Product Engineer (Backend)

Plain is redefining customer support for the next generation of B2B companies. We’re building the fastest, most powerful platform to help companies move beyond reactive support and build true customer relationships. Some of the world’s most forward-thinking companies - like Cursor, Raycast, and Granola - trust Plain to unify all customer interactions, enable faster team collaboration, and supercharge their workflows with AI. B2B customer support is undergoing a seismic shift. AI is transforming the way companies engage with customers, shifting support from a siloed function to a company-wide effort across Slack, Discord, and any other channel you talk to customers in. The old way - slow, manual, and disconnected - no longer works. This role is hybrid with 3 days a week in our London office. WHAT YOU’LL DO We’re in a rare spot. Plain is already powering support for high-growth companies with a lightning-fast, deeply scalable platform. AI has been part of our DNA from the start. Now we’re doubling down to define how it shapes every layer of modern customer support. This is where you come in. We're looking for a customer obsessed Senior Product Engineer to help take our AI ambitions to the next level. You’ll build the Infrastructure that helps us build AI features quickly, safely and with confidence. You’ll own problems end to end, and play a key role in building a category-defining product. In this role, you will: * Own and ship AI features such as our agents, assistants, insights and more - from concept to production. * Design and implement backend services for AI integrations and data pipeline. * Build robust APIs and abstractions for internal AI usage * Drive system scalability: caching, optimization, processing patterns, etc * Contribute and drive the evolution of the infrastructure for AI including architecture, evaluation and iteration pipelines. * Solve complex AI challenges around AI accuracy, reliability, and quality – using rigorous quantitive approaches. * Advocate for our work externally – writing, speaking, and engaging with the broader AI community. THIS IS A GREAT FIT IF YOU… * Are an experienced backend engineer who’s shipped AI products in the OpenAI & Anthropic era – especially text and data-heavy AI applications. * Have hands-on experience with LLM orchestration and LLM API optimization (costs, latency, etc) * Are familiar with RAG architectures and technologies * Have AI observability experience. AI isn’t just about shipping features that sound great – it’s about knowing, conclusively, if they’re making things better. * Have a clear perspective on what makes a great AI product. We want to apply AI with precision, nuance, and real impact – not as a gimmick. * Are product minded and customer focused in how you build software. * Measure your success by user impact, not just technical elegance. * Are self-driven, impact-focused, and thrive in a fast-moving, high-ownership environment. * Bonus: you have experience with agent frameworks and design. THIS WON’T BE THE RIGHT ROLE IF YOU… * Have primarily academic or research ML/AI experience. This role’s looking for someone with Applied AI experience. * Think AI for support is just chatbots and deflection. B2B support is complex – we want AI through the platform, not just automating conversations. * Struggle with ambiguity. We’re an early-stage startup. Things change quickly, and often, knowing what to work on is as important as the work itself. * Prioritize working with the latest tech for its own sake. We choose tools that make us productive – not what’s trendy. (Postgres is great.) * Are looking for a management role. This is a hands-on individual contributor position. * Want the perks of a big company. We offer competitive compensation for our stage, but we’re a startup – this is a different kind of opportunity.

About 9fin 9fin 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 Senior II Product Engineer to take broad technical ownership of the backend systems powering 9fin's platform. You will sit between our Senior and Staff engineering tracks. You will own meaningful systems end to end, lead multi sprint initiatives across a product domain, and shape how we build for scale. You will operate as a strong senior IC, leading design, raising the engineering bar through review and mentorship, and partnering closely with Product, Data, and our financial and legal domain experts. This role suits someone who has already grown beyond a typical senior remit. You enjoy diving into distributed systems problems, and you have firsthand experience scaling services under real production load. WHAT YOU'LL DO * Own the design and delivery of significant backend systems and services across a product domain, from data modelling through API surface to operational concerns. * Lead complex, multi sprint engineering initiatives that span several engineers and stakeholders. * Drive scaling work: identify bottlenecks, propose architectural changes, and execute migrations safely in production. * Make pragmatic decisions about distributed systems concerns including consistency, idempotency, partitioning, retries, and observability. * Contribute to platform level discussions on service boundaries, data ownership, and how systems evolve as we enter new markets. * Run thorough code reviews, mentor senior and mid level engineers, and help define what good looks like in the codebase. * Partner with cross functional teammates from Product, Data Engineering, and our editorial and legal domain experts to scope work and ship the right thing. * Improve developer experience by investing in tooling, testing, observability, and the paved road so the whole team moves faster. * Ramp on legacy areas of the system, find the highest leverage cleanup, and execute it without stalling delivery. * Use AI assisted engineering to move faster on understanding, prototyping, and shipping code. OUR BACKEND STACK * Python is the dominant backend language. * Flask and FastAPI, with strong typing and Pydantic powering automated documentation and API definitions. * PostgreSQL at the heart of most services. * AWS for cloud hosted services, with heavy use of AWS Lambda serverless. * Docker on AWS ECS for uniform development and deployment. * Internal developer platform with a standardised paved road from code to production, plus managed preview environments and feature flags. WHAT WE'RE LOOKING FOR You will be a great fit if you have: * Several years operating as a senior backend engineer, with a clear track record of leading technical initiatives end to end across a product domain. * Experience contributing to the design of distributed systems in production, including the operational realities such as failure modes, observability, data consistency, and graceful degradation. * A track record of solving scaling problems, whether database scaling, throughput, latency, or cost. You can talk through a real example end to end: how you measured the problem, what you changed, and what the outcome was. * Experience designing and operating event driven architectures in production, including message queues or streaming systems, with a clear understanding of delivery guarantees, idempotency, and failure handling. * Strong design skills around APIs, service boundaries, and the flow of data from persistent storage through to clients. * Strong Python experience, including modern web frameworks (we use FastAPI with Pydantic and strong typing). * Deep working knowledge of SQL and relational data modelling. * Excellent collaboration and communication, including the ability to explain complex systems and tradeoffs to non technical stakeholders. * A strong product and customer mindset, with the judgement to balance shipping value against long term system health. * A high bar for code quality, testability, and operational maturity, and the habit of raising that bar through review and mentorship. Nice to have: * Experience working on complex permissioning or access control systems (multi tenant, role or attribute based, fine grained, audit driven, or similar). * Experience with end to end document management systems, especially solving problems of scale and applying strong security practices such as encryption, access control, data lineage, and retention. * Experience modernising legacy systems and migrating workloads with minimal downtime. * Experience with a data pipeline orchestrator. * Experience working at AI startups, including integrating with and building on top of AI and LLM based solutions. COMPANY BENEFITS We’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 & Insurance * Competitive Salary(our salary bands are benchmarked at the top end of the market) * Pension (your minimum contributions are 4% with 9fin matching up to 7%) * Private Medical Insurance * Paid sick leave with Income Protection for long periods of illness * Group Life Assurance * Season Ticket Loan&Cycle to Work schemes Time off * 25 holiday days per year * Local 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 work * Work abroad for up to 3 months a year * 1 month paid sabbatical after 5 years of service * Enhanced parental leave&flexible working arrangements available Training & Culture * Professional learning and development budget * Quarterly team socials * Summer and Winter company social events 9fin is an equal opportunities employer At 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.