# Fastify v5.12.1

- Product: Fastify (https://whatsnew.fyi/product/fastify)
- Vendor: Fastify
- Date: 2026-08-18
- Version: v5.12.1
- Original notes: https://github.com/fastify/fastify/releases/tag/v5.12.1
- Permalink: https://whatsnew.fyi/product/fastify/releases/v5.12.1

What's New is an index, not a publisher: every entry below links to the vendor's own release notes, which are the authoritative source. Entries are labelled where they are hand-curated sample data, pre-releases, or drawn from a secondary source such as a developer blog.

Reuse: the summaries, labels and curation here are © What's New. Quote freely with attribution and a link back; wholesale republication of the corpus is not permitted — terms: https://whatsnew.fyi/terms. The vendors' own release notes remain their publishers'.

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- **security** — Fix for GHSA-w2qp-rph6-63g4
- **security** — Fix for GHSA-3m5p-2c4r-xxw2
- **fixed** — Fix reply.mediaType test
- **fixed** — Run preClose hook exactly once per declaring instance
- **fixed** — Report canonical route url for hidden prefix slash route

##### ⚠️  Security release

* Fix for https://github.com/fastify/fastify/security/advisories/GHSA-w2qp-rph6-63g4
* Fix for https://github.com/fastify/fastify/security/advisories/GHSA-3m5p-2c4r-xxw2

##### What's Changed
* [Backport 5.x] test: fix reply.mediaType by @github-actions[bot] in https://github.com/fastify/fastify/pull/6950
* [Backport 5.x] ci: allow backport in pr title by @github-actions[bot] in https://github.com/fastify/fastify/pull/6952
* [Backport 5.x] fix: run preClose hook exactly once per declaring instance by @github-actions[bot] in https://github.com/fastify/fastify/pull/6954
* [Backport 5.x] fix: report canonical route url for hidden prefix slash route by @github-actions[bot] in https://github.com/fastify/fastify/pull/6961


**Full Changelog**: https://github.com/fastify/fastify/compare/v5.12.0...v5.12.1