# Prometheus v3.15.0

> Observability · 2026-09-25

A feature-rich release adds scraping, logging, memory-limit, and storage capabilities, alongside broad correctness, performance, and stability fixes. Operators should review the deprecated interfaces and the newly enforced PromQL constraints.

## Action needed
- **[breaking]** PromQL range-selector offset and `@` modifier validation
  - PromQL now rejects duration-expression offsets and `@ start()` or `@ end()` modifiers placed before range selectors. This matches the existing rejection of literal offsets and `@ <timestamp>` in that position.

## Check if affected
- **[breaking]** AWS service discovery `request_concurrency` validation
  - Applicability is not stated in the release notes.
  - AWS service discovery now rejects non-positive `request_concurrency` values instead of allowing the service discovery process to hang indefinitely.

## Plan ahead
- **[deprecated]** The `--log.level` flag, deprecated
  - Applies if you set `--log.level`.
  - The `--log.level` flag is deprecated. Use the `runtime.log_level` configuration to set the default log level, including through configuration reloads.
- **[deprecated]** XOR2 float chunk encoding configuration, changed
  - Applies if you enable `--enable-feature=xor2-encoding` or use XOR2 float chunk encoding.
  - The `--enable-feature=xor2-encoding` flag is deprecated for XOR2 float chunk encoding. Use `storage.tsdb.chunk_encoding.floats: xor2` instead, and verify that software reading the TSDB directly, such as a Thanos sidecar, supports XOR2.

## Other recorded changes
- 76 (defect_corrected 50, value_changed 14, added 10, default_changed 1, constraint_changed 1)

[Full analysis](https://ratatosk.io/en/releases/prometheus/v3.15.0)

[Original release notes](https://github.com/prometheus/prometheus/releases/tag/v3.15.0)