Mobile Engineer

Hydrogen UK Ltd; City Of London; Hybrid; GBP 92,000 - 129,000; Full time

Job summary

Hydrogen UK Ltd is seeking an experienced Android Engineer for an initial 8-week engagement with potential extension. The role is hybrid and outside IR35, requiring Kotlin and modern Android experience in large production codebases.

You will work across Android, backend API interaction, and architecture reconnaissance to deliver robust mobile solutions. The position involves navigating large codebases, analyzing CI/CD pipelines, and collaborating with cross-functional teams under time pressure.

Qualifications

• Experience delivering Android apps with Kotlin and Jetpack Compose in large production codebases.
• Ability to read large codebases and map architectural layers.
• Fluent professional English, both written and spoken.
• Experience with REST/GraphQL/gRPC and API contracts.

Responsibilities

• Develop Android features using Kotlin and Jetpack Compose in hybrid/production environments.
• Navigate large codebases, perform static analysis and dependency mapping.
• Trace UI flows through networking to backend using REST/GraphQL/gRPC.
• Assess and integrate third-party SDKs and security considerations (OAuth/OpenID).
• Collaborate with engineers to drive architectural consistency and quality.

Skills

Kotlin

Jetpack Compose

MVVM

MVI

Clean architecture

Codebase navigation

CI/CD

OpenAPI

Tools

OpenAPI

GraphQL

gRPC

REST API

OAuth 2.0 / OIDC

Job description

My client has a need for an experienced Android Engineer to join their team for an initial 8 week piece of work with potential to extend. This is a hybrid role and is outside of IR35

Android (primary depth)

• Kotlin and modern Android in large production codebases; Jetpack Compose alongside legacy View-based UI
• Multi-module builds: reading, untangling and mapping them, including convention plugins and version catalogues
• DI frameworks and the ability to derive actual runtime structure from wiring
• Architectural patterns in practice (MVVM, MVI, Clean-style layering), and the ability to identify what a codebase genuinely does

Code archaeology and analysis

• Navigating an unfamiliar large codebase quickly and systematically
• Static analysis, dependency graph extraction, module dependency visualisation, code search at scale
• Reading CI/CD pipelines as a source of architectural truth: Fastlane, Bitrise, GitHub Actions, Jenkins, Gradle/Xcode build logic
• Reconstructing runtime behaviour from instrumentation, logging and observability tooling (Crashlytics, Sentry, Dynatrace, Firebase or similar)

Backend and API

• Tracing a request from UI through the mobile networking layer to the backend service and back
• REST/GraphQL/gRPC, OpenAPI and contract inspection, API gateway and BFF patterns
• OAuth 2.0 / OIDC token flows as implemented on device
• Third-party SDK inventory and assessment: analytics, payments, chat, fraud, marketing, feature flags

Banking-specific mobile concerns

• Secure storage, biometric authentication, device binding, certificate pinning as implemented in code
• Feature flags, remote config, phased/staged rollout, kill switches
• Offline behaviour, caching and state management, and what happens at the edges

Essential non-technical skills

• Structured interviewing – runs a technical interview that yields specifics rather than a tour of opinions, and captures it in a form the team can use
• Rigorous note-taking and traceability – findings can be traced back to a source; distinguishes what was observed in code from what someone said
• Collaborative, non-territorial — will be reviewing other engineers' work in front of them; needs to do it without triggering defensiveness
• Self-directed under time pressure — 8 weeks means little supervision and no ramp-up period
• Fluent professional English, spoken and written

Nice to have

• Financial services or other regulated-sector mobile delivery
• Kotlin Multiplatform, React Native, or hybrid/webview integration experience
• Diagramming from code: Structurizr, PlantUML, Mermaid, dependency-graph tooling
• Accessibility and performance profiling experience
• Consultancy or contracting background with multiple client environments
• iOS working knowledge (second depth) - Swift, SwiftUI/UIKit, SPM and Xcode project structure — sufficient to read the codebase and validate the Architect's iOS findings