Senior C# Engineer – .Net, Typescript, Lambda, API Gateway, SQS, SNS, Cognito – OUTSIDE IR35

£700 – £800 per day Outside IR35 • 100% Remote • 8-month rolling contract

Play a pivotal role in architecting and delivering a major greenfield B2B SaaS platform as a senior, hands-on, backend C# engineer. Expect serious backend engineering, high-performance event-driven systems, and production-grade AWS serverless delivery with an elevated focus on API engineering, identity & access, and secure-by-default service design. You’ll also gain hands-on experience with Claude Code and modern agentic development workflows.

Role Overview:

You’ll act as the technical spearhead for a greenfield product build, reporting to the Director of Engineering and working closely with their IAM Technical Lead. In addition to deep backend C#, API engineering, and AWS skills, without being a security expert, you’ll ideally have some experience of AWS Cognito, AWS IAM, and some Typescript.

What You’ll Bring:

• Senior/Staff-level backend mastery in C# and.NET 9/10, capable of architecting and leading distributed REST microservices at scale.
• Advanced AWS serverless expertise including Lambda, API Gateway, SQS, SNS, Step Functions & EventBridge.
• Serious API engineering capability: versioning, documentation, throttling, rate-limiting, contract stability, DX-first design, and building APIs that other engineers love to consume.
• Some frontend experience with Typescript will be advantageous.
• Some real-world identity engineering: familiarity with AWS IAM, permission modeling, least-privilege design, cross-service auth patterns, and secure-by-default access controls.
• Some hands-on experience with AWS Cognito: user pools, identity pools, OAuth2/OIDC flows, JWT validation, token lifecycles, and integrating Cognito-backed auth into API Gateway and Lambda.
• Data expertise across Aurora MySQL and DynamoDB: schema design, access patterns, indexing, query tuning, and pragmatic trade-offs around consistency, scale, and cost.
• Event-driven architecture experience with SQS: idempotency, retries, ordering, DLQs, and operational resilience.
• Product-first mindset: comfortable shaping requirements, partnering with BAs, and operating without a traditional PO.
• Excellent communication skills and confidence in ad hoc client interaction.
• High engineering standards: Git-based workflows, trunk-based development, CI/CD (GitHub Actions), IaC (CDK/Terraform), comprehensive testing, and secure-by-default coding.
• Outcome-oriented pragmatism: comfortable with ambiguity and biased toward shipping.