Senior Software Engineer — Tax OS Founding Builder

About Tax OS Tax OS is the autonomous global tax compliance platform. The platform combines a microservices core, an AI-native classification and interpretation layer, and a jurisdiction-aware calculation and filing engine. You will work directly under the leadership and guidance of the founders driving product and technical direction. This is a small, trust-based team. You will have significant ownership, direct access to the founder, and a real say in how the platform is built. The Role We are looking for one senior software engineer who can take a well-defined product specification and turn it into a running, production-grade system. You will be a founding builder: writing the core of the platform, making architectural decisions with the rest of the founders, and shipping the MVP through to the first real filings. This is a hands-on role. You will spend most of your time writing code, reviewing your own work rigorously, and owning features end-to-end — from data model to deployed service. We are not looking for someone to manage a team. We are looking for someone who builds. What You Will Build

• The core microservices of the Tax OS platform: Legislation service, Calculation Engine, User service, Integration service, and API Gateway.
• The AI-assisted interpretation layer that parses tax legislation and converts it into computable rules, with a human-in-the-loop review workflow.
• Jurisdiction-aware calculation logic, starting with the Phase 1 jurisdictions and extending into multi-state and multi-country coverage.
• Integrations with accounting systems, bank feeds, and government filing gateways.
• The web application and API surface that clients — accountants, SMEs, and firm partners — will actually use.
• The infrastructure, CI/CD pipeline, observability, and security controls that make the system safe to run with real taxpayer data.

Technical Environment The Reference Stack For Tax OS Is

• Backend: Python 3.12 (FastAPI) for calculation, legislation, and interpretation services; Node.js 20 (NestJS) for user, integration, and notification services.
• Frontend: React 18 / Next.js 14 with TypeScript; React Native for mobile.
• AI/ML: LangChain, LLM orchestration, retrieval and semantic search over legislation.
• Data: PostgreSQL 16, MongoDB 7, Redis, Elasticsearch.
• Infrastructure: AWS, Kubernetes (EKS), Kafka for events, OpenTelemetry + Prometheus/Grafana for observability.
• Security: OAuth 2.0 / OIDC, encryption at rest and in transit, audit logging, SOC 2 readiness.

We are pragmatic. If you have a strong reason to adjust a choice for the first 12 months, we want to hear it. What we care about is that the system is correct, auditable, secure, and fast enough to meet the SLAs in the PRD. Who You Are

• 3-5 years of professional software engineering experience, with a clear track record of shipping production systems that handle sensitive data or regulated workloads (fintech, tax, healthcare, payments, legal tech, or similar).
• Strong in at least one of Python or TypeScript/Node, and comfortable moving between both. Comfortable owning backend services, data models, and the APIs that sit on top of them.
• Comfortable with cloud-native architecture: Kubernetes, event-driven services, and the operational discipline that comes with running them.
• Practical experience integrating or building with modern AI/ML tooling — LLM APIs, embeddings, retrieval pipelines, or classification models. You do not need to be a researcher; you need to ship AI features that behave well in production.
• You write clean code, you test what matters, and you care about correctness. For a tax platform, a silent 0.5% calculation error is a serious failure.
• You can work independently. Remote-first work over a 12+ month horizon requires someone who sets their own pace, communicates clearly in writing, and does not need to be managed.

Trust and Alignment This is the most important section of the description, and we want to be direct about it. Tax OS is being built by a small team under a founder. The intellectual property — the architecture, the models, the legislation corpus, the calculation rules — is the company. We need someone who understands that, takes it seriously, and moves in the same direction as the rest of the team. In Practice, That Means

• You will sign a mutual non-disclosure agreement covering all product, business, and technical information before any confidential material is shared. Confidentiality obligations survive the engagement.
• You will sign an intellectual property and inventions assignment agreement. All code, models, designs, documentation, and derivative works created in connection with the role are the sole property of the company.
• You will agree to reasonable non-solicitation and non-competition terms scoped to the tax compliance and tax automation space, for a defined period after the engagement ends.
• You will not take, retain, or reuse company code, data, models, or documents outside the engagement. On separation, all company material is returned or destroyed and access is revoked.
• You will treat disagreements as something to work through with the fractional CTO directly, not as a reason to fork direction quietly. We want a builder who flags concerns early and commits once a decision is made.

We are protective of what we are building because we believe it is genuinely valuable. The right person will read this section and recognize themselves in it. Engagement and Compensation

• Location: Fully remote within Europe or the United States. You choose your working hours within reason, provided there is meaningful overlap with the team for real-time collaboration.
• Schedule: Flexible, but this is a full-time role. We expect your primary professional focus for at least 12 months and beyond.
• Base salary: Competitive cash compensation, benchmarked to senior engineering roles in your region. Paid on a regular cadence.
• Equity: Shares in the company, granted and vesting based on progress against defined milestones (Phase 1 delivery, first filings, Phase 2 readiness). The equity grant is meaningful — this is a founding-builder role, not a contractor role.
• Duration: 12-month minimum commitment. The expectation on both sides is that this becomes a long-term role if the first year goes well.

How to Apply Send a Short Note Directly To Us And Include

• A brief description of one or two production systems you have personally built that are comparable in complexity — what the system did, what you owned, and what you would do differently today.
• Links to code, technical writing, or projects that represent your work. A working prototype or sample matters more than a polished résumé.
• Your location, notice period, and target base salary range.
• One sentence on why Tax OS in particular — not why startups in general.

We will reply to every serious application. First conversations are with the fractional CTO directly.