# Bun 1.4.4 adds a built-in formatter that reads Prettier configs — but output is not always identical Bun's October 22 release makes code formatting part of the runtime, including staged-file hooks, Prettier/oxfmt configuration reuse and multi-language support. Bun's dramatic speed claims are reproducible in principle, but remain vendor benchmarks and real-world output compatibility is imperfect. Bun 1.4.4 introduces bun fmt, accelerates bun check and preserves Yarn Berry lockfiles during migration. Teams should test formatter output and unsupported plugins before replacing Prettier in CI. - Status: Active - Published: 2026-10-27T06:12:25+13:00 - Updated: 2026-10-27T06:12:25+13:00 - Categories: Web Development, JavaScript, Developer Tools - Tags: Bun, code formatting, developer tooling, JavaScript, Prettier, TypeScript, Yarn - Canonical HTML: https://beyondthe.news/dossiers/bun-1-4-4-bun-fmt-prettier-oxfmt-formatter-yarn-berry-migration ## What changed On October 22, 2026, Bun released v1.4.4 with bun fmt, a built-in formatter covering JavaScript, TypeScript, JSX, CSS, HTML, Vue, Svelte, JSON, YAML, TOML, Markdown and other formats. It reads existing Prettier and oxfmt configurations and supports --check, --staged and --changed workflows. The release also speeds up the built-in bun check TypeScript checker, migrates Yarn 2+ (Berry) lockfiles without re-resolving pinned dependencies, and includes security and runtime fixes. This is distinct from Bun 1.4.3's initial built-in type checker. ## Why it matters Formatting is a recurring cost in developer workflows and CI, particularly in large repositories. Integrating it into a runtime can remove an extra formatter dependency and make pre-commit checks practical on large trees. But compatible configuration does not guarantee byte-identical output: formatter swaps can create noisy diffs or fail on syntax and plugins. Bun users can evaluate consolidation without automatically rewriting the codebase. ## Formatter reuse without a wholesale config rewrite bun fmt reads .prettierrc, prettier.config, .editorconfig, .prettierignore and oxfmt configuration. It includes Tailwind class sorting, Svelte formatting and package.json key sorting, and can format staged files and restage them for Git hooks. Some unsupported Prettier plugins trigger an error and exit code 2, while languages needing an installed plugin such as Astro can fall back to Prettier. The older bun format command still runs a project's format script, so scripts and direct CLI invocations are not interchangeable. ## Speed is promising, but the headline numbers are Bun's Bun reports bun fmt --check taking 59.7 ms versus 17.1 seconds for Prettier 3.9.10 on Prettier's 2,616-file repository, using 48 MB rather than 459 MB peak memory. It publishes pinned commits and reproduction commands. The same release reports 3–6x speedups on several oxfmt workloads, with one fumadocs comparison much higher. Independent articles corroborate the release and note the caveats but do not provide independent reproduction of those timings. Hardware, cache warmth, project composition and options matter. ## Configuration compatibility is not output identity Bun says 9,648 of 9,731 Prettier fixture tests produce identical output, but across 74 real repositories using Prettier only 57 full runs (77%) were byte-for-byte identical. Differences include older formatter versions, plugins, Markdown and unsupported syntax; in one Next.js test one Babel-only proposal file prevented an otherwise mostly identical run. For existing repositories, compare git diffs and run bun fmt --check and --debug-check on a pinned version before switching CI. ## Lockfiles and type checking also change bun install now migrates Yarn Berry lockfiles, retaining pinned versions rather than re-resolving the dependency graph, including workspaces and aliases. Bun also reports substantial improvements to bun check on larger repositories and monorepo project-reference builds, alongside broad TLS, WebSocket, database-client and package-manager security fixes. The release does not by itself establish that a particular earlier vulnerability is remediated without a specific advisory. ## Key details - Release: Bun v1.4.4, October 22, 2026. - New built-in bun fmt supports --check, --staged, --changed, existing Prettier and oxfmt configurations, and common frontend languages. - Bun vendor benchmark: 59.7 ms versus 17.1 seconds on Prettier's own 2,616-file repository; not independently reproduced. - Bun's own output tests: 9,648/9,731 Prettier fixtures identical, but only 57/74 full Prettier-repository comparisons identical. - Unsupported formatter plugins or Babel-only syntax can block adoption; bun format remains the package script invocation. - Yarn 2+ Berry lockfiles migrate with pinned dependency versions retained. - Bun reports faster bun check and broad protocol/client security fixes. ## Builder takeaways - Try bun fmt --check on an existing branch before replacing Prettier or oxfmt; review changed files and unsupported plugins. - Pin Bun and compare output on the repository's actual config, including generated files and Markdown. - Use bun fmt --staged in a pre-commit hook only after validating file selection and re-staging behaviour. - For Yarn Berry migration, diff bun.lock against the old resolution graph and test reproducible installs. - Treat Bun's speedup figures as vendor measurements until independently reproduced in your workload. ## What to watch - Independent cross-platform benchmarks and compatibility testing of bun fmt. - Formatter plugin and syntax coverage in subsequent releases. - Whether large projects actually remove separate formatter dependencies without churn. - Security advisories tying the broad 1.4.4 fixes to specific affected versions and CVEs. ## Uncertainties - No independently reproduced timing benchmark was located for the release. - Byte-identical compatibility varies by project, config, plugin and language. - Broad security-fix descriptions are not a substitute for CVE-specific remediation evidence. ## Sources - [Bun v1.4.4 release notes](https://bun.com/blog/bun-v1.4.4) — Bun · primary_release · 2026-10-22T00:00:00+13:00. Canonical feature, configuration, benchmark methods, compatibility data and migration details. - [Bun 1.4.4 adds bun fmt, a Prettier-compatible formatter](https://techaiwire.com/articles/bun-1-4-4-bun-fmt-prettier-compatible-formatter/) — Tech AI Wire · independent_reporting · 2026-10-22T00:00:00+13:00. Release corroboration and explicit warning that vendor benchmark is unverified. - [Bun ships a built-in formatter 286 times faster than Prettier](https://azeemhassni.com/blog/wire-bun-fmt-built-in-formatter/) — Azeem Hassni · independent_commentary · 2026-10-23T00:00:00+13:00. Independent developer explanation and practical CI/formatting caveat, not an independent benchmark.