PromLens is a PromQL query builder and visual debugger for Prometheus. It was originally built by Julius Volz at PromLabs, then donated by PromLabs and Chronosphere to the Prometheus project and open-sourced under Apache-2.0 in 2022.
Its distinguishing feature is a tree view of the PromQL AST: as you type a query, PromLens decomposes it into its sub-expressions and evaluates each node independently against a connected Prometheus server, so you can see the intermediate result of every selector, aggregation, and binary operation in one place. Each node can be inspected as a table or graph, making it straightforward to understand why a rate() produces an unexpected value or where a join is dropping series. It also provides metric and label autocompletion backed by the Prometheus metadata API, a “link to this query” share feature, and direct integration with Grafana panels via a copy-to-Grafana action.
PromLens is typically run as a standalone Docker container or static bundle served next to a Prometheus deployment and is a common tool for SREs learning PromQL or reverse-engineering complex alerting rules. Much of its query-construction UX has since been absorbed back into the Prometheus project’s own web UI.