Principal Software Engineer

Principal Full Stack Engineer, Washington DC

X4 is partnered with a scaling healthcare AI company building products for healthcare payers and providers. They’re looking for a Principal Full Stack Engineer based in Washington DC to support a government department.

Candidates must be US citizens living in the DC area and must be able to successfully go through the government security clearance process. The ideal candidate will already hold a US security clearance and/or have worked with government authorities or government customers in the past.

About the role

We’re looking for a Principal Full Stack Engineer to lead the design and delivery of secure, scalable web applications supporting a government department. The role bridges traditional full-stack development and AI integration. You’ll build scalable applications, implement autonomous agents, and integrate Large Language Models into production workflows, moving across the front end, back end, cloud infrastructure and AI layers. You’ll own architecture decisions, set technical direction, and help make sure the product is robust and secure.

The role is primarily remote, with occasional in-person attendance in the Washington DC area. Some interview rounds may take place on-site.

What you’ll do

• Own the architecture of full-stack applications, from front-end interfaces through to back-end services and cloud infrastructure.
• Design, build and maintain scalable, secure web applications and APIs.
• Provision, manage and optimise cloud infrastructure, including government cloud environments where applicable.
• Design and implement robust integrations with LLMs and other ML models, including autonomous agents where appropriate.
• Set and uphold CI/CD pipelines and infrastructure-as-code standards.
• Mentor engineers, lead technical design reviews, and set standards for code quality, security and testing.
• Work with product management and other engineers to define requirements and deliver high-quality products.

What we’re looking for

We’re looking for a well-rounded engineer. We’re not fixed on a specific tech stack, so experience with an equivalent technology is just as valuable. The core areas are:

• Full-stack experience: 10+ years of professional software engineering experience, including significant full-stack work on customer-facing applications. Candidates with 10 to 15 years’ experience are particularly well suited.
• Strong front-end experience: for example React, Next.js or TanStack, using JavaScript and TypeScript, or equivalent.
• Strong back-end experience: for example Python (FastAPI) or the Node.js ecosystem, or equivalent. Proven experience designing secure, scalable REST and GraphQL APIs is essential.
• Cloud experience: hands-on experience with AWS, Azure or GCP, including relational and NoSQL databases such as PostgreSQL or DynamoDB, or equivalent.
• AI and LLM experience: experience building applications that consume LLM APIs, with working knowledge of prompt engineering and patterns such as chain-of-thought, ReAct and RAG, plus context window management. Experience integrating AI and ML models into production systems is preferred.
• Technical leadership: demonstrated experience setting architecture direction, mentoring engineers, and making pragmatic trade-offs.

Preferred

• Existing US government security clearance.
• Experience working with, or supporting, government authorities or government customers.
• Experience building and maintaining CI/CD pipelines (GitHub Actions, Bitbucket Pipelines or similar).
• Infrastructure-as-code experience with Terraform, Pulumi or AWS CDK.
• Startup mindset: a quick learner with a bias for action, ownership and pragmatism over perfection.

What’s on offer

• Competitive salary
• Healthcare
• 401(k)
• Bonus
• Remote, with occasional in-person attendance