British AI startup StackOne raises €17.6 million to reinvent SaaS and AI agent integrations

London-based StackOne, an AI-powered platform fuelling enterprise AI agents and SaaS integrations – has raised €17.6 million in a Series A funding round in order to build StackOne’s tool-calling LLM, invest in R&D, and further expand the number of integrations and depth of actions available.

The round was led by GV (Google Ventures). Workday Ventures, XTX Ventures, existing investors Episode 1 and Playfair, and angels from OpenAI, Deepmind, Microsoft and Mulesoft also participated. The funding takes the total raised by StackOne to €21.1 million.

Romain Sestier, Co-founder and CEO, StackOne said: “For over a decade, Guillaume and I have felt the acute pain of integrations. We’d see teams burn months rebuilding or refusing requests and nothing on the market eased this pain. So we, alongside a super talented and passionate team, built it ourselves. StackOne doesn’t just reinvent how integration platforms can and should be built – with depth, accuracy and security by default – but we believe AI agents are the missing piece in finally delivering enterprise-grade integrations at scale.

“It’s the platform we’d always dreamed of, built for the AI future of SaaS, and it’s a vision shared by the whole team. This shared experience has shaped a culture of innovation, collaboration and deep care for the product we’re building and it shows.”

Founded in 2023, StackOne’s integration platform provides SaaS vendors and AI Agent builders with a universal interface for enterprise-grade integrations. The StackOne solution provides platforms with a way to connect with B2B SaaS tools. Its real-time architecture enables powerful bi-directional integrations while protecting sensitive data.

Integrations are so important, they’re now a top priority for 83% of B2B buyers, yet 71% of SaaS firms still take three weeks or more to launch a single connection meaning engineering teams are losing valuable time and resources, and SaaS firms are losing out on contracts.

Guillaume Lebedel, Co-founder and CTO, StackOne said: “Integrations are now table stakes for winning and keeping customers in B2B SaaS, especially for the ever-growing swathe of AI agents. You only need to look at the rise of standards like Model Context Protocol (MCP) to see the demand, but these protocols just aren’t fit for enterprise use by themselves – they’re a small piece of the puzzle They lack security, accuracy and scale. They don’t work with multi-tenant systems, they’re too restrictive for the complex needs of big business, and they don’t go deep enough. StackOne solves all of this in a single layer; one that marks the birth of a new generation of integrations platform for an AI-first world.”

StackOne’s Founders – Romain Sestier (CEO) and Guillaume Lebedel (CTO) – saw this first-hand during 10+ years of working together, building SaaS products for Google, Oracle and Yieldify. With the rise of AI agents demanding a new kind of integration interface, this challenge is only getting harder and while emerging standards, like Model Context Protocol (MCP), show where the market is heading, they lack the security, depth, and scalability needed for real enterprise use.

StackOne looks to solve this by reinventing the way integrations are built – combining a proprietary AI agent and real-time engine to give teams the speed, coverage and reliability to finally connect SaaS and AI agents to the enterprise tech stack.

Luna Schmid, Partner at GV: “What impressed us most about StackOne is its ambition and clarity. Romain and Guillaume aren’t building just another SaaS integration platform. They’re creating infrastructure that modern software and the entire AI agent ecosystem can rely on.The depth of secure integrations, the pace of delivery, and the team’s foresight into AI’s future uniquely position StackOne to redefine this category”

According to StackOne, developers spend weeks combing through messy enterprise APIs, writing custom logic just to get a single integration to work. StackOne’s proprietary AI agent takes on this heavy lifting and automatically builds use-cases on top of complex APIs, connecting AI and SaaS tools to their customers’ entire tech stack – reportedly in a fraction of the time, and with higher accuracy than leading LLMs.

Through the platform, product teams get access to 3,000+ actions on 200+ connectors instantly, from HR to CRM, ticketing, messaging, and IAM.

StackOne aims to be one of the fastest-moving players in the €15.4 billion integration space, having recently surpassed 1 billion API calls, and with customers including Drata, Attensi, Localyze and others across three continents.

Barbry McGann, Managing Director and SVP at Workday Ventures: “We’ve been impressed by how quickly and deeply StackOne integrates with complex enterprise systems – and now, with their focus on agent-to-agent interoperability, they’re unlocking even more powerful use cases for customers. In a space where speed and scale often trade off with reliability, compliance, and functionality, StackOne delivers all of the above in a universal layer – without compromise.”

StackOne is also building a growing AI developer community, including its own AI Demo Days that bring together researchers, builders, and infrastructure teams, and its own open-source contributions.

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