Principal Software Engineer

Principal Software Engineer (AI Agents & Model Context Protocol) UK Remote | Permanent | High-Growth Fintech We are currently partnering with an innovative, high-growth fintech in their search for a Principal Software Engineer to lead the development of cutting-edge AI infrastructure that will transform how merchants, partners, and internal teams interact with payment technology. This is a rare opportunity to take ownership of a strategically important platform at the forefront of AI and fintech innovation, helping build the secure foundations that enable AI agents to interact with real-world payment systems. The Opportunity As AI-powered interfaces become an increasingly important way users engage with technology, our client is investing heavily in the development of Model Context Protocol (MCP) servers that securely expose platform capabilities and data to AI agents and developer tooling. In this hands-on Principal Engineering role, you'll own the architecture and delivery of these services end-to-end. You'll define technical direction, write production code, establish engineering standards, and help shape the future of AI-enabled payments. The environment is fast-moving, highly collaborative, and focused on outcomes over process. Engineers are trusted to solve complex problems, innovate, and make meaningful technical decisions. What You'll Be Doing Leading the architecture and delivery of a suite of MCP servers exposing platform capabilities to AI agents and developer tooling.Designing transport and session models, including streaming interactions, subscriptions, sampling, progress updates, and user elicitation workflows.Building secure authentication and authorisation capabilities using OAuth 2.0/OIDC and least-privilege access principles.Designing and operating gateway and edge services using technologies such as Cloudflare Workers, Durable Objects, or comparable serverless platforms.Creating reusable standards, patterns, and frameworks that enable scalable adoption across engineering teams.Driving security, reliability, observability, performance, and operational excellence across the platform.Working closely with engineering, product, security, and compliance teams to deliver robust, customer-focused solutions.Mentoring engineers, reviewing architecture and code, and helping shape technical strategy across the organisation. What We're Looking For We're looking for someone who is genuinely experienced in building modern AI-powered systems and understands how agentic applications work in practice. You'll bring: 8+ years of software engineering experience with a track record of owning and delivering complex systems.Strong backend engineering expertise with TypeScript/Node.js and/or Python.Extensive experience building and operating secure APIs and distributed services.Strong understanding of OAuth 2.0, OIDC, JWT validation, and identity-based security models.Experience with edge and serverless technologies such as Cloudflare Workers, AWS Lambda, Cloud Run, or similar.Strong knowledge of databases, systems integration, caching, messaging platforms, and modern software architecture.A security-first mindset, ideally gained within fintech, payments, or other regulated environments. AI & MCP Experience The ideal candidate will have practical experience working with: Model Context Protocol (MCP) servers, tools, resources, and prompts.Well-defined input/output schemas for reliable LLM interactions.Agent frameworks, tool-calling patterns, and multi-step AI workflows.MCP capabilities including elicitation, sampling, and agent-driven interactions.Modern AI SDKs and LLM application development.Security considerations surrounding AI systems, including prompt injection risks, tool permissions, and safe execution of model-driven actions. What Makes This Role Different? This organisation believes that small, empowered teams achieve exceptional results. Engineers are encouraged to experiment, challenge assumptions, and continuously improve their craft while working on real-world problems that deliver measurable value. If you are a senior technical leader with deep backend expertise and hands-on experience building AI-enabled systems, please apply ASAP.