Vercel Build Failed or Deployment Blocked? Two Causes I Hit

A push that doesn't turn into a new production version has two very different shapes on Vercel. In one, the build runs and fails, and you get an error with logs. In the other, the build never starts, there are no logs, and the only visible symptom is that your change isn't live. I hit both on this site within a few weeks of each other, and the second one was quiet enough that my own monitoring reported everything as healthy.

This post separates the two, shows where to look first for each, and covers the build cache questions that come up while you debug. The Vercel details were checked against its documentation in November 2026; the incidents are from this site's own history.

First question: did a build start?

Open the deployment in the dashboard.

• A build log exists and ends in an error. This is a build failure. Go to the first section.
• No build log, and an overlay with a message instead. The build was stopped before it began. Vercel's docs list the typical reasons: an invalid vercel.json was committed, an Ignored Build Step skipped it, the commit came from someone who isn't a team member, or a Marketplace integration resource failed to provision first. Go to the second section.

That one distinction saves a lot of time. People spend an hour reading a build log that doesn't exist.

Case 1: builds locally, fails on Vercel

Vercel's own advice is to run the build on your machine first, which catches problems in your code or dependencies. It's good advice and it still isn't enough, because a local build reuses whatever is already in your node_modules. Vercel installs from scratch.

On this site, that gap showed up through the lock file. I had a Next.js 16 project where a script on a server with an older npm ran a clean install and rewrote package-lock.json: it removed a block of libc fields, 161 lines in total. The local build passed, because node_modules was already there. The Vercel build failed, because it installs from the lock file in the repository and the changed file wasn't accepted.

I didn't keep the exact error text from that day, so I won't quote it. What I can say is the cause and the check:

git diff --stat -- package-lock.json package.json

If you didn't intend to change dependencies, neither file should show up in that diff. If the lock file changed and you don't know why, restore it:

git checkout -- package-lock.json

The rule I took from it: never run an install or update with a different npm version than the one the lock file was produced with, and look at the lock file in every diff, even when the change you meant to make was only content. If you want a local check closer to what Vercel does, delete node_modules and build from a clean install before you push. It's slower, and it's the only version of the test that matches the real one.

Where the error actually is

Vercel's troubleshooting guide notes that the last line is often not the useful one. A log that ends with npm run build exited with 1 only says the build command failed. Scroll up to the first red "Error" line. That's usually the cause.

Other build-time limits worth knowing, from the same documentation:

Builds that exceed memory or disk are canceled, and Vercel prints a system report in the log when that happens. Setting VERCEL_BUILD_SYSTEM_REPORT=1 as an environment variable gives you the report for every build, which helps you notice out-of-memory events that didn't kill the build outright.

Case 2: deployment blocked, site still returns 200

This is the quiet one. A push goes to GitHub, nothing deploys, and the site continues to serve the previous version without an error. Anything that only checks "does the page return 200" will say all is well.

On this site the commits from an automated job used a git email that matched no GitHub account. Vercel could not tie the commit to a user, and two deployments in a row stayed Blocked. The site kept answering 200 with the old content.

The documented rule is that for a private repository, a commit is only deployed if its author has access to the Vercel project. On a Hobby team, the commit author must be the owner of that Hobby team, and Vercel verifies this by comparing the owner's login connections with the commit author. Pro and Enterprise teams check team membership instead. Hobby teams don't support collaboration on private repositories at all.

The practical checklist when a deploy is blocked:

1. Run git config user.email on the machine that made the commit.
2. Check that this address is a verified email on your Git provider account. Your provider attributes commits to users by matching it.
3. Make sure your Git provider is linked to the Vercel account that owns the project (Account Settings, Login Connections).
4. If you commit under several addresses, add the extra ones to your Vercel account's email settings and verify them.
5. Push again. An existing blocked deployment doesn't fix itself; the next commit with a correct author is what goes out.

For automation, the fix is to give the job a commit identity that belongs to a real account. In my case the author name stayed distinguishable (so I can still tell automated commits in the history) while the email was changed to one registered on the account.

Your monitoring probably can't see this

The more useful lesson is about verification. A deploy check that requests a page and expects 200 will pass on a blocked deployment, because the previous deployment is still being served. I had this for two nights before noticing.

The fix is to check that the content you just changed is actually live. Pick something that reflects the new commit, like a title or a version string, and search for it:

curl -s https://example.com/some-page | grep -F "text from the new commit"

One further wrinkle: on pages with revalidate, the CDN can hold a stale copy in a region even after a successful deploy, so a page you just changed may look old for a while. A route that is never cached, such as a dynamic text endpoint generated from the same content, is a more reliable thing to test against.

The build cache, and when to bypass it

Once a build is failing in a way that makes no sense, someone will suggest clearing the cache. Here is what the docs say it does.

• A successful build saves the cache (dependencies, framework files). The next build restores it before the install command runs.
• A failed build does not modify the existing cache. So a failed build can't poison it.
• The cache key is built from your account or team, project, framework preset, root directory, Node.js version, package manager and Git branch. Changing any of those starts from a different cache.
• The cache is retained for one month, and you can't configure which files are cached.

To build without it, once:

• In the dashboard, Redeploy and leave Use existing Build Cache unchecked.
• With the CLI, vercel --force.

To skip restoring it on every build, set the environment variable VERCEL_FORCE_NO_BUILD_CACHE to 1. A successful build still uploads a fresh cache afterwards. VERCEL_FORCE_NO_BUILD_CACHE_UPLOAD=1 does the opposite: it leaves the existing cache untouched.

In my experience clearing the cache is the last thing to try, not the first. It rarely explains a failure that has a clear error message, and it makes every following build slower. It is worth doing when the failure looks like stale state, such as a dependency that resolves differently than it does locally. If the cause were in the cache, redeploying without it would fix it; if redeploying without it fails the same way, the cache wasn't the problem and you can stop looking there.

Summary

• Decide first whether a build started. Build log with an error: read up from the last line to the first real error. No log and an overlay: the build was blocked before it began.
• A passing local build proves less than it seems. Vercel installs from scratch, so check package-lock.json in your diff, and don't use a different npm version than the lock file's.
• A blocked deployment keeps serving the old version with 200. For a private repository, the commit author must have access to the Vercel project, and on Hobby that means being the team owner. The git email has to match a verified address on the linked account.
• Verify a deploy by finding the new content live, not by status code.
• A failed build never changes the cache. Redeploy without cache once to rule it out, and don't turn it off permanently.