Turbopack panic: inner_of_upper_lost_follower in aggregation_update.rs

[Krish-2025]

Summary

Turbopack terminated the Next.js development server with an internal panic while navigating through an authentication and setup flow.

Environment

• OS: Windows
• Next.js: 16.3.2
• Package manager: npm
• Command: npm run dev
• Project: Next.js App Router application
• Turbopack: default Next.js development bundler

Steps

1. Start the development server with npm run dev.
2. Open the application.
3. Navigate through /join, /, /login, and /setup.
4. Trigger the authentication/setup flow.
5. Turbopack panics and aborts the server.

Expected behavior

The development server should continue running or report a recoverable error.

Actual behavior

The server terminated with this panic:

thread 'tokio-rt-worker' (21852) panicked at turbopack\crates\turbo-tasks-backend\src\backend\operation\aggregation_update.rs:2021:17:
inner_of_upper_lost_follower is not able to remove follower TaskId 117424 (TaskId { id: 117424 } content_changed) from TaskId 117404 TaskId { id: 117404 } any_output_changed as it doesn't exist as upper or follower edges
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

turbo-tasks: an internal panic occurred outside the per-task panic boundary. This is a bug in turbo-tasks/Turbopack — please report it at https://github.com/vercel/next.js/discussions and include the panic message and stack trace above.

Aborting.

Relevant request log

GET /join 200 in 1065ms
GET /icon?3203102822f39c02 200 in 488ms
GET / 307 in 1236ms
GET /login 200 in 258ms
GET /setup 200 in 558ms
GET /api/setup 200 in 1988ms
GET /auth/callback?error=server_error&error_code=unexpected_failure&error_description=Database+error+saving+new+user 307 in 89ms
GET /login?error=missing_code 200 in 599ms

Reproduction status

The panic occurred once. I attempted to reproduce it twice with:

$env:RUST_BACKTRACE = "full"
npm run dev

but the panic did not occur again.

The full Rust backtrace was not captured during the original occurrence.

The authentication error shown in the request log appears to be a separate issue from the Turbopack panic.

I also searched for similar discussions before submitting this report.

[edwardsong08]

This looks like a Turbopack compiler bug, especially since the panic happens inside turbo-tasks while navigating the auth flow. As a temporary workaround, try running the dev server with Webpack instead of Turbopack. It’s also worth updating to the newest Next.js patch release and clearing .next before retrying. Since this only happened once, keeping the full panic output and a minimal reproduction would be especially useful if it happens again. The database error in the request log may be a separate issue from the Turbopack crash.