nextjs - ✅(Solved) Fix @next/mdx: Turbopack rule uses `as: '*.tsx'` causing unnecessary double-processing of MDX output [1 pull requests, 4 comments, 2 participants]

nextjs2026-03-09 14:48:13

ON THIS PAGE

Recommended Tools

×6

Utilities matched from this issue’s tags and category — try them while you read without losing context.

GitHub issue graph ai analysis

Paste a GitHub issue URL. We fetch that issue, discover linked issues from bodies/comments/timeline, collect linked pull requests, and produce a structured English report.

The report is written in English Markdown for sharing and archival.

GitHub issue URL

Helpful · Quick feedback

GitHub stats

vercel/next.js#91103•Fetched 2026-04-08 00:18:59

View on GitHub

Comments

Participants

Timeline

Reactions

Author

314systems

Participants

314systems

timneutkens

Timeline (top)

commented ×3cross-referenced ×1issue_type_added ×1labeled ×1

Fix Action

Fixed

Fixed by PR: fix(mdx): use as: '.js' instead of '.tsx' in Turbopack rule (https://github.com/vercel/next.js/pull/91134)

PR fix notes

PR #91134: fix(mdx): use as: '.js' instead of '.tsx' in Turbopack rule

Repository: vercel/next.js
Author: sleitor
State: open | merged: False
Link: https://github.com/vercel/next.js/pull/91134

Description (problem / solution / changelog)

Fixes #91103

Problem

The Turbopack loader rule for .mdx files sets as: '*.tsx', which routes already-compiled JavaScript through Turbopack's full TypeScript/JSX pipeline a second time.

@mdx-js/mdx already compiles MDX to plain JavaScript using the automatic JSX runtime (_jsx()/_jsxs() calls). The output contains no JSX syntax and no TypeScript, so the extra pipeline is pure overhead.

For large MDX files (e.g., rehype-katex on a math-heavy page: ~1.9 MB), the redundant source-map merging alone adds ~50 s of compilation time.

Fix

Change as: '*.tsx' → as: '*.js' so Turbopack treats the loader output as plain JavaScript and skips the unnecessary TypeScript/JSX pipeline.

Changed files

packages/next-mdx/index.js (modified, +1/-1)

Code Example

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Sun Aug  6 20:05:33 UTC 2023
  Available memory (MB): 4102
  Available CPU cores: 2
Binaries:
  Node: 20.12.1
  npm: 10.5.0
  Yarn: 1.22.19
  pnpm: 8.15.6
Relevant Packages:
  next: 16.2.0-canary.85 // Latest available version is detected (16.2.0-canary.85).
  eslint-config-next: N/A
  react: 19.2.4
  react-dom: 19.2.4
  typescript: 5.9.3

RAW_BUFFERClick to expand / collapse

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/staging-monad-ns775c

To Reproduce

Create a Next.js app with @next/mdx and Turbopack enabled (default in canary).
Add a page like app/heavy/page.mdx that uses rehype-katex with several math equations.
Run next dev.
Observe that compilation of the MDX file takes significantly longer than expected.

The slow path is triggered by as: '*.tsx' in the Turbopack rule inside packages/next-mdx/index.js, which routes already-compiled JavaScript through Turbopack's TypeScript/JSX pipeline a second time.

Current vs. Expected behavior

Current: The Turbopack loader rule for .mdx files sets as: '*.tsx', which tells Turbopack to re-process the loader output through its full TypeScript/JSX pipeline (type-stripping → JSX transform → source-map merge). @mdx-js/mdx already compiles MDX to plain JavaScript using the automatic JSX runtime (_jsx()/_jsxs() calls). The output contains no JSX syntax and no TypeScript, so the extra pipeline is pure overhead. For large outputs (e.g. rehype-katex on a math-heavy page: ~1.9 MB), the redundant source-map merging alone adds ~50 s of compilation time.

Expected: The rule should use as: '*.js' so Turbopack treats the loader output as plain JavaScript and skips the unnecessary TypeScript/JSX pipeline. The fix is a one-line change:

- as: '*.tsx',
+ as: '*.js',

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Sun Aug  6 20:05:33 UTC 2023
  Available memory (MB): 4102
  Available CPU cores: 2
Binaries:
  Node: 20.12.1
  npm: 10.5.0
  Yarn: 1.22.19
  pnpm: 8.15.6
Relevant Packages:
  next: 16.2.0-canary.85 // Latest available version is detected (16.2.0-canary.85).
  eslint-config-next: N/A
  react: 19.2.4
  react-dom: 19.2.4
  typescript: 5.9.3

Which area(s) are affected? (Select all that apply)

Markdown (MDX)

Which stage(s) are affected? (Select all that apply)

next dev (local)

Additional context

No response

extent analysis

Fix: Change the Turbopack rule for MDX output from `.tsx` → `.js`

The MDX loader already emits plain JavaScript (no JSX/TS).
Telling Turbopack to treat the result as *.tsx forces an unnecessary TypeScript/JSX pass, which adds tens of seconds for large pages.

1️⃣ Edit the MDX plugin source

File: packages/next-mdx/index.js (or wherever the Turbopack rule is defined)

// … existing imports …

module.exports = {
  // …
  webpack(config, { dev, isServer }) {
    // other rules …

    config.module.rules.push({
      test: /\.mdx?$/,
      // The loader that turns MDX → JS
      use: [
        {
          loader: require.resolve('./mdx-loader'), // ← your custom loader
          options: {
            // … loader options …
          },
        },
      ],
-      // ❌ Old: forces a second TS/JSX pipeline
-      as: '*.tsx',
+      // ✅ New: treat the output as plain JavaScript
+      as: '*.js',
    });

    return config;
  },
};

If the rule lives in a separate next-mdx package, open that package’s index.js and apply the same diff.

2️⃣ Re‑install / rebuild

# From the repo root
pnpm install   # or npm/yarn
pnpm run build # optional – just to be sure the compiled assets are fresh

3️⃣ Run the dev server and measure

next dev
# Open the heavy MDX page and watch the terminal.
# Expected: compilation finishes in < 1 s (instead of ~50 s).

You can also time it:

time node_modules/.bin/next dev

4️⃣ Optional: Guard against future regressions

Add a comment next to the rule so future contributors know why *.js is required:

// MDX loader already outputs plain JS; using *.tsx would re‑run the TS/JSX pipeline
// and cause massive slow‑downs for large pages (e.g. rehype‑katex output).
as: '*.js',

5️⃣ Verify the

Vote matrix · Quick signals

Works

Did the solution work? Tap to confirm.

Easy Fix

Was it a quick fix?

Time Saver

Did it save you time?

Blocking

Was it severely blocking?

Common Issue

Are others likely hitting this too?

Flaky / Intermittent

Is it intermittent?

Verified / Reproducible

Can you reproduce it reliably?

#api #ssr #installation #tensor shape #autograd error #SSR setup #ISR setup #authentication setup #request error #file not found

Still need to ship something?

×6

Another batch ranked right after the header list — different links, same matching logic.

Data

Security

Network

Code

UI/UX

Text

System

Multimedia

Protocol

API

Engineering

nextjs - ✅(Solved) Fix @next/mdx: Turbopack rule uses `as: '*.tsx'` causing unnecessary double-processing of MDX output [1 pull requests, 4 comments, 2 participants]

Recommended Tools

GitHub issue graph ai analysis

Fix Action

Fixed

PR fix notes

PR #91134: fix(mdx): use as: '.js' instead of '.tsx' in Turbopack rule

Description (problem / solution / changelog)

Problem

Fix

Changed files

Code Example

Link to the code that reproduces this issue

To Reproduce

Current vs. Expected behavior

Provide environment information

Which area(s) are affected? (Select all that apply)

Which stage(s) are affected? (Select all that apply)

Additional context

extent analysis

Fix: Change the Turbopack rule for MDX output from `.tsx` → `.js`

1️⃣ Edit the MDX plugin source

2️⃣ Re‑install / rebuild

3️⃣ Run the dev server and measure

4️⃣ Optional: Guard against future regressions

5️⃣ Verify the

Still need to ship something?

TRENDING

nextjs - ✅(Solved) Fix @next/mdx: Turbopack rule uses `as: '*.tsx'` causing unnecessary double-processing of MDX output [1 pull requests, 4 comments, 2 participants]

Recommended Tools

GitHub issue graph ai analysis

Fix Action

Fixed

PR fix notes

PR #91134: fix(mdx): use as: '.js' instead of '.tsx' in Turbopack rule

Description (problem / solution / changelog)

Problem

Fix

Changed files

Code Example

Link to the code that reproduces this issue

To Reproduce

Current vs. Expected behavior

Provide environment information

Which area(s) are affected? (Select all that apply)

Which stage(s) are affected? (Select all that apply)

Additional context

extent analysis

Fix: Change the Turbopack rule for MDX output from *.tsx → *.js

1️⃣ Edit the MDX plugin source

2️⃣ Re‑install / rebuild

3️⃣ Run the dev server and measure

4️⃣ Optional: Guard against future regressions

5️⃣ Verify the

Still need to ship something?

RELATED_DISCOVERY

TRENDING

Fix: Change the Turbopack rule for MDX output from `.tsx` → `.js`