Software Engineer, Agents

About SimSim is the open-source platform to build, deploy, and orchestrate AI agents. Over 70,000 developers — from early-stage startups to the U.S. Government — use Sim to turn ideas into production-grade agentic workflows on a visual, Figma-like canvas. We connect to 1,000+ apps and LLMs, and our Copilot is the state of the art for building workflows with natural language.We're a team of 5, YC X25, and backed by a $7M Series A led by Standard Capital with participation from Paul Graham, Perplexity, SV Angel, and Y Combinator. We're based in San Francisco and build in-person every day.Our mission is simple: agents will run the world, and Sim is the primary means by which that happens.About The RoleYou'll own end-to-end development of our core agentic workflow engine and Copilot — the AI assistant that helps developers build and debug workflows using natural language, turning high-level intent into executable, reliable agents. This spans our backend (Next.js), orchestration layer, and all integrations with LLMs and external APIs.Copilot is currently the best-in-class tool for building workflows with natural language. Your job is to keep it there and push it further — making it faster, more reliable, and capable of handling increasingly complex agentic architectures.This is a foundational role. You'll set architecture decisions, reliability standards, and coding patterns that define how the core product works. On a team of 5, your work ships to tens of thousands of developers on fast release cycles.What You'll DoOwn the agentic workflow engine: the runtime that executes multi-step, tool-using agent workflows in productionBuild and evolve Copilot — our natural language interface for creating, editing, and debugging workflowsDesign and maintain integrations with LLM providers (OpenAI, Anthropic, Google, local models via Ollama) and external APIsArchitect the orchestration layer that turns visual flows into reliable, observable agent executionsSet technical standards for the codebase — reliability, testing, code quality, and architecture patternsWork across the stack in our Next.js monorepo, shipping to production dailyDebug hard problems at the intersection of LLMs, distributed systems, and developer toolingWhat We're Looking ForStrong generalist engineer who has shipped complex distributed systems or developer tools to productionDeep experience with TypeScript/JavaScript and Bun; comfort working in a Next.js monorepoTrack record of owning and operating production systems — you've been paged, you've debugged the weird ones, you've made things reliableExperience integrating with LLMs or building on top of foundation model APIsHigh agency and ownership — on a team this small, you define the roadmap as much as you execute itStrong opinions on architecture, loosely held — you care about building things right but move fastExperience with AWS infrastructure is a plusContributions to open-source projects are a plusWhy SimFoundational impact. You're not joining a team — you're building one. Every decision you make shapes the product and the company.Real users, real scale. 70,000+ developers use Sim today. Your work ships fast and matters immediately.World-class backers. Series A - Standard Capital, Paul Graham, Perplexity, SV Angel, and Y Combinator believe in what we're building.Open source at the core. We've been open source from day one and we're proud of it. 26,000+ GitHub stars and growing.Competitive salary + meaningful equity. Early team, significant ownership.Full benefits. Comprehensive health insurance, relocation.Visa sponsorship available.