Back End Developer at CalCoach, Manchester Area, 12-16 Weeks, £Contract Rate

Senior Backend / Full-Stack Developer

(React Native Experience Preferred)

** You have to be a UK-based business (grant requirements) **

Commitment: 1–2 days per week (2 days preferred)

Contract Length: 12–16 weeks

Start Date: ASAP

Remuneration: Competitive / TBA

Summary

CalCoach is seeking an experienced senior backend or full-stack developer to join our product team on a part-time contract. You will report directly to the CTO and take ownership of architecting, securing, and delivering the backend for our rebuilt mobile and web platform.

You’ll design and implement scalable backend systems, build our API, handle authentication, integrate LLM workflows, set up cloud infrastructure, implement DevOps pipelines, and ensure the system is production-ready and capable of scaling reliably. The frontend will be handled in-house; your role is to provide the backend foundation and, as a bonus, support building out frontend logic where helpful.

We already have a working web-based MVP (35+ coaches, 300+ end users) and validated UX. Our coaches are paying monthly customers, and we are already forming early partnerships with major gym groups. You will build a robust backend that allows us to retire the MVP and launch CalCoach V1 — our first true native app — alongside the PT dashboard, built to support tens of thousands of active users as we scale.

About Us

CalCoach exists to solve the real problem in nutrition behaviour: people know what to do, but staying consistent is the challenge. Our focus is helping people stay on track with their nutrition in a sustainable, behaviour-first way.

Our mission is to build a next-generation, AI-driven alternative to MyFitnessPal — a conversational, ChatGPT-style nutrition coaching platform that keeps clients consistently on track and gives coaches an at-a-glance view of exactly who’s slipping and how to support them.

Where we are now?

✅ 35+ paying personal trainers using the MVP

✅ 300+ end users

✅ Strong behaviour-validated usage

✅ Innovate UK grant secured

✅ Early partnerships forming with major gym groups

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Role Overview

As Senior Backend Developer, you will:

• Lead the architecture, setup, and implementation of the new backend and database

• Build all backend logic, data models, and REST API endpoints

• Implement robust authentication and authorisation (Firebase Auth or equivalent)

• Integrate LLM workflows (OpenAI / Anthropic / Gemini), including orchestration and cost-aware context handling

• Maintain an accurate OpenAPI specification as the source of truth, with Swagger documentation generated from it

• Support frontend integration; bonus if you can support building frontend logic when needed

• Build notification infrastructure (Firebase, SES, Twilio)

• Implement system-wide analytics, logging, and observability (e.g., Sentry, Mixpanel, Firebase Analytics)

• Architect for scale and reliability, ensuring high performance, caching, pagination, and long-term maintainability

• Own DevOps: CI/CD pipelines, staging/production environments, deployments, and cloud infrastructure

• Lead the migration from the current MVP to the new platform

• Ensure the platform meets security best practices across backend and infrastructure

• Support QA, testing, and production releases including App Store and Play Store submissions

This is a highly autonomous role working directly with the CTO to deliver a stable and scalable V1.

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Technical Responsibilities:

Backend Architecture & Setup:

• Design and implement the backend from scratch

• Evaluate and recommend the optimal infrastructure approach for CalCoach — whether serverless or managed servers — and whether AWS or GCP is the best fit for long-term scalability

• Build scalable database schemas suitable for the chosen architecture

• Set up CI/CD pipelines and DevOps workflows

• Configure monitoring, alerting, logs, and tracing using the tools most suitable for scale

API Development:

• Build all REST API endpoints for mobile and web

• Maintain the OpenAPI specification

• Ensure performance, reliability, caching, and robust error-handling

Authentication & Security:

• Work from our baseline security plan and refine it for real-world scale

• Options on the table include (but are not limited to):

• JWT/session-based auth
• Firebase Auth
• Role-based access control (PTs, clients, admins)
• Middleware for secure validation
• IAM least-privilege frameworks
• Firewalls and WAF
• Cloudflare protections
• Rate limiting and private networking

• Your job is to implement the right combination of these layers, identify what is essential vs overkill, and justify your decisions in terms of scalability, reliability, security, and cost

LLM Integrations:

• Food log parsing

• Behavioural insights and recommendations

• Conversation agents

• Orchestration of multi-provider LLMs with fallback strategies

Testing, QA & DevOps:

• Comprehensive unit testing across backend components

• CI/CD and environment management

• Support staging/production builds and QA cycles

• Implement error tracking, analytics, and performance monitoring using tools such as Sentry, Mixpanel, or Firebase Analytics

Migration:

• Map MVP data to new data models

• Build safe import and migration scripts

• Execute cutover with minimal downtime

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Requirements

Essential:

• Proven experience building and deploying large-scale backends

• Strong DevOps experience: CI/CD, infrastructure, environments, deployments

• Strong backend security experience (firewalls, WAF, IAM, rate limiting, private networking)

• 2+ years building production APIs and backend services end-to-end

• Strong database design experience

• Ability to work independently and take ownership of the backend

• Experience implementing observability (logs, monitoring, analytics)

Preferred:

• Ability to support frontend logic (React / React Native)

• Experience with LLMs, OpenAI APIs, Anthropic, or agent frameworks

• Experience with notification systems (Firebase, SES, Twilio)

• Early-stage startup or MVP → V1 rebuild experience

• Interest in fitness, nutrition, or health technology

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Output of This Contract

By the end of this engagement, we aim to deliver:

• Fully built backend and database

• Mobile and PT Dashboard apps fully integrated with the backend

• Complete, reliable OpenAPI documentation

• Secure, scalable cloud infrastructure with CI/CD

• Successful migration from MVP → V1

• App deployed on iOS + Android

• CalCoach V1 launched, stable, and ready to scale to tens of thousands of active users

• Legacy MVP fully retired