Gives Claude a query language for inspecting and transforming structured data without guessing at the schema. You can ask it to extract fields from JSON, YAML, TOML, HCL, CSV, or Markdown, and it will tell you the shape at each pipe stage before executing. The explain operation shows you what formats the result can serialize to, and when a query produces something incompatible with your target format, it suggests bridging fragments. Useful when you're working with unfamiliar API responses, auditing configuration files, or need Claude to pull specific fields from data it hasn't seen before. The query language is intentionally bounded: no recursion or lambdas, which is the trade-off that makes static shape inference possible.