Harness Engineer

Harness Engineer Location: San Francisco (in-person, in-office, full-time ONLY) Compensation: $375K–$625K total annual compensation | $130k-$200k base + $25K-$50K cash bonus About Virio Virio is building the next-generation B2B GTM stack. Backed by investors and operators from LinkedIn, YouTube, HubSpot, Rippling, and Google, we help the fastest-growing B2B companies go to market through executive-led content that drives revenue. We’re growing extremely quickly, added $1M ARR in 30 days, and are onboarding customers faster than we can manage—so we’re looking for a Growth Engineer to build internal tools and workflows to augment our team. What You'll Do As a Harness Engineer, you'll own the intelligence layer that sits between our AI models and product—the system prompts, tool definitions, context management, and evaluation framework that makes agents actually work in production. Specifically: Build and refine the system prompts, tool integrations, and context windows that shape how our models behave across our platform—you'll see how small prompt tweaks cascade into measurable product impact. (Nice to have: background in English, literature, creative writing, or humanities) Design the evaluation framework (LLM-as-judge evals, deterministic tests, production monitoring) that lets us ship with confidence. You'll define what success looks like and build the metrics to measure it. Architect the abstraction layer between our product (file systems, artifacts, skills) and the model's capabilities—making complex multi-step workflows feel natural to the model and reliable to users. Own prompt versioning, experimentation, and iteration. You'll A/B test prompt variations, measure their impact on agent quality, and ship improvements across our production system. Collaborate with product and engineering to identify signal—where our agents are failing, where users are stuck—and translate that into prompt and architecture improvements. What Success Looks Like After 30 days: You understand our agent stack—the system prompts, available tools, context window strategy, and current failure modes. You've shipped your first prompt improvement and measured its impact. After 90 days: You've designed and implemented our core evaluation framework. Our team uses it as the source of truth for agent quality. You've shipped 2-3 meaningful prompt iterations that improved measurable outputs, and you're comfortable making architecture trade-offs. After 6 months: You own the harness layer end-to-end. You've architected improvements that reduced hallucination, improved context retention, or increased model reliability. You're driving architectural decisions about how we expose capabilities to the product team. You're writing technical content about how we've solved these problems—attracting the next generation of harness engineers to the company. Traits That Thrive at Virio High agency & self-direction – Acts without waiting for requirements or detailed instructions Small-team intensity comfort – Comfortable working intensely and closely with a small, high-output team Unorthodox problem solving – Sees non-obvious paths forward and challenges conventional approaches Quality-driven execution – Detail-oriented with high standards for correctness and quality Low ego & accountability – Takes full ownership and accountability for outcomes Benefits Work directly with the founders Competitive total compensation aligned to impact and ownership Meaningful equity upside as part of the founding team Medical, dental, and vision insurance 401(k) plan All working meals covered Relocation support for San Francisco Company-wide annual off-site High ownership, high trust, and fast career progression alongside world-class client partners In-person culture with deep commitment to excellence This is a full-time, on-site role based in San Francisco. We do not offer hybrid or remote positions.