fix(dev): let a restarted fork reload changed .env values

[productdevbook]

Editing .env during nuxt dev prints .env changed. Restarting Nuxt in a new process..., but the new process keeps serving the old values.

Cause

ForkPool.createFork passes { ...process.env } to the fork. By then the parent's process.env already contains whatever the previous .env set.

setupDotenv in c12 only overwrites a variable when it is undefined or when c12 set it itself, and it tracks which ones it set in globalThis.__c12_dotenv_vars__. That record is per process, so inside the fork the stale values look like real shell variables. The fresh .env never wins: a changed value is ignored, and a removed one stays.

Fix

Before forking, drop the variables that c12 recorded as set by .env files in this process (withoutDotenvVars in dev/pool.ts). The fork then loads .env from scratch. Variables that came from the shell are not in that record, so they are passed through unchanged. If the record is missing (another c12), the env is passed as before.

Reproduction

The fixture's nuxt.config.ts has runtimeConfig: { public: { benchMarker: '' } }, and the dev server runs with CI=1:

echo NUXT_PUBLIC_BENCH_MARKER=before > .env
nuxt dev --port 33992 &
curl -s localhost:33992/ | grep -o 'public:{[^}]*}'   # public:{benchMarker:"before"}
echo NUXT_PUBLIC_BENCH_MARKER=after > .env           # ".env changed. Restarting Nuxt in a new process..."
curl -s localhost:33992/ | grep -o 'public:{[^}]*}'

	after the 1st edit (after)	after the 2nd edit (third)
main	benchMarker:"before" ❌	—
this PR	benchMarker:"after" ✅	benchMarker:"third" ✅

Checks

• eslint and tsc --noEmit pass
• vitest run packages/nuxt-cli/test/unit plus e2e dev-restart and dev: 134 files, 1881 tests passed

🤖 Generated with Claude Code

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Reviewing files that changed from the base of the PR and between f140924 and d8ab4cf.

📒 Files selected for processing (1)

• packages/nuxt-cli/src/dev/pool.ts

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[pkg-pr-new]

• nuxt-cli-playground

  npm i https://pkg.pr.new/create-nuxt@1592
  

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commit: d8ab4cf

[github-actions]

CLI benchmark

@nuxt/cli v4.0.0-alpha.1 (baseline) vs v4.0.0-alpha.1 (this PR)

Metric	baseline v4.0.0-alpha.1	head v4.0.0-alpha.1	Delta
nuxt --version wall time (median)	51 ms	52 ms	+0.6%
nuxt --help wall time (median)	105 ms	103 ms	-2.0%
nuxt dev --help wall time (median)	81 ms	81 ms	+1.0%
nuxt --version modules loaded	35	35	0.0%
nuxt --version built-ins loaded	27	27	0.0%
nuxt --help modules loaded	134	134	0.0%
nuxt --help built-ins loaded	87	87	0.0%
nuxt dev --help modules loaded	63	63	0.0%
nuxt dev --help built-ins loaded	87	87	0.0%
Installed node_modules	2.43 MB	2.43 MB	+0.0%
Published tarball (packed)	239.6 kB	239.8 kB	+0.1%

Full report

@nuxt/cli v4.0.0-alpha.1 (baseline) vs v4.0.0-alpha.1 (head)

Setting	Value
Baseline	ref:f140924fb88992febb831316ac01bce275609336 (v4.0.0-alpha.1)
Head	local packages/nuxt-cli at dcaf00c (v4.0.0-alpha.1)
Node	v24.21.0
OS	Linux 6.17.0 (kernel 6.17.0-1022-azure)
CPU	INTEL(R) XEON(R) PLATINUM 8573C x 4
Memory	15.6 GB
Load average at start	1.34, 0.37, 0.13
Run started	2026-10-03T17:10:45.375Z

Cold CLI startup

Median of 15 interleaved runs per command, one warmup discarded.

Command	baseline v4.0.0-alpha.1 median	head v4.0.0-alpha.1 median	Delta	baseline v4.0.0-alpha.1 min / p95	head v4.0.0-alpha.1 min / p95
nuxt --version	51 ms	52 ms	+0.6%	48 ms / 55 ms	49 ms / 58 ms
nuxt --version (first output byte)	49 ms	49 ms	+1.0%	46 ms / 52 ms	46 ms / 55 ms
nuxt --help	105 ms	103 ms	-2.0%	101 ms / 108 ms	98 ms / 108 ms
nuxt --help (first output byte)	103 ms	100 ms	-2.4%	99 ms / 105 ms	96 ms / 105 ms
nuxt dev --help	81 ms	81 ms	+1.0%	78 ms / 82 ms	76 ms / 84 ms
nuxt dev --help (first output byte)	78 ms	78 ms	+0.7%	75 ms / 80 ms	74 ms / 81 ms
nuxt <unknown-command> (no-op)	110 ms	111 ms	+0.4%	106 ms / 114 ms	105 ms / 115 ms
nuxt <unknown-command> (no-op) (first output byte)	107 ms	107 ms	+0.1%	103 ms / 111 ms	102 ms / 111 ms

Module load cost

Counted with a module.registerHooks load hook, compile cache disabled. Counts every JS module actually evaluated on that code path (native addons excluded). Built-ins loaded after bootstrap are counted separately, including the internal modules they load.

Command	baseline v4.0.0-alpha.1 modules	head v4.0.0-alpha.1 modules	Delta	baseline v4.0.0-alpha.1 source bytes	head v4.0.0-alpha.1 source bytes	Delta	baseline v4.0.0-alpha.1 built-ins	head v4.0.0-alpha.1 built-ins	Delta
nuxt --version	35	35	0.0%	297.8 kB	297.8 kB	0.0%	27	27	0.0%
nuxt --help	134	134	0.0%	839.9 kB	840.4 kB	+0.1%	87	87	0.0%
nuxt dev --help	63	63	0.0%	453.0 kB	453.4 kB	+0.1%	87	87	0.0%

Install footprint and published tarball

Each version installed on its own into an empty project with nothing but @nuxt/cli as a dependency, so the tree is exactly the CLI and its transitive dependencies. npm cache is warm and the registry is only consulted for metadata, so install wall time is indicative, not a network benchmark.

Metric	baseline v4.0.0-alpha.1	head v4.0.0-alpha.1	Delta
Direct dependencies of @nuxt/cli	23	23	0.0%
Packages in the installed tree (unique name@version)	39	39	0.0%
Unique package names	39	39	0.0%
Package directories on disk (cross-check)	32	32	0.0%
Installed node_modules on disk	2.43 MB	2.43 MB	+0.0%
Installed files	434	434	0.0%
Install wall time (warm npm cache, median of 3)	1.03 s	1.03 s	-0.5%
Published tarball (packed)	239.6 kB	239.8 kB	+0.1%
Published tarball (unpacked)	775.2 kB	775.7 kB	+0.1%
Files in tarball	99	99	0.0%

Interleaved runs on a shared runner: trust the deltas, not the absolute timings. The dev, restart and build suites run locally via pnpm bench:cli.

[codspeed]

Merging this PR will not alter performance

✅ 2 untouched benchmarks

---

_{Comparing productdevbook:fix/dev-fork-dotenv (d8ab4cf) with main (f140924)}

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@f140924). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
packages/nuxt-cli/src/dev/pool.ts	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1592   +/-   ##
=======================================
  Coverage        ?   83.39%           
=======================================
  Files           ?      177           
  Lines           ?    11311           
  Branches        ?     3244           
=======================================
  Hits            ?     9433           
  Misses          ?     1582           
  Partials        ?      296           

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