth of expertise while still providing unified program visibility.

The Payoff: Reliability in Action

Fewer Incidents, Faster Response, Smaller Blast Radius

Preparation didn’t replace vigilance — it empowered it. We were confident in the alerting and dashboards we built and we watched them intently throughout BFCM. What we saw was the payoff of many months of work across Klaviyo.

Under our revamped incident process, which is set up to catch and escalate potential issues before they impact our systems or customers, we had 32 internal incidents that had no customer impact. Teams were proactively catching issues over an hour before they would have triggered paging or caused customer impact. This is exactly the behavior our refined incident framework was designed to encourage.

We did have one external incident declared on our status page during BFCM: 18 minutes related to services which didn’t impact our Marketing Product, it was fully resolved with no lasting impact. A clear measure of the successful planning and preparation of the entire organization!

What Engineering Actually Does During BFCM

The culture of BFCM at Klaviyo is one of the most unique parts of the weekend.

As graphs spike and throughput curves rise, engineers are monitoring dashboards, watching queues and traffic patterns, and validating autoscalers. We’re also grabbing treats from the snack cart stocked with customer products, watching our annual turkey-themed internal dashboard, and celebrating the day’s numbers and wins together. Side note — we run an annual contest at Klaviyo to design the turkey for our turkey themed dashboard, and this year’s winning bird was Alan Turking.

Alan Turking — our BFCM mascot for 2025

This year we added a dedicated Slack channel for real-time “engineering highlight reels”: screenshots, metrics, and dashboards from teams proud of how their systems were performing under unprecedented scale. It became a running celebration of both technical achievement and teamwork. BFCM is serious engineering work, but it’s also a moment when the entire organization comes together to take pride in the foundations they’ve built.

Looking Ahead to BFCM 2026

As we celebrate this year’s performance, our focus has already shifted to 2026. We’re continuing to invest in our AI-powered platform, expand on the features we offer, and strengthen the culture that makes BFCM possible.

The bar rises every year and that’s exactly what makes Klaviyo Engineering such an exciting place to build.

Interested in joining us? Take a look at our open roles here!