Senior Platform Engineer

Platform Engineer Marketing teams still ship campaigns by hand. We're replacing that with agents that write, test, and runs campaigns itself, and the systems you'd build are what make that possible at scale. About JustAI We're a YC-backed team building the AI platform that runs marketing for the world's best companies. Marketers describe a campaign in plain language, and our platform turns it into running code that generates, tests, and executes personalized messaging across email and push at scale. About the founders We were at X during the Elon takeover. I had to rewrite the backend infrastructure that now powers the core user and tweet services. You'll get to work closely with us. The role You'll own the layer that sits between raw infrastructure and the product surface marketers touch: the campaign execution engine, the multi-tenant platform APIs, durable and stateful orchestration, and the SDKs and interfaces other teams (and our customers) build on. This is a high-leverage role, since the abstractions you design determine how fast everyone above you can move. In your first quarter you might design how we isolate and fairly schedule work across tenants, or define the SDK contract every product team builds against. What you'll do Design and build the platform primitives that power campaign execution: durable workflows, per-tenant isolation, scheduling, and state management. Define the API and SDK contracts between infra and product, and keep them clean as the product surface grows. Build for multi-tenancy from the ground up, including isolation, fairness, quotas, and safe rollout of per-customer logic. Own reliability and correctness of the execution path: idempotency, retries, and exactly-once semantics where it matters. Partner with product engineers to turn ambiguous product needs into stable platform capabilities. What we're looking for 5+ years building large-scale distributed systems, with a track record of systems that sit between infrastructure and product (platform/API/SDK teams, execution engines, workflow orchestration, internal platforms). Deep experience with stateful, concurrent systems, with a strong grasp of consistency, idempotency, and failure modes. You think in abstractions and contracts, and you've built interfaces other engineers depend on. Comfort operating across the stack, from low-level systems concerns up to the developer experience of the people consuming your platform. We care more about how you think about systems than which specific tools you've used.