watch mode triggers unsolicided messages to Workers

Version

24.19.0, 26.6.0

Platform

Linux 05338a186e89 6.18.33.1-microsoft-standard-WSL2 #1 SMP PREEMPT_DYNAMIC Fri Jun  5 01:12:21 UTC 2026 x86_64 Linux

Subsystem

worker

What steps will reproduce the bug?

given the following two files:

// main.mjs
import { Worker } from 'worker_threads'
const worker = new Worker('./worker.js')
worker.on('message', console.log)

// worker.js (can be empty)

it can be reproduced using node --watch main.mjs.

The behavior is identical when using an equivalent main.cjs instead.

How often does it reproduce? Is there a required condition?

reproduces consistently

What is the expected behavior? Why is that the expected behavior?

applications should not receive unexpected (and undocumented) messages by the engine.

What do you see instead?

Unsolicided "watch:require" messages are sent.

Additional information

This issue effects v24 and v26, as can be seen in the following log (tested with node.js v22.23.1, v23.11.1, v24.19.0, v25.9.0, v26.6.0; note that I aborted each run with ctrl+c)

$ docker run --rm -v .:/app -w /app node:22-alpine   node --watch main.mjs
Completed running 'main.mjs'. Waiting for file changes before restarting...

$ docker run --rm -v .:/app -w /app node:23-alpine   node --watch main.mjs                                                 
Completed running 'main.mjs'                  

$ docker run --rm -v .:/app -w /app node:24-alpine   node --watch main.mjs
{ 'watch:require': [ '/app/worker.js' ] }
Completed running 'main.mjs'. Waiting for file changes before restarting...

$ docker run --rm -v .:/app -w /app node:25-alpine   node --watch main.mjs
Completed running 'main.mjs'. Waiting for file changes before restarting...

$ docker run --rm -v .:/app -w /app node:26-alpine   node --watch main.mjs
{ 'watch:require': [ '/app/worker.js' ] }
Completed running 'main.mjs'. Waiting for file changes before restarting...

Though robust message handling code should always validate and check received messages, such unexpected messages can lead to unexpected behavior, e.g. crashes. This unexpected node.js behavior leads to the following issue downstream with an XML schema validating library: https://github.com/noppa/xmllint-wasm/issues/37

Start by running the reported reproduction with main.mjs, worker.js, and node --watch, then trace the worker and watch-mode handling that emits the "watch:require" message. Done means the worker receives no unsolicited message in this case, with regression coverage for the consistently reproducible behavior.