Senior C# Engineer - .Net, Typescript, Lambda, API Gateway, SQS, SNS, Cognito - OUTSIDE IR35 at Smart-Sourcer, Remote (United Kingdom only), 8 Months, £700-£800 per day

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, Back End C# engineer. Expect serious Back End 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 Back End 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 Back End 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 Front End experience with Typescript will be advantageous
• Some real-world identity engineering: familiarity with AWS IAM, permission modelling, 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.

What You'll Own:

• Architecting, building and shipping high-performance event driven services in C#/.NET 9/10 on AWS Lambda.
• Designing secure, scalable API architectures using API Gateway, IAM, Cognito, and modern identity patterns.
• Delivering resilient event-driven systems using SQS and related AWS messaging primitives.
• Owning features end-to-end: product thinking, scoping, trade-offs, sequencing and delivery.
• Championing engineering excellence: testing discipline, peer review, CI/CD quality, observability, performance and secure coding.
• Developing hands-on skills with Claude Code: agentic workflows, smart refactoring, automated context management, and productivity-boosting AI patterns.

Contract Details:

• £700-£800/day
• Outside IR35
• 100% Remote (UK only)
• 8-month rolling contract