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Full-Stack API Developer – Merchant Integration at Hyphen Connect Limited. . Location: China. Job Summary. . We are hiring for an innovative crypto wallet and payment network startup who are looking for a Full-Stack API Developer to build and maintain merchant integration APIs, client portals, and SDKs for our crypto wallet app’s merchant payment system. You will develop backend services to support crypto-only payments via QR/NFC and POS, contributing to the January 2026 rollout. This remote role requires expertise in API development, merchant systems, and blockchain technology.. . Responsibilities. . . Design and develop merchant-facing APIs and SDKs to enable crypto payments via wallet app.. . Build and maintain merchant, admin, and customer portals for transaction management and reporting.. . Integrate backend services with smart contract-based payment systems for secure transactions.. . Collaborate with the Tech Lead and Integration Engineer to align APIs with merchant requirements.. . Ensure scalability, security, and performance of APIs for online and physical POS environments.. . Document APIs and SDKs for external developers and internal teams.. . . Qualifications. . . 5+ years of experience in full-stack development, with a focus on API and SDK development.. . Proven experience building merchant or client portals for payment systems (e.g., fintech, e-commerce).. . Strong proficiency in backend technologies (e.g., Node.js, Python, Go) and frontend frameworks (e.g., React, Vue.js).. . Familiarity with blockchain technology and crypto wallets (e.g., Solidity, Web3.js, Ethers.js).. . Experience with RESTful APIs and integration with payment gateways or POS systems.. . Ability to work remotely and align with Hong Kong/Singapore time zones.. . Bachelor’s degree in Computer Science, Engineering, or a related field (or equivalent experience).. . . Preferred Skills. . . Experience with crypto payment gateways (e.g., Coinbase Commerce, BVNK) and QR/NFC systems.. . Knowledge of smart contract development and security (e.g., Hardhat, Foundry).. . Familiarity with POS hardware integration for in-store payments.. . Strong documentation skills for developer-facing APIs and SDKs..