Response body object should not be disturbed or locked" error with Node.js middleware and large request bodies

Link to the code that reproduces this issue

https://github.com/platformshape/nextjs-body-bug-repro

To Reproduce

1. npm install && npm run dev
2. chmod +x test.sh && ./test.sh 500 && ./test.sh 500

The issue is intermittent and never fails on the first request after launching the server. It does however fail after a few tries.

Current vs. Expected behavior

Next.js v15.5.2 throws a fatal error when using Node.js runtime middleware with large request bodies (>350MB):

TypeError: Response body object should not be disturbed or locked
    at extractBody (node:internal/deps/undici/undici:5574:17)
    at new Request (node:internal/deps/undici/undici:9801:48)
    at new NextRequest (/path/to/next/server/chunks/node_modules_next_dist_*.js)

This results in a 500 Internal Server Error and prevents processing of large file uploads through middleware.

Expected: Large file uploads processed successfully through middleware to API route.

Actual:

• ✅ Files ≤350MB: Work correctly
• ❌ Files >350MB: Fatal error with 500 status

Test Results

File Size	Status	Notes
100MB	✅ Works	No issues
300MB	✅ Works	No issues
350MB	✅ Works	Threshold
375MB	❌ Fails	Consistent failure
400MB+	❌ Fails	Always fails

Error Details

Console Error:

TypeError: Response body object should not be disturbed or locked
    at extractBody (node:internal/deps/undici/undici:5574:17)
    at new Request (node:internal/deps/undici/undici:9801:48)
    at new NextRequest (/private/tmp/my-app/.next/server/chunks/node_modules_next_dist_eb0c80da._.js:5368:14)
    at NextRequestAdapter.fromNodeNextRequest (/private/tmp/my-app/.next/server/chunks/node_modules_next_dist_eb0c80da._.js:5539:16)
    at handler (/private/tmp/my-app/.next/server/chunks/node_modules_next_dist_eb0c80da._.js:8127:246)

Server Logs:

Middleware: POST http://localhost:3000/api/upload
⨯ [TypeError: Response body object should not be disturbed or locked] {
  page: '/api/upload'
}
POST /api/upload 500 in 1063ms

Provide environment information

- **Next.js version**: 15.5.2
- **Node.js version**: v23.1.0 (affects all Node versions)
- **Operating System**: macOS (affects all platforms)
- **Runtime**: Node.js middleware (not Edge runtime)

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

Middleware

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

next dev (local)

Additional context

Analysis

Code Path Investigation

The error originates from Next.js's Node.js middleware body cloning mechanism:

1. Entry Point: packages/next/src/server/next-server.ts:1715

   body: hasRequestBody ? requestData.body.cloneBodyStream() : undefined,
   

2. Body Cloning Logic: packages/next/src/server/body-streams.ts:75-88

   • Creates PassThrough streams for body cloning
   • Sets up event listeners synchronously

3. Error Location: undici's extractBody function detects stream as "disturbed"

   • Occurs during new NextRequest() construction
   • undici validates stream state before processing

Behavioral Observations

• Size Dependency: Error threshold consistently around 350-375MB
• Race Condition: Intermittent failures suggest timing-related issue
• Runtime Specific: Only affects runtime: "nodejs", not "edge"
• Middleware Required: Error only occurs when Node.js middleware is present
• Stream State: Middleware executes successfully before error occurs

Undici Interaction

The error originates from Node.js's internal undici module checking if the request body stream has been "disturbed" (consumed or locked). This suggests a conflict between:

• Next.js's body cloning PassThrough streams
• undici's stream state validation
• Timing of large body processing

Impact

This bug affects any Next.js application using:

• Node.js runtime middleware
• Large request bodies (file uploads, API proxies, form data)
• Body processing in API routes after middleware

Environment Variants Tested

• ✅ Works: Edge runtime middleware
• ✅ Works: No middleware
• ✅ Works: Small request bodies with Node.js middleware
• ❌ Fails: Large request bodies with Node.js middleware

Additional Notes

• Bug appears to be related to body stream management in Node.js runtime
• Error is not recoverable - results in complete request failure
• No client-side JavaScript required to reproduce
• Issue is server-side only

_NEXT-4732

Start with the reproduction at https://github.com/platformshape/nextjs-body-bug-repro and run npm install && npm run dev, then ./test.sh 500 twice. Read packages/next/src/server/next-server.ts around line 1715 and packages/next/src/server/body-streams.ts around lines 75-88, focusing on the Node.js middleware body clone path. Done means requests above 350MB reach the API route without the undici disturbed-or-locked error or a 500 response.