Module not found on Vercel, but it works locally

Module not found: Can't resolve './components/Header'

# or, at runtime in a Node function:
Error: Cannot find module '../lib/Utils'

Why it happens

Windows and macOS use case-insensitive file systems by default, so an import of ./components/Header happily loads header.tsx on your machine. Vercel runs builds on a case-sensitive Linux file system, where those are two different paths, so the build cannot find the file.

Two related causes give the same error: a file that never got committed (it is in .gitignore, or you forgot to add it), and a package you use but never added to package.json. AI coding agents hit all three often, because they create and rename files quickly.

Fix it by hand

1. Find the exact path that failed. Open the failed deployment's build log and copy the path after Can't resolve or Cannot find module, plus the file that imports it.
2. See the name git actually stored. Vercel builds what is in git, not what is on your disk. Search it case-insensitively:

   git ls-files | grep -i header

   Compare it letter by letter with the import. components/header.tsx in git and ./components/Header in the import will fail on Vercel.
3. Fix the import or rename the file. Changing the import to match the file is the simplest fix. If you want to rename the file instead, do it in two steps through a temporary name, because a single case-only git mv can fail silently on Mac and Windows:

   git mv components/header.tsx components/header-tmp.tsx
   git mv components/header-tmp.tsx components/Header.tsx
   git commit -m "Fix Header file name case"

4. Make git report case changes from now on.

   git config core.ignorecase false

   This only changes what git reports going forward. It does not fix names already committed, which is why step 3 matters.
5. If the file is missing from git entirely, check .gitignore (git check-ignore -v path/to/file tells you which rule matches), then add and commit the file.
6. If it is a package, confirm it is listed in package.json and that the lockfile is committed. Install it with npm install the-package so both files update, then commit them.
7. Redeploy and check the build log passes the step that failed.

Stop it happening again

• Run the production build on Linux before you merge, for example in a CI job, so case problems fail there and not on deploy day.
• Keep core.ignorecase set to false in every clone of the repo.
• Pick one naming style for files (for example PascalCase for components) and have your agent follow it in your project rules file.
• After an agent renames files, run git status and check the rename was recorded.