Full Stack AI Builder in London

Can you help us fix the world's most broken supply chain? The fashion industry produces over 100 billion garments a year. 60% end up in landfill, not because people don't want these clothes, but because the supply chain for getting secondhand fashion from donation to resale is completely offline, manual, and broken. Fleek is fixing this. Our mission is to make secondhand the first choice. We are building the infrastructure powering the global secondhand clothing trade, through a B2B marketplace and the AI systems digitising the supply chain behind it.

Through Fleek, vintage stores, online resellers, and retail buyers source graded secondhand inventory directly from wholesalers and rag houses across multiple countries. Our key enabler is FleekSort, an AI model fine-tuned on secondhand fashion, that can grade, price, and categorise items with just a photo, turning what has historically been an opaque and manual trade into a structured, searchable, and tradable global inventory layer. Since 2022, we have saved 9 million items from landfill and tripled in size year-on-year. We have raised ~$50M from a16z, Y Combinator, Burda Principal Investments, HV Capital, and others, and have recently closed our Series B. Our advisors and private investors include operators behind Depop, Shopify, and Postmates. Today, we serve thousands of resellers and retailers across the UK, Europe, and the US. With the secondhand fashion market projected to reach $350B by 2027, Fleek is building the AI-native operational and financial backbone for secondhand commerce globally.

We are looking for people who want to build something that matters, and move fast doing it. Sound good? Keep reading.

About the Role

We are looking for a deeply technical, hands-on Full-Stack Builder to embed across our AI and operations pods, building the agents, internal tools, and infrastructure that run Fleek end-to-end. You will work alongside our CTO and Head of AI, jumping between teams to ship the systems that compound: agentic workflows that automate manual ops, internal tools that make every team 10x faster, and the connective tissue between our marketplace, our data, and our AI models. One week you might be building a sourcing agent for the commercial team. The next, productionising an integration with our warehouse partners. The next, hardening FleekOS — our internal operating system — so the next thing we build on it lasts.

This is the role for someone who has shipped real systems for 3-4 years, is bored of building the same thing in slightly different shapes at a big company, and wants range, ownership, and proximity to the part of AI that's actually useful right now: production agentic infrastructure inside a real business with real revenue.

What Makes You a Fleek Builder?

• Full-Stack in the Real Sense: You move comfortably across the stack. Backend services, frontend interfaces, integrations, data pipelines, and AI/agent infrastructure. You have favourite tools but no religion about them.
• AI-Native: You use AI as your default toolkit, not a curiosity. You have built with LLMs in production, understand how agents actually work and break, and know the difference between a demo and a system that survives contact with real users.
• Builder Mindset: You ship. You prefer working code over perfect plans, and you know how to scope something to ship this week rather than next quarter. You have opinions about what's worth abstracting and what isn't.
• Pod-Hopper: You get energy from context-switching between problems. One week customs and ops, the next merchant tooling, the next internal infrastructure. You see the through-line.
• Commercially Wired: You connect what you build to business outcomes. You can tell a good idea from a real one because you understand who the user is and what changes for them.
• Comfortable with Ambiguity: Fleek is being built in real time. Specs are loose, the data is messy, and the right answer often isn't written down. You thrive there.
• Action-Oriented Communicator: You translate technical decisions into 'here is what we should do and why.' Your teammates never have to guess what you're doing or what the trade-offs were.
• Startup Pace: You have worked somewhere that ships fast and where velocity is a competitive advantage, not a slogan.

What You'll Do

• Build Across Pods: Embed with different teams (commercial, ops, supply, product) to build the tools and agents they need. You are not assigned to one squad. You go where the leverage is.
• Ship Production Agents: Design and ship agentic systems that automate real work. Sourcing, classification, customer ops, internal workflows. Things that need to run reliably and not break when the on-call engineer is asleep.
• Extend FleekOS: Contribute to and improve our internal operating system, the platform every team uses to build their own tools. Make it more reliable, more extensible, easier for non-engineers to build on top of.
• Own End-to-End: Take features from 'someone mentioned a problem in Slack' through scoping, building, shipping, and iterating. You own the outcome, not just the code.
• Maintain What You Build: Treat reliability and maintainability as first-class. The reason this role exists is that things built without engineering rigour don't last. Yours will.
• Use AI to Move Faster: Automate your own workflows, speed up your own analysis, and ship more in a week than most engineers ship in a month. Live the 10x-er pitch we make to candidates.
• Stay Ahead: Track what's actually working in agent infra, internal tooling, and AI-native development. Turn new ideas into production systems before they become trend pieces.

What You Bring

• 3-5 years of professional engineering experience. You have shipped production systems and learned what breaks. You are not a junior, but you are still hungry to learn fast.
• Strong full-stack chops. Comfortable across backend, frontend, and the glue in between. Specific languages matter less than the ability to pick up new ones quickly.
• Hands-on experience building with AI. Production work with LLMs, agents, retrieval, or AI-driven features. Not theoretical.
• Startup or scale-up experience. You have worked somewhere with fast cycles, evolving infrastructure, and high ownership. Or you have built your own thing and shipped it.
• A bias toward shipping. You finish what you start, and you know how to make something good enough to ship rather than perfect and unshipped.

Nice to Have

• Experience with TypeScript, Python, and modern frontend frameworks (React, Next.js).
• Familiarity with data warehousing tools (BigQuery, Snowflake, dbt) and event pipelines.
• Experience building internal tools or developer platforms.
• Open source contributions or a public portfolio of side projects.
• Background in B2B SaaS, marketplace, or supply chain products.

Ready to build the systems that run one of the fastest-growing marketplaces in Europe? If you want range, ownership, and a front-row seat to AI-native infrastructure in production, we would love to hear from you.

How We Work

Fleek is an in-office company. We believe the best work happens when people are together, the quick decisions, the cross-functional energy, the ability to solve problems in real time. Most of the team works from our London HQ 4-5 days a week.

The Practicalities

• Location: London HQ, Commercial St, E1 (Liverpool St / Aldgate)
• Work model: In-office, typically 4-5 days per week.
• Stage: Post-Series B, hyper-growth.
• Team: 100+ across London, India, and Pakistan (~20 in London office).
• Culture: High ownership, non-hierarchical, fast-moving. Founder-led with strong opinions loosely held.

Our Values

• Dream Big and Disrupt Yourself: Push the most ambitious version of what you are building. Step out of your comfort zone. Nothing is out of reach.
• Absolute Ownership: The buck stops with you. Drive initiatives to completion. Experiment, learn, persevere. The job is not done until the job is done.
• Curiosity Leads the Way: Do not accept anything at face value. Ask questions you do not know the answer to. That is how you innovate.
• Talk to the Customer: Deeply understand their challenges, motivations, and ambitions. Every decision starts with the customer at the centre.
• Embrace Diversity: A global team building for a global supply chain. Bring your authentic self and embrace the diversity of everyone you encounter.