Principal Software Engineer

SR2 Clean Energy; Greater London; On-site; GBP 120,000 - GBP 160,000; Full time

Job summary

A leading clean energy firm in Greater London is seeking a Principal Software Engineer to build and enhance backend systems using FastAPI. The role involves collaborating closely with stakeholders to solve complex problems and taking ownership of features from idea to production. Ideal candidates should have 5+ years of experience in backend engineering, a strong proficiency in Python, and a keen interest in AI and LLMs. This position promises high impact and an opportunity to work on significant engineering challenges.

Qualifications

• 5+ years of backend engineering experience.
• Proficient in Python with a focus on system quality.
• Interest in working with AI and LLMs.

Responsibilities

• Build and improve backend systems using FastAPI.
• Collaborate with technical stakeholders to solve complex problems.
• Take ownership of end-to-end features from design to production.

Skills

Backend engineering

Python

Code quality

System thinking

Interest in AI

Job description

Principal Software Engineer | London | £120k to £160k + benefits | Python

This is an exceptional individual contributor role for engineers who want to work on problems that actually matter and be in an environment where you’re not the smartest person in the room.

You will be building backend systems that support advanced AI research and deployment decisions. The software you work on influences how complex models are tested, understood, and monitored before they reach the real world.

This is serious engineering in a fast moving space, with high expectations around quality, reliability, and judgement.

What you will work on

• Building and improving backend systems used by internal teams every day using FastAPI
• Working on tools that operate at scale and need to be both fast and dependable
• Helping shape how new systems are designed and brought into production
• Collaborating closely with technical stakeholders to solve ambiguous problems
• Taking real ownership over the software you build

Most of the work is Python, but this is not scripting. You will be designing systems that need to be correct, reliable, and maintainable under real load.

What makes this role different

• You are close to the problem, not buried under layers of abstraction
• You work directly with stakeholders, researchers and influence how tools are designed
• You will own features end to end, from idea through to production use
• Engineering standards are high, but speed still matters
• This is a role where judgement counts more than dogma

What they are looking for

• Backend engineers who enjoy complexity and responsibility and have at least 5 years+ experience
• People who care about code quality but know when to move quickly
• Engineers who think in systems, not just features
• Experience with or an interest in LLMs
• Those motivated by the impact of AI and how it is developed

Experience with LLMs, agents, cloud platforms, or security is a bonus, not a requirement.

This is a career defining role for someone who wants to stay deeply technical and work on problems that will only become more important over the next decade.

If you’re looking for a role where you can make tangible impact, work on an interesting product, and be part of an exceptional team, get in touch.