Backend Integration Engineer - Secure API Integration for a Voice Assistant

We're a voice-AI agency. We've completed phase one of a build for an enterprise client and need a backend engineer for phase two. This is a pure backend integration project - we own all conversation design, prompts and the client relationship. You will not be writing prompts or doing any AI/ML work. WHAT YOU'D BUILD A middleware service that sits between a conversational voice platform and a client's internal systems. It is the trust boundary: the voice platform never holds client credentials and never makes decisions about access. - REST/webhook integration against a documented third-party API - A server-side verification state machine: session status, attempt counters, and lockouts that persist beyond a single session - Tool endpoints with strict JSON schemas, returning minimum-necessary fields - Idempotent write operations (records must not duplicate on retry or dropped connection) - A read-only data adapter with field-level allowlisting and staleness handling - Full audit logging of every call, every access decision and every failure - A webhook handler for post-session events STACK Node/TypeScript or Python (your choice, whichever you are fastest in), Postgres, Redis, OAuth2/JWT, structured logging. HOW IT STARTS The first several weeks are built against a documented API contract and a mock server, so you can be productive immediately without waiting on client environment access. ENGAGEMENT - Contract, roughly 300-350 hours across two phases - Start early September 2026. First milestone early November, second early December. - Remote. Some overlap with UK working hours needed for standups and reviews. REQUIREMENTS - Production experience integrating third-party REST APIs, including OAuth2 or equivalent, retry/backoff and idempotency - Comfortable designing audit trails and access-control logic - Postgres and Redis to a production standard - Disciplined about negative-path testing. You test the failure cases, not just the happy path. - Willing to sign an NDA, non-circumvention and IP assignment (standard; the end client is in a regulated sector) NICE TO HAVE - Experience with regulated data (finance, healthcare or similar) where audit logging and access control were requirements, not nice-to-haves - Any voice-agent platform exposure (ElevenLabs, VAPI, Retell, LiveKit). Genuinely optional; the work is backend. TO APPLY Tell me briefly about one integration you built where the failure handling was the hard part: retries, partial failures, duplicate prevention, or access control. Skip the generic intro, I'll read that answer first. Shortlisted candidates get a paid 4-hour trial task before any offer.