About Cheerful Monster Limited

Cheerful Monster is a small, London-based AI digital studio that builds software for clients, mostly in healthcare. Small senior teams own the work end to end, from discovery to production, and use AI to remove the grind rather than cut corners. We intend to find the new layout of the software workshop in the AI era, and to publish what we learn, failures included, so plainly that others can follow. We are a responsible services provider rather than a body shop. We aim to punch well above our weight through honesty, craft and quality, and have fun along the way.

If you’re a senior engineer with strong fundamentals looking to retool for AI-native development — or already an AI-native developer looking for an environment designed around it — read on.

Cheerful Monster is a London-based development studio building software with a smile. We deliver world-class AI and software for clients.

We combine product thinking, technical strategy, and software delivery in one small, senior team. Rather than passing work down a chain of specialists, our engineers work directly with clients and own outcomes from discovery to production.

We’re hiring a senior engineer to deliver client work now and grow out our small engineering team. This is a full-time role, working primarily from our Canary Wharf office in London, but with some ability to flex.

The role

You’ll work as a product engineer in a small, mission-driven pod, directly with clients, taking problems from early ambiguity through design, implementation, and deployment to production outcomes.

Our first projects will use Elixir, Phoenix, and LiveView. Experience of using this stack on at least one production system would be helpful.

Coding agents will be a core part of the normal development environment: exploring codebases, developing plans, implementing features, generating tests, reviewing changes, and investigating production faults.

You remain accountable for their output and whether it is fit to deploy. AI makes writing code cheap, but it cannot decide what should be built, understand the client’s situation, make sound architectural trade-offs, or establish that plausible-looking output is actually correct.

What you will do

Turn ambiguous client needs into clear product and technical direction, separating the outcome a client needs from the first solution someone happens to suggest.
Design and build production systems with Elixir, Phoenix, and LiveView, working across domain modelling, data, interfaces, integrations, background processing, testing, deployment, and observability as each project requires.
Decide what to do yourself, what to delegate to agents, and what evidence is needed before the result can be trusted.
Run investigations in parallel, discard generated work when needed, and recognise when AI is not the right tool for the job.
Review and develop other engineers, helping establish how we capture context, structure work, verify generated changes, manage cognitive load, and demonstrate increasing autonomy.
Contribute to our shared engineering practice and the development harnesses that support it.
Help rediscover the craft and joy in software development as the tools and practices evolve.

What we’re looking for

You may already use coding agents as a routine part of production delivery and can explain how you frame work, supply context, delegate meaningful implementation, review what comes back, and catch output that is convincing but wrong.
Or you may be a strong senior Elixir engineer whose organisation has not yet given you the opportunity to work this way, but who has begun experimenting seriously with AI-assisted development.
In either case, you bring strong engineering judgement, production experience, and a willingness to make your working process visible and improve it.
Substantial experience building and operating software with Elixir and Phoenix.
A strong grasp of OTP, supervision, concurrency, Ecto, and relational data modelling.
The ability to spot generated Elixir that compiles but is unidiomatic, poorly structured, or based on a mistaken model of the runtime.
Comfort working directly with clients, shaping ambiguous work, explaining trade-offs, and taking responsibility for what happens after deployment.
Clear writing skills across plans, specifications, constraints, acceptance criteria, and review comments.

Technology and useful experience

The initial environment centres on Elixir, Phoenix, LiveView, OTP, Ecto, Ash, and PostgreSQL, alongside testing, Git, CI/CD, cloud deployment, and observability tooling.

Experience in other languages is welcome but not required. Python is particularly useful, whether you’ve used it for services, data, automation, scientific computing, or AI integrations. Experience in another ecosystem is also valuable where it demonstrates real depth and an ability to move intelligently between technologies.

Experience with Oban, Broadway, distributed Elixir, data engineering, LLM integrations, regulated domains, consultancy, early-stage products, or coaching other engineers would all be useful. We don’t expect one person to bring all of them.

Benefits

Competitive salary: £80-90,000
Company pension: 3%
25 days of paid annual leave
A fun and flexible working environment
A modern, dog-friendly office located in Canary Wharf

Senior AI-Native Product Engineer