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Memory leak in `@tailwindcss/oxide` — RSS grows 0 → 15 GB in 12 seconds, reproduces with both `next build` and `@tailwindcss/cli` standalone (Tailwind v4.3.3)

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
css, node.js, rust
Domain
build-system, cli, performance

Research direction

Start with the minimal globals.css reproduction and run npx @tailwindcss/cli -i ./globals.css -o ./generated.css; inspect the @tailwindcss/oxide bootstrap path reached by the standalone CLI. Compare its behavior with the documented RSS growth and confirm that the compile completes with bounded memory on the 23-byte input.

Written by the indexing model from the issue text.

Description

needs reproduction

Summary

The @tailwindcss/oxide Rust binary used by Tailwind v4 (via @tailwindcss/postcss and @tailwindcss/cli) exhibits unbounded memory growth when processing any input containing @import "tailwindcss";. RSS climbs from ~0 to ~15 GB in 12–60 seconds and is killed by SIGKILL. This is not a Turbopack or Next.js issue — we reproduced the same OOM with the standalone @tailwindcss/cli, which is the official supported build path. The leak is in Oxide itself.

Two independent reproductions, identical OOM signature

Repro A: next build (via @tailwindcss/postcss)

next build (Turbopack) spawns a PostCSS worker that invokes @tailwindcss/postcss, which calls Oxide. Worker RSS grows 30 MB → 15 GB in 56 seconds.

Repro B: @tailwindcss/cli (standalone, no Next.js involved)

The official Tailwind CLI invokes Oxide directly with no Next.js, no Turbopack, no PostCSS, no bundler. RSS grows 0 → 15 GB in 12 seconds, then SIGKILL (exit 137).

This proves the bug is in Oxide, not in Next.js or any integration layer.

Memory profile (Repro B — @tailwindcss/cli)

Host state immediately before compile: 15 GB available RAM, no other Node processes running.

Memory monitor sampled every 2 seconds during tailwindcss -i ./src/app/globals.css -o /tmp/generated.css:

Elapsed Available RAM Top process RSS Process
0 s 15108 MB 276 MB next-server (unrelated)
2 s 14066 MB 1168 MB node (CLI startup)
4 s 10850 MB 4386 MB node
6 s 7984 MB 7239 MB node
8 s 4228 MB 10975 MB node
10 s 1260 MB 13974 MB node
12 s 619 MB 14618 MB node
14 s 63 MB 15170 MB node ← SIGKILL

Growth rate: ~1.7 GB per 2 seconds — consistent with unbounded AST/scan allocation without release, not a slow leak.

Memory profile (Repro A — next build)

Same host, same globals.css, same 15 GB free before build. Sampled every 5 seconds:

Elapsed Available RAM Top process RSS Process
0 s 15433 MB 30 MB caddy (unrelated)
5 s 14721 MB 1404 MB next-build
20 s 13057 MB 2106 MB next-build
25 s 3032 MB 5674 MB node (PostCSS worker spawned)
56 s 28 MB 15198 MB node ← SIGKILL
65 s 15271 MB 275 MB next-build (parent retries)

The PostCSS worker is a child process of next-build:

next-build (v16.3.1)
└─ node ./.next/build/chunks/pool_entry-[turbopack-node]_transforms_postcss_ts_0tp-k2v._.js

The worker file name (transforms_postcss_ts) confirms it is the PostCSS transform pipeline, which calls @tailwindcss/postcss, which calls Oxide.

Reproduction

Repro B (simplest — no Next.js required)

package.json (relevant deps)

{
  "devDependencies": {
    "tailwindcss": "4.3.3",
    "@tailwindcss/cli": "4.3.3"
  }
}

globals.css (23 bytes — minimum that triggers the issue)

@import "tailwindcss";

Command

npx @tailwindcss/cli -i ./globals.css -o ./generated.css

Expected: compile completes, RSS stays bounded (a few hundred MB at most).

Actual: RSS climbs 0 → 15 GB in 12 seconds, killed by SIGKILL (exit 137).

Repro A (Next.js 16 + Turbopack)

src/app/layout.tsx

import './globals.css'

export default function RootLayout({ children }: { children: React.ReactNode }) {
  return (
    <html lang="en">
      <body>{children}</body>
    </html>
  )
}

src/app/globals.css (23 bytes)

@import "tailwindcss";

postcss.config.js

module.exports = {
  plugins: {
    "@tailwindcss/postcss": {},
  },
}

Command

npx next build

Actual: PostCSS worker RSS climbs 0 → 15 GB in 56 seconds, killed by SIGKILL.

Environment

Component Version
Tailwind CSS 4.3.3
@tailwindcss/postcss 4.3.3
@tailwindcss/cli 4.3.3
@tailwindcss/oxide-linux-x64-gnu 4.3.3
Node.js v22.x (also tested on v20.20.2 — same issue)
OS Ubuntu 22.04 (kernel 5.15), x86_64, glibc 2.35

Diagnostic findings

1. Even a 23-byte input triggers the leak

@import "tailwindcss"; alone is sufficient. Custom CSS, @theme inline, @source directives, and project-specific utility classes are not required. The leak is triggered by Oxide's bootstrap, not by user-authored CSS.

2. Not resource contention

Tested with all production services stopped (pm2 Next.js + 8 systemd microservices), 15 GB available RAM, no other Node processes running. The CLI still OOMed at the same RSS ceiling.

3. Tailwind v3 syntax works on the same machine

Replacing globals.css with v3 syntax (@tailwind base; @tailwind components; @tailwind utilities;) makes the build complete successfully with stable memory (~2 GB peak).

4. Not a musl/glibc binary mismatch

Two Oxide native packages were installed (@tailwindcss/oxide-linux-x64-gnu and @tailwindcss/oxide-linux-x64-musl). Removing the musl binary does not resolve the issue. The host is glibc 2.35.

5. Not a Next.js or Turbopack bug

Repro B reproduces the OOM without any Next.js, Turbopack, or bundler involvement. The leak is in @tailwindcss/oxide itself, invoked through @tailwindcss/cli.

Workaround

We have not found a workaround that allows Tailwind v4 to compile on hosts with less than ~32 GB RAM. Current mitigations:

  1. Build on a separate machine with ≥32 GB RAM and deploy the artifact (rsync .next or generated CSS) to production.
  2. Downgrade to Tailwind v3 — works on the same host with stable memory.

Why we're filing this

This issue makes Tailwind v4 effectively unusable on standard 16 GB VPS hosts — a very common deployment target — regardless of whether the user is using Next.js, Turbopack, or the standalone CLI. The growth pattern (0 → 15 GB in 12 seconds on a 23-byte input) is consistent with an unbounded allocation bug in Oxide's bootstrap path, and we believe it is actionable upstream.

Reproduction repo

A fresh project with npm install tailwindcss @tailwindcss/cli + the 23-byte globals.css reproduces the issue. No application code, no framework, no bundler required.

Tags

area: oxide, area: cli, area: postcss, type: bug, severity: blocker

Dominant language
TypeScript
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