Fintech & Financial Services
Software engineering for Banking
Software for banks, built by senior engineers who understand that in this sector downtime and errors are regulatory events, and that incremental modernisation beats the big-bang core replacements that keep making the news.
Why the domain matters
Banking software is a distinct discipline, and it is not the same thing as consumer fintech. This is the incumbent, enterprise side of finance: the core-banking platforms that hold customer accounts and balances, the ledgers that must reconcile to the penny, the payment rails that move money between institutions, and the reporting that regulators read. The defining fact about all of it is that the stakes are asymmetric. A feature shipped a week early is worth little; an hour of downtime, a mispriced transaction, or a reconciliation break is worth a great deal, sometimes a regulatory event, sometimes a headline, sometimes both. Everything about how good banking software is built follows from that asymmetry.
The other defining fact is legacy. Most established banks run core systems that are decades old (frequently mainframe and COBOL, often older than the engineers maintaining them), and those systems work. They hold the record of truth for millions of accounts and have processed transactions correctly for thirty or forty years. That reliability is real and it is why they persist. It is also why modernising a bank is one of the hardest jobs in software: you are not building on a blank field, you are changing the engine of an aircraft that is not allowed to land. The organisations that have tried to replace a core in one big move have, repeatedly and publicly, come unstuck. The ones that succeed do it incrementally.
We build and operate software for banks and financial institutions with that reality front of mind. We are senior engineers only (there are no juniors learning the payment rails on your production ledger), and we are blunt about trade-offs, because in this sector the honest trade-off is usually the safe one. We favour incremental modernisation over risky rewrites, legible systems over clever ones, and tested resilience over assumed resilience. Where a slower, more careful path is the right one, we will say so plainly, and we will explain why the speed you could have would cost you more than it is worth.
The challenges in banking
Decades-old core systems that cannot simply be replaced
The core-banking platform is the record of truth, it is frequently mainframe and COBOL, and it works, which is exactly why it is so hard to change. Big-bang replacements have a long history of high-profile failure. The challenge is modernising safely and incrementally around a system nobody can afford to break, not heroically ripping it out.
Downtime and errors are regulatory events, not just incidents
Under the UK operational-resilience regime, a bank sets impact tolerances for its important business services, and breaching them is something regulators care about directly. An outage is not merely an engineering embarrassment; it is a reportable event with consequences. That changes what “good enough” reliability means at a fundamental level.
Integration with entrenched legacy is the hardest, riskiest part
The interesting work is rarely the new service; it is the boundary between the new service and a core system whose interfaces were designed in another era, batch files, fixed-width records, screen-scraping, nightly cycles. Underestimating this integration is the single most common way banking projects overrun and fail.
Payment rails leave no room for approximation
Faster Payments, CHAPS, SWIFT and the card networks each have their own message formats, timing rules, cut-offs and settlement mechanics. A payment either moves correctly or it becomes a real-money problem with an audit trail and a complaint attached. There is no partial credit for a transfer that mostly worked.
Reconciliation and auditability are load-bearing, not features
Every movement of money has to be traceable, every balance has to reconcile, and every action has to leave a record that stands up to inspection months later. Systems that treat the audit trail as an afterthought create exactly the discrepancies that turn into regulatory findings and painful manual investigations.
Fraud and financial crime are an adversarial, moving target
Fraud, money laundering and sanctions screening are not static rule sets; the people you are defending against adapt continuously. Detection systems have to catch genuine crime without drowning operations in false positives, and they have to do it while the tactics on the other side keep changing under them.
What we build for banking
The systems this sector most often needs, built by engineers who understand the domain, not just the code.
Core-banking modernisation, done incrementally
We modernise around the core using the strangler-fig pattern, building new capability alongside the legacy system, routing traffic across piece by piece, and retiring the old paths only once the new ones are proven. Each step is reversible and verified against real data, which is the opposite of the big-bang cutover that keeps failing in public.
Customer-facing banking channels
Web and mobile banking, onboarding and servicing journeys that sit on top of the core and the payment rails. The visible surface is only part of the job; the harder part is making those channels reliable, secure and consistent with a back end that was never designed to be spoken to in real time.
Back-office and operations systems
The internal tooling that runs a bank, case management, reconciliation, exceptions handling, servicing workflows, reporting dashboards. Unglamorous, high-value software where correctness and a clean audit trail matter far more than novelty, and where good tooling directly reduces operational risk and cost.
Fraud and AML platforms
Transaction monitoring, sanctions and PEP screening, and financial-crime case management, tuned to catch what matters while keeping false positives at a level operations can actually work through. We build these as evolving systems, because the threat evolves, and we make the decisions they reach explainable to compliance and to regulators.
Regulatory and financial reporting
The pipelines that turn the bank’s data into the returns the PRA and FCA expect, and the capital and liquidity reporting behind Basel requirements. Accuracy, repeatability and a defensible lineage from source data to submitted figure are the whole point; a report you cannot trace back is a liability.
API and open-banking layers over legacy cores
Clean, well-governed API layers that expose the core’s capabilities to modern channels and, where required, to the open-banking ecosystem, without letting every consumer reach into the mainframe directly. This is often the highest-leverage modernisation move: it decouples new work from the legacy core and buys room to change the core later.
Where we help
An open-banking-grade API layer over a legacy core
A bank needs modern channels and third-party access, but its core exposes only batch files and green-screen interfaces. We build a governed API layer in front of it (translating, caching and rate-limiting against the core’s real constraints), so new development stops being hostage to the mainframe’s interfaces.
Strangling a legacy back-office system into retirement
A decades-old operations platform is expensive to run and impossible to hire for. Rather than a rewrite, we peel off one capability at a time behind a stable interface, prove each in production alongside the old system, and shrink the legacy footprint gradually until what remains is small enough to switch off safely.
A transaction-monitoring platform that operations can live with
An AML system is generating more alerts than analysts can clear, and the real cases are hidden in the noise. We rework the detection and tuning so genuine risk surfaces and false positives fall to a workable level, with every decision explainable to compliance and every alert traceable end to end.
A regulatory-reporting pipeline with defensible lineage
Reporting is assembled through spreadsheets and manual steps that nobody can fully reconstruct under scrutiny. We rebuild it as a repeatable pipeline with clear lineage from source system to submitted figure, so a regulator’s question about how a number was derived has a documented, verifiable answer.
How we build for banking
We start from the stakes, and in banking the stakes push almost every decision towards caution, correctly. We favour incremental modernisation over rewrites: the strangler-fig approach, where new capability is grown alongside the legacy core and the old paths are retired only once the new ones are proven against real data. We do this because the alternative (the big-bang core replacement), has failed publicly and expensively enough times that recommending it would be malpractice. Slow, reversible and verified beats fast and irreversible when the thing you are changing is the ledger that has to reconcile to the penny.
Concretely, that means mapping the integration surface before promising a timeline, because the boundary with legacy is where the risk lives and where estimates go wrong. It means designing the audit trail, the reconciliation and the failure handling in from the start rather than bolting them on. It means treating impact tolerances and recovery objectives as engineering targets and testing against them, not asserting them. And it means being honest when the careful path is slower: we will tell you plainly that banking software moves the way it does because the stakes and the regulation demand it, and that the speed you might want elsewhere would, here, cost more than it saves.
The engineers who build your system are the ones who run it. That is not a slogan; it is the incentive structure that keeps banking software safe. The person who designs the payment integration is the one who gets paged when a settlement file is late, so they design for the edge cases rather than the happy path. The person who builds the reconciliation is the one who has to explain a break, so they make it explainable. Senior engineers, operating what they build, in a sector where getting it wrong is a regulatory event. That combination is the whole point.
Regulation
Banking in the UK is supervised by the Prudential Regulation Authority and the Financial Conduct Authority, and the software sits squarely inside that supervision. The PRA cares about the safety and soundness of the institution: capital and liquidity under the Basel framework, which your systems have to calculate and report accurately and repeatably. The FCA cares about conduct, market integrity and the fair treatment of customers, which shapes everything from onboarding to complaint handling. We are engineers, not your compliance function, and we do not pretend otherwise; what we do is build systems that give compliance and risk the accuracy, traceability and control they are accountable for, and we work to their requirements rather than around them.
The regime that most directly shapes engineering is operational resilience. UK regulators require firms to identify their important business services, set impact tolerances for the maximum tolerable disruption to each, and be able to demonstrate they can stay within those tolerances through severe but plausible scenarios. That is not a compliance box; it is a design constraint that reaches into architecture, failover, dependency mapping and testing. We treat impact tolerances as real engineering targets, designing systems to meet them, and testing against the scenarios that would breach them, because in this sector an untested resilience claim is not resilience, it is an assumption waiting to become a reportable event.
Integration
Integration is where banking projects are won or lost, and we treat it as the central risk rather than a final phase. On one side sit the core-banking platforms, mainframe systems, COBOL applications, and the packaged cores that hold accounts and balances, whose interfaces were shaped by the era they were built in: batch cycles, fixed-width and delimited files, message queues, and sometimes nothing more modern than a terminal screen. Reaching these systems reliably, without overloading them or corrupting the record of truth, is skilled and careful work. We design the boundary deliberately (with clear contracts, back-pressure, idempotency and reconciliation built in), so the fragility of a legacy interface does not leak into everything built on top of it.
On the other side sit the payment networks, each with its own exacting rules. Faster Payments for near-instant domestic transfers, CHAPS for high-value same-day settlement, SWIFT for cross-border messaging, Bacs for bulk direct debits and credits, and the card networks with their own authorisation and settlement flows. The industry is converging on ISO 20022 as the message standard, which is a genuine improvement and a substantial migration in its own right. Each rail has message formats, cut-off times and settlement mechanics that have to be honoured exactly, because a payment that is late, malformed or duplicated is a real-money incident. We integrate to these rails as they actually behave (timing, edge cases, failure modes and all), not as a simplified diagram suggests they behave.
Security and resilience
In banking, security and resilience are the same subject as regulatory standing, and the audit trail is where it becomes concrete. Every movement of money and every consequential action must leave an immutable, timestamped record that can be reconstructed and inspected long after the fact, not because it is tidy, but because reconciliation, dispute resolution, fraud investigation and regulatory inspection all depend on it. We build systems where the audit trail is a first-class part of the design: complete, tamper-evident, and detailed enough that “how did this happen?” always has an answer. Alongside it, we apply the fundamentals without exception: least-privilege access, strong authentication, encryption in transit and at rest, segregation of duties, and secrets kept out of code and configuration.
Resilience is the other half, and the operational-resilience regime makes it non-negotiable. Disaster recovery in a bank is not a document; it is a tested capability: backups that are proven to restore, failover that is exercised under realistic conditions, and recovery objectives that map to the impact tolerances set for each important business service. We design for the failures that will happen (a data centre lost, a dependency down, a payment rail unavailable), and we test the recovery, because an untested standby or an unexercised runbook is a hope, not a plan. That is what “we operate what we build” means here: the engineers who design the recovery are the ones who would have to make it work at three in the morning, which is precisely the incentive that keeps it honest.
What changes
Modernisation without a bet-the-bank cutover
You move off legacy incrementally, in reversible steps proven against real data, rather than staking the institution on a single big-bang replacement. The core keeps running correctly throughout, and the risk at each step stays small enough to manage.
Resilience you can evidence to a regulator
Impact tolerances and recovery objectives become tested engineering targets, not assertions. Failover and restore are exercised rather than assumed, so when a regulator (or an actual incident), asks whether you can stay within tolerance, the answer is demonstrated rather than hoped for.
Payments and reporting that reconcile and trace
Money moves correctly across the rails, balances reconcile, and every figure and every action carries a lineage that stands up to inspection. The discrepancies and unexplained numbers that turn into regulatory findings and manual investigations stop being generated in the first place.
What we build for banking
From a first platform to modernising what you already run. The disciplines this sector draws on most.
How we deliver
- 01
Discover
We map the system, the constraints and the business it serves, including the parts nobody documented.
Architecture brief
- 02
Architect
Decisions get made, written down and defended before a line of production code exists.
Decision records
- 03
Build
Short cycles against working software. You see progress in the product, not in a status deck.
Shipping increments
- 04
Operate
Monitoring, incident response and iteration. The system is alive, so the engagement is too.
Runbooks & SLOs
Building something for banking?
Tell us the problem and the constraints you are working under. A senior engineer will give you a straight view on what it would take, and say so plainly if we are not the right team for it.
Technologies we work in
Chosen per problem, not per fashion. A selection of the stack we most often reach for.
Why teams in banking choose us
Senior engineers only, on a system that cannot afford juniors
There are no trainees learning the payment rails or the core-banking boundary on your production ledger. The people on your work have operated regulated financial systems, and that judgement (knowing which edge cases bite and which risks are real), is exactly what this sector punishes the lack of.
Honest about why banking moves carefully
We will not sell you a fast core replacement, because the fast path here is the dangerous one. We recommend incremental modernisation and tested resilience even when a slower plan is a harder sell, and we explain the trade-off plainly rather than promising a speed the stakes do not allow.
We operate what we build
The engineer who designs the failover is the one who would have to make it work under pressure; the one who builds the reconciliation is the one who has to explain a break. That incentive produces tested recovery, legible systems and complete audit trails. The things that matter when downtime is a regulatory event.
We treat integration as the real work
We know the hardest, riskiest part of a banking build is the boundary with entrenched legacy and the payment networks, so we map it before we promise a date and design it deliberately. That is where projects overrun and fail, and it is where our attention goes first rather than last.
Related sectors
Part of Fintech & Financial Services. Adjacent sectors we also know.
Common questions
Can you replace our legacy core-banking system?
We can help you move off it, but almost never in one big move, and we will be blunt about why. Big-bang core replacements have a long, public history of failure, because you are changing the record of truth for millions of accounts with no room to get it wrong. The approach that works is incremental: growing new capability alongside the legacy core using the strangler-fig pattern, routing traffic across piece by piece, and retiring old paths only once the new ones are proven against real data. It is slower and it is far safer, and in this sector safer is the right trade. We start by mapping what the core actually does and what depends on it, then modernise around it in reversible steps.
How do you handle integration with old mainframe or COBOL systems?
As the central risk, not a final phase. Legacy cores expose interfaces from another era (batch files, fixed-width records, message queues, sometimes only a terminal screen), and reaching them reliably without overloading them or corrupting the record of truth is skilled, careful work. We design the boundary deliberately: clear contracts, idempotency so a retry cannot double-post, back-pressure so we do not overwhelm a batch-oriented system, and reconciliation so any drift is caught rather than hidden. We map this surface before we commit to a timeline, because underestimating legacy integration is the single most common way banking projects overrun and fail.
How do you deal with the PRA, FCA and operational-resilience requirements?
We are engineers, not your compliance function, and we do not pretend to be. You own the regulatory relationship. What we do is build systems that give your risk and compliance teams the accuracy, traceability and control they are accountable for, and work to their requirements rather than around them. Operational resilience is the regime that most shapes the engineering: we treat impact tolerances as real design constraints, map dependencies for important business services, design failover to meet recovery objectives, and test against the severe-but-plausible scenarios that would breach tolerance. For PRA capital and liquidity reporting under Basel, and FCA conduct requirements, we build pipelines and controls that are accurate, repeatable and defensibly traceable from source to submitted figure.
Which payment rails and standards do you integrate with?
Faster Payments for near-instant domestic transfers, CHAPS for high-value same-day settlement, Bacs for bulk direct debits and credits, SWIFT for cross-border messaging, and the card networks with their own authorisation and settlement flows. The industry is moving to ISO 20022 as the common message standard, which is a real improvement and a significant migration in its own right. Each rail has message formats, cut-off times and settlement mechanics that must be honoured exactly, because a payment that is late, malformed or duplicated is a real-money incident with an audit trail attached. We integrate to how these rails actually behave (timing, edge cases and failure modes included), rather than to a simplified diagram of them.
What do you do about fraud and financial-crime systems?
We build and improve transaction-monitoring, sanctions and PEP screening, and financial-crime case-management platforms, and we build them as evolving systems, because the people you are defending against adapt continuously. The hard part is rarely detecting something; it is detecting genuine crime without burying your analysts in false positives, so the real cases are found rather than lost in the noise. We work on that balance directly, tuning detection so operations can actually clear the alerts, and we make the decisions these systems reach explainable: traceable end to end and defensible to compliance and to regulators. A fraud system nobody can explain is a liability, however clever it is.
Building for banking?
Tell us what the system has to do and what it cannot get wrong. A senior engineer reads it, and if banking is not a domain we know well enough to be useful in, we will say so rather than learn it on your budget.
- 01A senior engineer reads it. Not a form queue, and not an account manager.
- 02We reply either with questions or with a straight answer that we are not the right fit.
- 03If it looks like a fit, a technical call with the person who would actually run the delivery.
- 04Then scope, effort and risk in writing, before anyone signs anything.