Senior Software Engineer — Tax OS Founding Builder

Fiskl.AI is an accounting intelligence platform that is seeking a Senior Software Engineer to build a category-defining tax product. The role involves architecting the platform, designing the data model, and ensuring the product meets high standards of reliability and security.

Responsibilities:

• Architect the platform and own the substrate choices
• Design the ledger. taxOS needs a tax-aware data model that captures entity relationships, source documents, and the time dimension tax demands (transaction date, filing period, amendment history, audit trail). Graph, event-sourced, bitemporal, or some combination: that call is yours to make and defend
• Build the deterministic calculation engine and the agentic orchestration layer that surrounds it. The orchestrator decomposes practitioner intent into deterministic calculation steps and probabilistic classification steps, with explicit guardrails, retries, and human-in-the-loop checkpoints between them. Single-prompt chains are not the bar
• Ship document ingestion, classification, and the practitioner workspace through to a live filing season
• Stand up CI/CD, observability, security posture, and the regression discipline a tax product demands. A 0.5% silent calculation error is a serious failure, and the system has to make that class of error impossible
• Walk a practitioner through the product in two minutes. Selling it to a senior accountant is part of the job

Requirements:

• Production experience with frontier LLMs. Shipped systems, evaluation harnesses, a real cost-per-call number you can defend. Not prototypes
• Agentic orchestration in production. Multi-step agent workflows with tool use, retries, fallbacks, and human-in-the-loop checkpoints. You have a view on when to constrain an agent and when to let it reason
• Graph or event-based data modelling. Comfortable with graph databases, event-sourced ledgers, or bitemporal data, with a view on when each is the right tool. Tax is fundamentally relational and temporal, and the ledger has to reflect that
• A track record in a regulated or high-stakes domain. Fintech, healthtech, tax, payments, legal tech. You have internalised that probabilistic outputs need deterministic guardrails and that audit trails matter from day one
• Strong on either Python or TypeScript, willing to use whichever the project needs
• Comfortable owning the full stack: backend, data model, frontend, infra. The job is not divisible into roles
• You are excited about what AI changes for builders, not threatened by it. The right person sees Claude and similar tools as a lever, not a risk
• You are a strong communicator. You will translate the founders' vision into a working system, and translate back when something they want will not work. That conversation has to be crisp
• You overlap Europe or US East Coast working hours comfortably. Async is fine once the product is built. While you are still figuring out what you are building, same-ish hours matter
• You are senior enough to be opinionated and humble enough to be wrong. Both are the job
• You do not need a tax background. Domain expertise comes from the founders and the senior practitioner