Senior Platform Engineer

JustAI; San Francisco (CA); On-site; USD 130,000 - 180,000; Full time

Job summary

JustAI is looking for a Platform Engineer to design and build the infrastructure that powers marketing campaign execution. You will work on reliable and scalable systems that allow marketers to automate their campaigns across multiple channels.

The ideal candidate has over 5 years of experience in building distributed systems and understands the intricacies of platform-level design. Join a YC-backed team and work closely with the founders to take the marketing AI platform to the next level.

Qualifications

• 5+ years building large-scale distributed systems.
• Experience with stateful, concurrent systems.
• Proven skills in defining system abstractions.

Responsibilities

• Design and build platform primitives for campaign execution.
• Define API and SDK contracts for product interactions.
• Ensure reliability and correctness of the execution path.

Skills

Building large-scale distributed systems

Stateful concurrent systems

System abstraction and contracts

Reliability and correctness in systems

Job description

Platform Engineer

Marketing teams still ship campaigns by hand. We're replacing that with agents that write, test, and run campaigns themselves, 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.