Staff Backend Engineer, MPC Custody Platform

The Vault is an MPC-based crypto custody platform for institutional and retail clients, running threshold signing across HSM and Ledger hardware. We're scaling toward Series A, launching our productized platform in 2026, and publishing original cryptographic research alongside the product. We're hiring a Staff Engineer to design and ship the custody platform's hardest backend code. This is a hands-on IC role with real technical leadership — no people management — where you drive non-trivial initiatives through code, RFCs, and mentorship. You'll shape architecture in your area of ownership; platform-wide decisions sit with technical leadership. We're AI-forward (Claude Code, Cursor, Codex), and we need engineers who can read AI-generated code critically and catch what it misses — not just prompt it. You'll naturally gravitate toward either the security-critical core (MPC, threshold signing, HSM/Ledger integration, key lifecycle) or the faster-moving periphery (chain adapters, integrations, APIs/SDKs). What you'll do Design, write, and ship critical platform components: policy engine, signing orchestration, key lifecycle workflows, recovery flows, approvals, audit trails, multi-tenant wallet and account services. Lead non-trivial cross-cutting initiatives in your area — through written RFCs, design reviews, and code, not through hierarchy. Build high-load, multi-tenant SaaS systems: tenant isolation, quotas, rate limits, backpressure, concurrency control, predictable latency under spikes. Distributed systems & reliability: idempotency, retries, outbox/inbox, event-driven workflows, sagas and state machines, replay, graceful degradation. Security-by-design: least privilege, secret handling, mTLS/JWT, tamper-evident logs, audit-ready data models — designed for SOC 2, ISO 27001, and DORA scope from day one. Performance & data: profiling, load testing, benchmarking, contention analysis, GC tuning; strong relational modelling (Postgres or equivalent), migrations, indexing, clear consistency boundaries. AI-assisted development as a core competency: ship production code with Claude Code, Cursor, and Codex; build the prompt and agent patterns that work for your area; contribute to the company's AI engineering practices; know when to use AI heavily (periphery, integrations, tests, docs) and when not to (cryptographic core, signing paths) where blind generation is unsafe. Set patterns and shared libraries for the services your area touches; raise the bar through code review and example. Own features end-to-end: design notes / RFCs, code, tests, observability, rollout, on-call. No "throw it over the wall" handoffs. Mentor senior engineers: your reviews and your written designs are the strongest signal to the rest of the team.