ci: Comment the Vercel build log on PRs whose build fails

[marcleblanc2]

Problem

• When a PR's Vercel build fails, the Vercel bot comment and the Vercel check link to the Vercel inspector, which only Vercel team members can open
• Contributors who aren't Vercel team members see a red X and a login wall, with no hint what broke
• Example: test: PR checks on a broken PR, left broken (do not merge) #1913

Solution

• On vercel.deployment.error (the same repository_dispatch channel preview-links.yml uses), comment the tail of the build log on the PR, like the links / redirects / CSpell checks do
• On the next vercel.deployment.success, update that comment to ✅. The success path only reads PR comments; it does not touch Vercel

Token

• VERCEL_TOKEN repo secret is a token scoped to the sourcegraph-docs project only, expiring 2026-12-10
• Verified it can read this project's deployments and build events, and is denied team members, user info, and other projects
• Vercel has no read-only scope; project scope is the narrowest it offers
• Fork PRs are skipped, and repository_dispatch runs the workflow and script from main, so a PR can't change the code the token is handed to

Tested

• Dry-run against test: PR checks on a broken PR, left broken (do not merge) #1913's failed deployment produces the comment below; the success path on test: PR checks on a broken PR, left broken (do not merge) #1913 correctly exits with nothing to resolve
• Cannot test end-to-end before merge because GitHub only delivers repository_dispatch to the workflow on main. After merge: gh workflow run vercel-build-report.yml -f id=dpl_9VtUD163jZ6grMQwdYpnvQesAPeh -f state=error -f sha=<1913 head sha>

Dry-run output (abridged)

❌ The Vercel build failed for this PR

Vercel only shows build logs to members of its team, so here is the end of the log.
Run npm run build locally to reproduce.

Build log

Running build in Cleveland, USA (East) – cle1
...
❌ Found 2 dead link(s) in 1 file(s):
📄 docs/code-search/features.mdx
   Line 154: /code-search/no-such-page
...
❌ Failed checks: links, filenames
 ELIFECYCLE  Command failed with exit code 1.
Error: Command "pnpm run build" exited with 1

Amp thread

• Vercel build failure report

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated
sourcegraph-docs	Ready	Preview	Sep 11, 2026 11:07am UTC