Software Engineer

Job Title: Software Engineer

Department: Engineering & Product Development

Reports to: Engineering Lead

Location: Flexible but Singapore preferred

About Codex

Codex is building the infrastructure for stablecoin-based cross-border payments - moving money faster, cheaper, and more transparently than the legacy financial system. Backed by Circle, Coinbase, Dragonfly, Wintermute, and Cumberland, we're a small team solving hard problems at the intersection of crypto and global payments.

The Role

You'll own Codex's financial partner integrations end-to-end - from API contract and webhook design through to reconciliation and production hardening - and build the automation layer that makes our trading and treasury operations increasingly self-running.

This is a high-ownership role at the infrastructure layer of the business. You'll be adding new payment rails, eliminating manual operational workflows, and designing systems that handle real money reliably across fiat and crypto rails. You'll work directly with the founders and Engineering & Product Development team, and closely with Compliance, Operations, and Financial Partnerships.

Responsibilities

Financial Partner Integrations

• Own integrations with banks, payment providers, and liquidity venues end-to-end - from API contract and webhook design through to production hardening and reconciliation

• Build integrations that are robust to vendor reality: incomplete documentation, schema changes without warning, and validation rules that shift post-go-live

• Build messaging / notification systems with signature verification and replay protection; pin schemas against real sandbox behaviour; design for failure at every layer

• Maintain and evolve the reconciliation layer across all connected payment rails - fees, settlement windows, and exception handling as first-class concerns

Operational Automation

• Identify manual workflows in onboarding, compliance, and back-office operations and systematically replace them with product and automation

• Enable self-service for internal ops teams and customers, reducing engineering dependency for routine operational tasks

• Work closely with Compliance and Operations to understand where the friction is and build the tooling that removes it

Treasury & Liquidity Automation

• Design and build the automation layer for Codex's trading and treasury operations - automated position management, liquidity routing, and risk controls

• Design for failure: clear handling of automated decision failures, position unwinding, and safe degradation

Engineering Standards

• Set and maintain the engineering standard for how payment integrations are built at Codex - idempotency, audit trails, fail-closed design, exact decimal arithmetic, and deterministic durable orchestration

• Mentor engineers on the hard parts of money movement systems

• Continuously design and build for security, hardening our systems

• Ship and critically review AI-assisted code with the judgement to identify plausible but incorrect output or output detrimental to a product’s long-term health and sustainability

What We're Looking For

• 6 to 10 years of backend engineering experience in cross-border payments, stablecoins, FX infrastructure, or crypto custody

• Production experience with both crypto rails (on-chain settlement on major networks like Ethereum, Tron and Solana; bridging major stablecoins; DeFi protocols) and fiat rails (SWIFT, SEPA, local schemes such as FPS or CHATS)

• Must-have: Production experience with durable workflow orchestration - Temporal strongly preferred. Includes deterministic replay, safe versioning of live workflows, idempotency keys, and compensation logic

• Must-have: Strong TypeScript/Node and Postgres in a money context - fail-closed design, exact decimal arithmetic, append-only audit trails

• Must-have: Hands-on third-party API integration experience, including vendors with incomplete documentation, webhook reliability requirements, and schema instability