presets(vercel): hard link custom function dirs instead of copying

[RihanArfan]

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Instead of copying __server.func when there is modifications to .vc-config.json, we now hard link all files to it. We cannot symlink within a functions dir to another since on Vercel the builder cannot resolve the files, which is why we recursively hard link.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
nitro.build	Ready	Preview, Comment	Jun 29, 2026 9:27pm

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: c9c62fbe-90a6-4ca0-b69d-680383dddb37

📥 Commits

Reviewing files that changed from the base of the PR and between 6de81ee and d5bda43.

📒 Files selected for processing (2)

• src/presets/_utils/fs.ts
• src/presets/vercel/utils.ts

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📝 Walkthrough

Walkthrough

A new hardLinkDir function is added to the shared filesystem utilities, recursively recreating a directory tree using hard links (with symlink preservation and a copyFile fallback). The Vercel preset's createFunctionDirWithCustomConfig is updated to use this helper instead of fsp.cp, skipping .vc-config.json.

Changes

Hard-link based Vercel function directory creation

Layer / File(s)	Summary
hardLinkDir utility implementation src/presets/_utils/fs.ts	Adds join to the pathe import and exports a new hardLinkDir(src, dest, options?) function that recursively builds a directory tree at dest via hard links, re-creates symlinks, recurses into subdirectories, and falls back to copyFile on EXDEV/EPERM. Accepts an optional skip set to exclude named entries.
Vercel preset integration src/presets/vercel/utils.ts	Adds hardLinkDir to the fs utilities import and replaces the fsp.cp recursive copy in createFunctionDirWithCustomConfig with hardLinkDir(serverDir, funcDir, { skip: new Set(['.vc-config.json']) }).

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related issues

• vercel preset: functionRules forces full bundle copy per route, blowing up output size #4233: The new hardLinkDir helper directly replaces the fsp.cp call in createFunctionDirWithCustomConfig, which is the source of the full bundle duplication per functionRules entry described in that issue.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	Title follows conventional commits format with type prefix and clearly describes the main change of switching from copying to hard linking for Vercel function directories.
Description check	✅ Passed	Description is directly related to the changeset, explaining the motivation for replacing copying with hard linking in the Vercel preset's custom function directory handling.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch vercel-symlink-functions

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[pkg-pr-new]

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commit: 3675810

[pi0]

Have you tried fs.cp (https://nodejs.org/api/fs.html#fspromisescpsrc-dest-options) with COPYFILE_FICLONE flag? (https://nodejs.org/api/fs.html#fspromisescopyfilesrc-dest-mode) (it should have same CoW effect but hopefully faster since we don't do recursion ourselve.)