Job Title: Senior FullStack Engineer 

Reports to: Technical Lead

Team: Technical   

Type: Permanent, Full time | 

Location: Remote or hybrid, UK based, London hub

About B4B Payments

B4B Payments, part of the Banking Circle Group, is an award-winning provider of card issuing and embedded payment solutions. We are regulated as an E-Money Institution and hold Principal Membership with Mastercard and Visa, serving over 1,000 corporate clients across the UK, Europe and the US. We build the technology that lets organisations issue cards, manage spend and run payment programmes at scale

About the role

At the heart of our platform is a large Ruby on Rails monolith carrying fifteen years of domain logic and processing real money for real clients. We want a strong Ruby engineer to work at depth in it: shipping features, modernising what needs it, and improving test coverage and performance. This is hands-on work in mature Rails, not greenfield.

You will also own work across our Go and Node services and our frontend applications. We are hiring engineers, not language specialists.

We expect AI code assistants to be a normal part of how you work. You do not need to be a specialist, but you must already use them day to day, be able to show us concretely how, and want to help us push that adoption further.

This is a regulated environment (DORA, PCI DSS, SOC 2, EMI regulations,GDPR), which shapes our development lifecycle from change control to audit trails. We need people who navigate that intelligently and still move fast.

We calibrate seniority by demonstrated ability, not tenure. Ruby and Rails depth is the one thing we will not flex on.

Key Responsibilities

• Own features end to end across the full software development lifecycle: design, build, test, deploy and support.
• Work at depth in our Rails monolith, covering new features, performance work, bug fixing and production support, including in the older and more tangled parts of the domain.
• Write and maintain high-quality, well-tested Ruby: clear models and service boundaries, sensible use of ActiveRecord, solid RSpec coverage, and attention to performance and query behaviour at scale.
• Improve the monolith incrementally rather than working around it. Address technical debt in the areas you touch, and make deliberate calls about refactoring, extraction and where a service boundary is genuinely warranted.
• Improve how the Ruby codebase is documented and structured, so that it is easier for the next engineer, and for AI code assistants, to work in: architecture notes, context files and well-structured inline documentation.
• Contribute across the wider polyglot stack alongside the Rails work: maintaining and extending our Go and Node services, and frontend work with modern frameworks and web technologies.
• Use AI tooling as a normal part of how you build, review and QA software, and help raise the team practice as we move further in that direction.
• Convert product specifications into precise engineering specifications, with enough context and specificity that the work can be delivered efficiently, whether by you, by a colleague, or with a code assistant.
• Deliver within a lifecycle constrained by regulation, including change management, approvals, testing evidence and audit trails, and propose ways to improve delivery speed without weakening those controls.
• Build security into the lifecycle by default, from secure coding and threat modelling to dependency scanning and security review, and apply AI-assisted workflows without compromising the security of secrets, data or the supply chain.
• Build and run services on AWS, using containers on ECS and EKS, with GitHub for source control and CI.
• Work with data at a high standard: data analysis, ETL and data QA, handling complex financial data sets and transactions correctly.

Skills and Knowledge Required

Essential (Need to have) 

• Strong, current Ruby and Rails, with production experience in a large,  long-lived codebase. This is the core requirement.
• Solid testing and code quality in Ruby (RSpec or equivalent).
• Hands-on experience in at least one of Go or Node, good frontend skills,   and comfort picking up new languages.
• Established daily use of AI code assistants, with a clear view on where they help and where they need close review.
• Comfort in a regulated environment, with security built into how you  develop.
• AWS, containers (ECS and EKS), MySQL with some PostgreSQL.

Desirable (Nice to have): 

• Fintech or payments experience, 
• PCI DSS or SOC 2 environments,
• Extracting services from a Rails monolith, 
• Ruby open source contributions

The role offers:

• 25 days annual leave 
• Annual wellness day off 
• Access to one-to-one confidential counselling sessions through our Employee Assistance Programme 
• Access to retail & other discounts 
• Vitality health care plan  
• Access to ‘’Nous Finance-a smart way to save money’’   
• Learning & development support 
• Ride-to-work scheme

Make a difference:  

We’re always on the lookout for talented individuals to help take us further – it’s important to us to hire the right people and work with you to shape a role that fits your growth.

Our Values 

• Ownership & Accountability: We take responsibility for outcomes, not just tasks.
• Drive, Ambition & Initiatives: We move fast, think big, and don’t wait to be asked.
• Positive Mindset & Resilience: We solve problems with energy, even when the path isn’t clear
• Collaboration & Trust: We work together openly, with each other, with clients and with partners
• Customer & Commercial Focus: Every decision starts with the customer and ends with the values delivered.