horizon3ai - Senior Software Engineer, Agentic Systems
Requirements
• 5+ years building production software, with strong Python. • Hands-on experience building LLM-powered applications or agents, tool use / function calling, structured outputs, multi-step orchestration, and the glue that makes it all hold together. • A track record of making LLMs reliable in production, you've wrestled nondeterminism, designed around model limitations, and shipped something that worked when it mattered. • Real experience with evaluation: you've built or owned the harness that tells you whether a model or agent change is an improvement, not just a vibe. • Strong instincts for prompt and context engineering, and the judgment to keep the model's job small and well-scoped. • Solid software fundamentals — testing, observability, and the discipline to keep a complex agent debuggable. • Ownership mentality, comfortable owning a critical, fast-moving subsystem end to end. • Working knowledge of web application security, broken access control, IDOR/BOLA, SQLi, XSS, SSRF, SSTI, enough to collaborate fluently with offensive engineers. • Experience building eval harnesses or benchmarks specifically for agents (synthetic environments, CVE-based test targets, capture-the-flag-style scoring). • Experience with agent frameworks, and strong opinions about when not to reach for one. • Familiarity with graph data models (e.g., Neo4j) for representing application state and attack context. • What makes you stand out: • You've shipped an autonomous agent that did real, valuable work unattended in production, and you have scar tissue from making it trustworthy. • You've designed evaluation systems that actually drove improvement, closed the loop between "we changed something" and "it measurably got better." • You pair an offensive-security mindset (CTF, bug bounty, pentesting, or research background) with the engineering chops to turn that intuition into a reliable system. • You have hands-on experience with agent fine-tuning or RL (SFT, GRPO, reward design for tool-using agents) and a grounded view of when it's worth it versus improving the harness. • You've published or spoken on agent reliability, evaluation, or autonomous security tooling.
Responsibilities
• Please note this job description is not designed to cover or contain a comprehensive listing of activities, duties or responsibilities that are required of the employee. Duties, responsibilities, and activities may change at any time with or without notice. • Application Note • Application Note • In any materials you submit, you may redact or remove age-identifying information such as age, date of birth, or dates of school attendance or graduation. You will not be penalized for redacting or removing this information.
Benefits
• Inclusive Team: We value diversity and promote an inclusive culture where everyone can thrive. • Growth Opportunities: Be part of a dynamic and growing team with numerous career development opportunities. • Innovative Culture: Work in a collaborative environment that encourages creativity and out-of-the-box thinking. • Hybrid & Remote Work: We embrace a mix of remote and hybrid work models depending on role and location, including our Chicago office, where some roles require regular in-office presence. • Competitive Compensation: We offer competitive salary, equity and benefits. Our benefits include health, vision & dental insurance for you and your family, a flexible vacation policy, and generous parental leave. • At Horizon3, we believe that our people are our greatest asset, and our compensation philosophy reflects this core value. We are committed to fostering an environment where all employees feel valued, respected, and rewarded for their contributions. Our compensation structure is designed to be fair, competitive, and transparent, ensuring that every team member is recognized and compensated equitably across roles, levels, and locations. • In accordance with various State’s transparency regulations, we provide the following salary range information for this position: • Base salary range: $169,000 - $208,000 annually. The exact salary will be determined based on the selected candidate’s location, qualifications, experience, and relevant skills. • Additional compensation: All full-time roles are eligible for an equity package in the form of stock options. • You Belong Here
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