Fastify 5.9.0 release notes, 28 Jun 2026

What's Changed

feat: add request.mediaType by @climba03003 in https://github.com/fastify/fastify/pull/6653
docs: remove deprecated leveldb plugin and update ecosystem by @Tony133 in https://github.com/fastify/fastify/pull/6661
chore(sponsor): add bestforandroid by @Eomm in https://github.com/fastify/fastify/pull/6659
ci: drop Node.js 20 from yarn matrix in package-manager-ci.yml by @Tony133 in https://github.com/fastify/fastify/pull/6662
fix: prevent duplicate res.end in sendTrailer with sync callbacks by @climba03003 in https://github.com/fastify/fastify/pull/6676
fix: avoid duplicate closeIdleConnections call on native servers by @trivikr in https://github.com/fastify/fastify/pull/6669
fix: error.code not present on some routing errors by @mcollina in https://github.com/fastify/fastify/pull/6678
fix: correct isCustomSerializerCompiler flag check by @eddieran in https://github.com/fastify/fastify/pull/6657
fix: validate invalid route logLevel at registration by @maxpetrusenko in https://github.com/fastify/fastify/pull/6523
docs: update contribution rules by @Tony133 in https://github.com/fastify/fastify/pull/6670
fix: use ContentType parser for response schema lookup by @UlisesGascon in https://github.com/fastify/fastify/pull/6685
ci(ci): use shared quality workflow by @Fdawgs in https://github.com/fastify/fastify/pull/6688
fix(types): allow request.getValidationFunction() to return undefined by @trivikr in https://github.com/fastify/fastify/pull/6665
fix: do not trust forwarded host/proto when socket is missing by @mcollina in https://github.com/fastify/fastify/pull/6684
perf: defer ContentType parsing in getSchemaSerializer until needed by @aquie00t in https://github.com/fastify/fastify/pull/6692
perf: cache parsed ContentType objects in ContentTypeParser by @aquie00t in https://github.com/fastify/fastify/pull/6694
perf: add typeof guard before toString.call in send and onSendEnd by @aquie00t in https://github.com/fastify/fastify/pull/6693
chore: Bump pnpm/action-setup from 5.0.0 to 6.0.4 by @dependabot[bot] in https://github.com/fastify/fastify/pull/6704
chore: Bump actions/github-script from 8 to 9 by @dependabot[bot] in https://github.com/fastify/fastify/pull/6705
chore: Bump JustinBeckwith/linkinator-action from 2.4.0 to 2.4.2 by @dependabot[bot] in https://github.com/fastify/fastify/pull/6706
docs: correct return503OnClosing comment in route.js by @mcollina in https://github.com/fastify/fastify/pull/6712
fix: enable diagnostics tracking for async error handlers by @irzix in https://github.com/fastify/fastify/pull/6458
fix: ignore duplicate trailer completions by @mcollina in https://github.com/fastify/fastify/pull/6714
feat: add support of onMaxParamLength by @climba03003 in https://github.com/fastify/fastify/pull/6716
chore: introduce TSTyche for type testing by @mrazauskas in https://github.com/fastify/fastify/pull/6532
docs(reference): grammar and readability fixes by @Fdawgs in https://github.com/fastify/fastify/pull/6710
chore: update depedabot setting by @climba03003 in https://github.com/fastify/fastify/pull/6715
fix: include hint and docs URL in FSTWRN004 warning message by @aquie00t in https://github.com/fastify/fastify/pull/6723
ci(ci): do not pass secrets to reusable workflow by @Fdawgs in https://github.com/fastify/fastify/pull/6744
docs: add fastify-intlayer to ecosystem documentation by @aymericzip in https://github.com/fastify/fastify/pull/6594
chore: Bump concurrently from 9.2.1 to 10.0.0 by @dependabot[bot] in https://github.com/fastify/fastify/pull/6752
docs(Errors): fix incorrect usage of root fastify inside plugin scope by @Rpaudel379 in https://github.com/fastify/fastify/pull/6731
ci: add node 26 to test matrices by @Fdawgs in https://github.com/fastify/fastify/pull/6728
docs(Warnings): remove retired FSTWRN002 warning code by @leestana01 in https://github.com/fastify/fastify/pull/6754
fix: chunk large HTTP/2 buffer replies by @mcollina in https://github.com/fastify/fastify/pull/6746
chore: migrate type tests to TSTyche assertions (part one) by @mrazauskas in https://github.com/fastify/fastify/pull/6726
chore: migrate type tests to TSTyche assertions (part two) by @mrazauskas in https://github.com/fastify/fastify/pull/6727
docs: fix doubled braces in serializerCompiler signature by @DucMinhNe in https://github.com/fastify/fastify/pull/6747
docs: remove HackerOne reporting link by @jhcpeixoto in https://github.com/fastify/fastify/pull/6735
refactor(decorate): replace find with some in hasKey for correct boolean semantics by @aquie00t in https://github.com/fastify/fastify/pull/6759
fix: replace AssertionError with FST_ERR_PLUGIN_DEPENDENCY_NOT_REGISTERED in checkDependencies by @aquie00t in https://github.com/fastify/fastify/pull/6774
docs(ecosystem): add @inferdi/fastify by @maxrendel in https://github.com/fastify/fastify/pull/6742
docs(typo): Write-Plugin.md by @zakirimadullahprogrammer-tech in https://github.com/fastify/fastify/pull/6776
docs: fix duplicate anchor IDs causing broken TOC links by @AliMahmoudDev in https://github.com/fastify/fastify/pull/6770
docs: add fastify-ata as a JSON Schema validator option by @mertcanaltin in https://github.com/fastify/fastify/pull/6733
chore: rename type test files by @mrazauskas in https://github.com/fastify/fastify/pull/6762
docs: update ajv-errors guidance by @Herrtian in https://github.com/fastify/fastify/pull/6741
chore(warnings): correct duplicate 'not' typos in inline comments by @mixelburg in https://github.com/fastify/fastify/pull/6713
docs: add fastify-param-schema-validation to ecosystem by @Player1205 in https://github.com/fastify/fastify/pull/6760
docs(ecosystem): add @thecodepace/fastify-http-query by @Puppo in https://github.com/fastify/fastify/pull/6785
chore: Bump esbuild from 0.25.12 to 0.28.1 in /test/bundler/esbuild in the npm_and_yarn group across 1 directory by @dependabot[bot] in https://github.com/fastify/fastify/pull/6788
fix: clear socket._meta after response to prevent keep-alive leaks by @nerkoux in https://github.com/fastify/fastify/pull/6799
fix: avoid double slash when joining nested prefixes by @rohithvegesna in https://github.com/fastify/fastify/pull/6803
docs(sponsors): Update sponsors list by removing two entries by @Eomm in https://github.com/fastify/fastify/pull/6792
chore: Bump fastify-plugin from 5.1.0 to 6.0.0 by @dependabot[bot] in https://github.com/fastify/fastify/pull/6801
chore(package.json): fix delvedor's personal url by @Fdawgs in https://github.com/fastify/fastify/pull/6808
chore: replace http with https in urls by @Fdawgs in https://github.com/fastify/fastify/pull/6809
chore: add new sponsor by @Eomm in https://github.com/fastify/fastify/pull/6813
chore: Bump actions/checkout from 6 to 7 by @dependabot[bot] in https://github.com/fastify/fastify/pull/6812
fix: hasRequestDecorator/hasReplyDecorator misses constructor-assigned built-in properties by @LeSingh1 in https://github.com/fastify/fastify/pull/6753
docs: migrate Zod type provider to official @fastify package by @Tony133 in https://github.com/fastify/fastify/pull/6686
docs: add warning about empty string coercion with nullable types by @ritambh888 in https://github.com/fastify/fastify/pull/6452
docs: fix incorrect code examples in Hooks and Server reference by @rrodj in https://github.com/fastify/fastify/pull/6622
docs: update Serverless guide Dockerfile to a supported Node.js version by @harish885 in https://github.com/fastify/fastify/pull/6789
docs(types): mark request metadata accessors as untrusted input by @mcollina in https://github.com/fastify/fastify/pull/6572
Bumped v5.9.0 by @Eomm in https://github.com/fastify/fastify/pull/6816

Full Changelog: https://github.com/fastify/fastify/compare/v5.8.5...v5.9.0