Principal Engineer

About 9fin9fin is the AI platform powering global debt markets — the world’s largest asset class at over $145 trillion.Debt markets are vast, global, and mission-critical, yet still run on fragmented data, PDFs, and manual workflows. 9fin replaces this broken infrastructure with a single platform that centralises proprietary credit data, deep analysis, and high-value workflows across global markets.Today, 9fin powers teams at 300+ blue-chip institutions worldwide, including global banks, asset managers, private equity firms, law firms, and advisors. The business is scaling at exceptional speed, with rapid expansion in the US and best-in-class retention driven by deep workflow adoption.We’re at a defining inflection point. With proven product-market fit and strong, global market pull, 9fin is accelerating toward becoming the category-defining platform for debt markets worldwide.We're looking for a Principal Engineer to own the hardest technical problems at 9fin — the ones that cut across teams, don't belong cleanly to any single domain, and have the highest consequence if they go wrong.9fin sits at the intersection of financial data and AI. We ingest, model, and surface complex debt capital markets intelligence at scale. What we need now is someone who works across all of our areas (from AI, to Analytics, Platform, and Product Engineering) — setting the architectural standard, owning the foundational problems nobody else owns, and being the senior technical voice that makes the whole thing cohere.This is not a domain lead role. It's not advisory either. It's for someone who is energised by complexity and ambiguity, who wants to own the problems that are genuinely hard, and who can move between deep technical work and cross-org influence without losing credibility in either direction.What you'll ownThese are the cross-cutting problems that don't belong cleanly to any one team — but that everything else depends on.Core data model and entity resolution9fin's value depends on how well we understand financial entities and the relationships between them — issuers, instruments, deals, counterparties, legal documents. This is hard: data is messy, sources conflict, and the model has to serve analytics, AI, and product simultaneously. You'll own the decisions that shape this foundation and hold the standard for how we model and resolve financial data at scale.Foundational architectural decisionsWhen a decision is big enough to affect multiple teams and long-lived enough to be hard to undo, it lands with you. You'll write the RFCs, facilitate the debate, make the call, and be accountable for the outcome.What you'll overseeYou'll work closely with the Heads/Directors of Analytics, AI, Platform, and Product Engineering — not to run their domains, but to raise the bar across all of them.Bring architectural coherence to decisions made within individual domainsIdentify technical risks and compounding complexity before they become incidentsSet standards for engineering quality — system design, observability, reliability, API design — that every team builds toBe the senior technical voice in cross-team planning, roadmap discussions, and major build vs. buy decisionsMentor and stretch the most senior engineers across the orgWhat we're looking forYou're the person who:Has operated at principal or staff level in a data-intensive, fast-moving engineering org — and has the range to show for itCan credibly walk into a conversation about data pipeline architecture, RAG evaluation, API design, or system scalability — and add something useful in all of themOwns problems, not just opinions. You're not here to review and advise; you're here to make calls and be accountable for themWrites code when it matters — the proof of concept that validates a direction, the deep dive that unblocks the team, the PR that sets the standard others build fromHas strong opinions about what good engineering looks like and can defend them without being precious about being wrongIs energised by the problems nobody else can solve, not unsettled by themTechnical bar:Deep experience with complex data modelling, entity resolution, and data pipelines in production environmentsProven experience designing and shipping AI/LLM systems — RAG, structured extraction, evaluation frameworks — beyond the prototype stageStrong backend and distributed systems knowledge: APIs, service architecture, scalability, observabilityExperience making and owning foundational architectural decisions that affected how an entire engineering org workedProficiency in Python; comfort across the stack where it mattersNice to have:Experience with financial data, legal documents, or other semi-structured, domain-rich datasetsBackground as a founding engineer, early-stage CTO, or technical lead who has built systems from scratch and lived with the consequencesWhy this roleThe problems you'll own are genuinely hard — messy financial data, complex entity relationships, AI systems where correctness really mattersYou'll have real authority over foundational decisions, not just influence over them9fin's engineering org is strong and growing — you'll be working with and stretching great engineers, not compensating for weak onesHigh ownership, high impact, and the chance to shape how a serious data and AI company scales its technical foundationsBenefitsWe’re a scaling start up and we enjoy sharing our success, when the company succeeds, we always reinvest that in our people. We also offer huge amounts of responsibility, an abundance of opportunity for growth and a platform to truly excel.Financial & InsuranceCompetitive Salary (our salary bands are benchmarked at the top end of the market)Equity optionsPension (your minimum contributions are 4% with 9fin matching up to 7%)Private Medical InsurancePaid sick leave with Income Protection for long periods of illnessGroup Life AssuranceSeason Ticket Loan & Cycle to Work schemesTime off25 holiday days per yearLocal public holidays (with the ability to exchange them for alternative days)Hybrid working model, to allow you the flexibility to decide how, where and when you do your best workWork abroad for up to 3 months a year1 month paid sabbatical after 5 years of serviceEnhanced parental leave & flexible working arrangements availableTraining & CultureProfessional learning and development budgetQuarterly team socialsSummer and Winter company social events9fin is an equal opportunities employerAt 9fin we are dedicated to building and promoting a fair and inclusive workplace where everyone can reach their full potential and truly belong. We recognize that building diverse teams enables a more creative and productive environment. If you’re excited about this role but your experience doesn’t perfectly align with the job description, we encourage you to apply anyway. You might just be who we’re looking for — either for this role, or perhaps another.LocationLondonEmployment TypeFull timeDepartmentEngineering