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

New Contributors

• @eddieran made their first contribution in https://github.com/fastify/fastify/pull/6657
• @irzix made their first contribution in https://github.com/fastify/fastify/pull/6458
• @aymericzip made their first contribution in https://github.com/fastify/fastify/pull/6594
• @Rpaudel379 made their first contribution in https://github.com/fastify/fastify/pull/6731
• @leestana01 made their first contribution in https://github.com/fastify/fastify/pull/6754
• @DucMinhNe made their first contribution in https://github.com/fastify/fastify/pull/6747
• @jhcpeixoto made their first contribution in https://github.com/fastify/fastify/pull/6735
• @maxrendel made their first contribution in https://github.com/fastify/fastify/pull/6742
• @zakirimadullahprogrammer-tech made their first contribution in https://github.com/fastify/fastify/pull/6776
• @AliMahmoudDev made their first contribution in https://github.com/fastify/fastify/pull/6770
• @mertcanaltin made their first contribution in https://github.com/fastify/fastify/pull/6733
• @Herrtian made their first contribution in https://github.com/fastify/fastify/pull/6741
• @mixelburg made their first contribution in https://github.com/fastify/fastify/pull/6713
• @Player1205 made their first contribution in https://github.com/fastify/fastify/pull/6760
• @Puppo made their first contribution in https://github.com/fastify/fastify/pull/6785
• @nerkoux made their first contribution in https://github.com/fastify/fastify/pull/6799
• @rohithvegesna made their first contribution in https://github.com/fastify/fastify/pull/6803
• @LeSingh1 made their first contribution in https://github.com/fastify/fastify/pull/6753
• @ritambh888 made their first contribution in https://github.com/fastify/fastify/pull/6452
• @rrodj made their first contribution in https://github.com/fastify/fastify/pull/6622
• @harish885 made their first contribution in https://github.com/fastify/fastify/pull/6789

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