Senior Software Engineer

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The Role in 30 Seconds

1. First software engineer at a funded London startup (two founders with proven track record)
2. Apply a full-stack engineering skillset (Python & modern JS) to build the platform that will transform how subscription apps predict their future customer behaviour
3. Own the entire technical stack from frontend to infrastructure with direct business impact

What We Do

Day30 helps subscription apps improve paid acquisition ROI by providing predictive signals to optimise ad spend. We connect directly to mobile measurement partners (MMPs) to analyse behavioural event data, build ML models that predict high-value conversions weeks in advance, and deliver these predictions to advertising platforms without compromising user privacy.

We're a two-founder London startup combining deep expertise in performance marketing and machine learning. As our first software engineer, you'll be founder-adjacent, working directly with our CEO and CTO to transform our current capabilities into a scalable, automated platform that will power hundreds of clients.

This role offers rare technical autonomy: you'll work across the entire stack from platform development through data pipeline architecture, collaborate with the CTO and our Lead Data Scientist, and have direct input on all technical decisions. We're looking for someone who thrives on building systems from scratch and wants to see their work immediately impact real business outcomes.

What You'll Do
Platform Development

• Design and build our platform from prototype to production-ready system
• Create intuitive interfaces for configuring clients' data connections, system monitoring, and streamline client onboarding to reduce setup time
• Develop robust API integrations with 3rd party data sources. Example of data sources we work with include MMPs (such AppsFlyer, Adjust, Branch) and ad platforms (such as Meta, TikTok)
• Build reliable connectors with proper error handling and incremental loading

Data Engineering & Infrastructure

• Architect high-performance data pipelines that scale elegantly with growing data volumes
• Optimise BigQuery operations for cost and performance
• Establish monitoring for pipeline health and data quality

• Establish CI/CD pipelines with automated testing and staged rollouts
• Implement infrastructure as code using Terraform or similar tools
• Design for scalability: from a dozen clients today to 100+ within 12 months

Technical Leadership

• Partner with the CTO on technical strategy and architecture decisions
• Collaborate closely with the Lead Data Scientist to productionise ML workflows
• Establish engineering best practices and development standards
• Mentor junior engineers through code review and pairing as the team grows

Requirements

The ideal candidate must have...

• 4-6+ years building production systems with demonstrable business impact
• Strong full-stack capabilities: comfortable building frontend components and optimising data pipelines in the same day

• Deep expertise in Python and SQL with high code quality standards (e.g. ruff, mypy, uv, pytest, SQLFluff, pre-commit)

• Experience with backend API frameworks (Django preferred, FastAPI/Flask acceptable) and modern frontend frameworks (e.g. React, Vue, Svelte, Next.js, Tailwind)
• Proven track record with high-performance or real-time data processing
• Hands-on experience with cloud data warehouses (e.g. BigQuery, Snowflake)
• Right to Work in the UK (we cannot sponsor visas)
• Ability to work from Central London office 3 days/week (we believe in-person collaboration is crucial at this early stage)

You may be a great fit if you have experience with any of the following...

• Workflow orchestration tooling (e.g. Prefect, Airflow, Dagster)
• Data transformation tools (e.g. dbt) and data quality frameworks (e.g. Great Expectations)
• Infrastructure-as-Code (e.g. Terraform, Pulumi, or similar)
• Serverless compute (e.g. Cloud Run, AWS Lambda) and container orchestration (e.g. Docker, Kubernetes)
• Open source contributions, or conference talks
• Ad-tech / mobile app / subscription business experience

You don't need to be an expert in everything from day one. A good candidate will have deep expertise in at least one area with a desire to learn across the stack.

Our Interview Process

We respect your time and move quickly.

1. Application Review: We will look through your application (CV, screening questions, and code samples) to see if you meet the initial requirements for this role.
2. Initial Screen (20 mins with CTO): Short video call to assess mutual fit and technical background.
3. Practical Exercise (take home, up to 2 hours): We'll book you in for a 2-hour slot at any time. You will be given a real-world engineering challenge that mirrors our work at Day30. Use any tools you'd use on the job (including LLMs, Copilot, etc) - we care about approach and outcomes, not memorisation.
4. Technical Deep-Dive (60 mins, in-person): We will walk through your solution, discuss system design and architecture decisions, consider alternative approaches, and work through live problem solving.
5. Founder Conversation (30 mins): Meet both founders for us to understand your motivations and career goals, and for you to ask questions. We want to know that you'll be a great fit for our team, but we also want to help you achieve your goals too.

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Benefits
Compensation & Benefits

• Base Salary: Up to £80,000 per annum (depending on experience)
• Holidays: 25 days of annual paid leave, plus bank holidays
• Equity: Meaningful options as first engineering hire
• Flexibility: 3 days/week in Central London office, remote otherwise
• Equipment: Top-spec MacBook Pro and any tools you need
• Learning Budget: Conferences, courses, and resources to stay current

What Makes This Role Unique

Founder-Adjacent Position: Work directly with two second-time founders, participating in strategic decisions beyond just engineering. Your input will shape product direction and company culture.

Technical Ownership: Build core systems from near-scratch with full autonomy over technical choices. Balance pragmatism with excellence - we build robust solutions for real problems today while keeping tomorrow's needs in mind.

Immediate Impact: Your code ships to production quickly, directly affecting client performance and company revenue. No layers of bureaucracy or months-long deployment cycles.

Growth Trajectory: As we scale, you'll have the opportunity to shape engineering culture as our first engineering hire, defining best practices and building the team.