Senior Software Engineer, Backend (AI Platform)

Affirm

22h • $173,000 - $255,000 • Remote

About The Position

Affirm’s AI Platform team builds and operates the platform for developing, running, evaluating, and governing AI systems. Its customers include Affirm users, engineers, and production workloads. The team provides simple, governed interfaces to models, tools, and agents, with identity, cost attribution, reliability, and security built in. Its components include an LLM Gateway for approved models, an MCP Gateway for mediating and auditing tool calls, an Agent Runtime for bounded execution, and evaluation and observability capabilities. The platform is in an early stage, with the gateway in daily engineering use and the first production agent live, and the team is expanding toward production inference and agents.

The Senior Software Engineer will own technical decisions and projects from design through launch and on-call. The role involves designing and building gateway, runtime, and evaluation services; creating controls for AI identity, quotas, routing, caching, and failover; and partnering with security, infrastructure, and product engineering on guardrails and identity for agentic tool access. The engineer will own system operational health and metrics, raise engineering standards, use AI tools and agentic workflows, and mentor and support other engineers.

Affirm is a remote-first company. Most roles can be performed from almost anywhere within the country of employment, though some positions may occasionally require office attendance. New hires are invited to an in-person onboarding experience.

Requirements

• At least 4 years of experience designing, developing, and launching backend systems at scale using languages such as Python or Kotlin.
• Track record of developing highly available distributed systems using technologies such as AWS, MySQL, and Kubernetes.
• Practical production experience building with LLMs, agents, or AI frameworks, such as inference APIs, LLM gateways, MCP, tool calling, evaluations, or agent runtimes.
• Understanding of AI usage and the infrastructure needed to support and leverage AI workloads across disciplines.
• Experience defining a technical plan to deliver a significant feature or system component with an elegant, simple, and extensible design.
• Ability to write high-quality code that is easily understood and used by others.
• Proficiency making significant changes in a large codebase, supported by tools and practices that enable safe changes.
• Demonstrated ownership of professional growth, including proactively seeking feedback from teammates, managers, and stakeholders.
• Strong verbal and written communication skills for effective collaboration with a global engineering team.
• Equivalent practical experience or a bachelor’s degree in a related field.

Responsibilities

• Plan, execute, and own AI Platform technical decisions and projects from design through launch and on-call.
• Design and build gateway, runtime, and evaluation services to move AI workloads from pilot to production and scale them securely.
• Build identity, quota, routing, caching, and failover controls to make AI spending attributable and governable.
• Partner with security, infrastructure, and product engineering teams to establish guardrails for agentic workloads and the on-behalf-of identity model for tool access.
• Create and monitor system metrics and own operational health, including monitoring, alerting, runbooks, and on-call.
• Set and improve code review and design standards, and advocate for them through writing and tech talks.
• Use AI tools and agentic workflows in coding, design review, and operations, and help drive the capabilities of an AI-native team.
• Educate, mentor, and grow engineers, and promote best practices in design, coding, and testing.

Benefits

• 100% subsidized medical coverage for employees and dependents.
• Monthly stipends for technology setup and health and wellness options.
• Flexible time off and generous holiday calendars.
• Employee stock purchase plan allowing employees to buy Affirm stock at a discount.
• Dental and vision coverage for employees and dependents.

What This Job Offers

• Job Type: Full-time
• Career Level: Senior