[BUG] Vercel CLI shows Next.js error if page begins with api-, even if Next.js is not present in project

[Lissy93]

Hey Vercel team 👋
Wondering if someone could spare a second to help me on this:

I've got an Astro project, for which I am using the Vercel CLI.
There's no trace of React or Next.js in my repo project.
Yet when I have a route or component named /src/pages/api-troubleshooting.astro then Vercel CLI starts throwing tons of warnings, saying:

WARN!
When using Next.js, it is recommended to place JavaScript Functions inside of the
pages/api (provided by Next.js) directory instead of api (provided by Vercel). Other
languages (Python, Go, etc) should still go in the api directory.
Learn More: https://nextjs.org/docs/api-routes/introduction

If it helps, here's a screenshot of the issue.

---

Fix

I've submitted a fix for this in #11611
And would really appreciate a quick review, if any of you had a moment

---

Cause

Expand Root Cause

This is appears to be caused by fileName.startsWith('pages/api') in

vercel/packages/fs-detectors/src/detect-builders.ts

Lines 215 to 220 in 80e0bab

	if (
	!hasNextApiFiles &&
	(fileName.startsWith('pages/api') || fileName.startsWith('src/pages/api'))
	) {
	hasNextApiFiles = true;
	}

Which in turn throws the warning which is being repeatidly displayed in loop

vercel/packages/fs-detectors/src/detect-builders.ts

Lines 335 to 347 in 80e0bab

	if (
	hasNextApiFiles &&
	apiBuilders.some(b => isOfficialRuntime('node', b.use))
	) {
	warnings.push({
	code: 'conflicting_files',
	message:
	'When using Next.js, it is recommended to place JavaScript Functions inside of the `pages/api` (provided by Next.js) directory instead of `api` (provided by Vercel). Other languages (Python, Go, etc) should still go in the `api` directory.',
	link: 'https://nextjs.org/docs/api-routes/introduction',
	action: 'Learn More',
	});
	}
	}

This feature appears to have been added in #3784 (specifically, this commit specifically).

---

Bug Report

If you need a formal bug report before this can be reviewed or looked into, I've written one up below:

^{Bug Report: Vercel CLI shows Next.js error if page begins with `api-`, even if Next.js is not present in project}

Summary

When using the Vercel CLI with an Astro app (without React or Next.js), any pages which start with api, such as src/pages/api-setup-guide.astro will throw the following warning

WARN! When using Next.js, it is recommended to place JavaScript Functions inside of the pages/api (provided by Next.js) directory instead of api (provided by Vercel). Other languages (Python, Go, etc) should still go in the api directory.
Learn More: https://nextjs.org/docs/api-routes/introduction

Steps to Reproduce

1. npm create astro@latest
2. npx astro add vercel
3. touch src/pages/api-docs.astro
4. vercel dev

Minimal reproducible example: https://stackblitz.com/edit/github-yogmzg-d6z7xm

Expected Behavior

No warnings. Because everything is 200 okay :)

Actual Behavior

User sees a barage of warnings from the Vercel CLI, informing them not to put Next.js API routes in Astro pages. This occours on a totally vanillar Astro app, with no traces of either React nor Next.js, just because they have an Astro route beginning with pages/api-XXX

Environment Info

Vercel CLI: 34.2.0
Node: v20.13.1
Astro: 4.8.6
@astrojs/vercel: 7.5.4
OS: Arch Linux, 2024.05.01

Areas Affected

Vercel CLI

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Thanks for taking the time to check this out, really appreciate it ❤️
If you need any more details, please let me know, and I'll be happy to help :)