SynapS3 is an open-source S3-compatible gateway that enables existing object storage tools and applications to use Filecoin Onchain Cloud through the standard S3 API, minimizing migration effort from Web2 workflows. It provides a local caching and background upload pipeline with operator-focused tooling for retries, health checks, observability, and reproducible containerized deployment. By bridging familiar S3 interfaces to Filecoin-backed storage, SynapS3 lowers adoption friction and helps expand real-world utilization of Filecoin infrastructure.