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Docs Preview GitHub Action

How our composite GitHub Action deploys the docs preview to Vercel and posts a sticky PR comment.

Our repository includes a composite GitHub Action that:

  • Deploys the documentation site to Vercel (staging or production based on branch and inputs)
  • Posts or updates a sticky PR comment with a preview link
  • Optionally posts a commit comment on push events
  • Skips unnecessary deployments based on file changes or template changes, and explains why when it skips

Where it lives

  • Action entry: internals/c15t-github-action/action.yml (composite)
  • Runtime code: internals/c15t-github-action/src/main.ts and src/steps/*

When it runs

The workflow .github/workflows/deploy-docs.yml triggers on:

  • push on main and canary
  • pull_request on any branch
  • a nightly schedule (to check for upstream template changes)

Inputs (selected)

  • GITHUB_TOKEN: token used by the action (required)
  • setup_docs: fetches the private docs template into .docs (default: true)
  • only_if_changed: deploy only when relevant files have changed (default: false)
  • change_globs: newline-separated globs of files that trigger deploy
  • check_template_changes: also deploy when the upstream template changed
  • vercel_token, vercel_project_id, vercel_org_id: credentials for Vercel
  • target: production or staging
  • assign_alias_on_branch: branch name to assign the alias on (e.g. canary)
  • alias_domains: newline-separated alias domains to assign (supports {{PR_NUMBER}}, {{BRANCH}})
  • comment_on_push: also post a commit comment on push events (default: false)
  • skip_unchanged: don't update sticky comment if body unchanged (default: false)
  • post_skip_comment: when a deployment is skipped, post a skip comment on the PR (or a commit comment on push with comment_on_push: true)
  • skip_message: optional override body for the skip comment
  • post_skip_comment: when a deployment is skipped, post a skip comment on the PR (or a commit comment on push with comment_on_push: true)
  • skip_message: optional override body for the skip comment

Preview comment content

We render a consistent, branded Markdown comment using render-comment.ts:

  • ASCII art header (with special first-contribution banner on your first PR)
  • A preview table with status and timestamp
  • Footer with attribution

When a deployment is skipped, we still post a comment indicating "Skipped" and show the last successful Vercel URL when available.

How skipping works

  • If only_if_changed is enabled, the action looks at changed paths and skips when none match change_globs.
  • If check_template_changes is enabled, the action compares the latest upstream template commit against the last recorded deployment and may skip when they match.
  • On skip, if post_skip_comment is true, the action posts a sticky comment (on PR) or a commit comment (on push with comment_on_push: true). You can override the body with skip_message.

Local development

The action is executed using tsx directly (composite action), so no bundling step is required locally or in CI.

runs:
  using: composite
  steps:
    - name: Run c15t action (tsx)
      shell: bash
      working-directory: ${{ github.action_path }}
      run: pnpm -w exec tsx "$GITHUB_ACTION_PATH/src/main.ts"

Troubleshooting

  • No comment on push: set comment_on_push: "true" in workflow inputs.
  • No deploy on PR: ensure your changes match change_globs or disable only_if_changed.
  • Node engine warning during docs setup: the .docs template may require a newer Node for dev tooling, but CI uses its own Node for the action. This warning can be ignored if the install continues.

Related files

  • internals/c15t-github-action/src/steps/deployment.ts
  • internals/c15t-github-action/src/steps/render-comment.ts
  • internals/c15t-github-action/src/steps/comments.ts
  • internals/c15t-github-action/src/steps/push-comment.ts
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