Lookslike is an open-source library developed by Elastic for testing and schema validation in Go, designed to match the structure of Golang data structures in a more powerful and Go-like way compared to JSON Schema. It allows developers to precisely or loosely match data structures and reduce code duplication by composing different field definitions. The library emerged from the Heartbeat project at Elastic, which required validation of Elasticsearch documents produced by their Uptime solution. Lookslike's architecture is built around two main types: Validator and IsDef, with Validator being used to compile schemas and IsDef for matching individual fields. The library supports flexible, composable, and nestable schemas, and includes a test helper for clear test output, making it particularly useful in testing contexts. Elastic invites contributions to expand the set of IsDefs and has provided extensive documentation on godoc.org for further learning.