Senior/Staff Fullstack Software Engineer to work with one of our most important clients in London. This role is hybrid in Fitzrovia (London) with four days in the office and one day remote. Responsibilities include designing and implementing services that support revenue workflows, payment logic, and invoicing pipelines; building robust APIs and full-stack features; integrating with internal systems (e.g., Salesforce, NetSuite) and external providers; implementing financial logic; integrating AI/LLM capabilities; building and deploying infrastructure in AWS; translating complex business rules into backend logic; enabling data flows between Client Processing, Deal Management, and Authoritative Data domains; collaborating with product managers, architects, and finance stakeholders. Must-have: 7+ years software engineering experience; experience with a TypeScript-native ORM (Drizzle or Prisma); Node.js, GraphQL, PostgreSQL; AWS services; event-driven systems (Kafka, SQS); data modeling and secure architecture. Nice-to-have: experience with financial systems, Salesforce/NetSuite, serverless, multi-party payments, Claude API, Cursor; AI tooling proficiency. Benefits include high-impact environment, professional development, flexible culture, global opportunities, and more. Location is hybrid in Fitzrovia (London) with 4 in-office days and 1 remote day.