Substack Post

Substack Post

Automates publishing a local markdown blog file to Substack using Playwright MCP browser tools. Defaults to saving as a draft and confirms with the user before publishing.

Prerequisites

This skill requires the Playwright MCP server to be installed and configured.

Install browsers:

npx playwright install

Add to your Codex MCP config:

{
  "mcpServers": {
    "playwright": {
      "command": "npx",
      "args": ["@playwright/mcp@latest"]
    }
  }
}

See https://github.com/microsoft/playwright-mcp for full setup instructions.

Credentials Setup

Credentials come from environment variables. Before using this skill, ensure these are set:

• SUBSTACK_EMAIL — Substack account email
• SUBSTACK_PASSWORD — Substack account password (only works if explicitly set — magic-link-only accounts will fail)
• SUBSTACK_PUBLICATION (optional) — publication subdomain, e.g. myblog for myblog.substack.com

One-time setup:

1. Create a .socials file in the project root (add it to .gitignore)
2. Add:

   SUBSTACK_EMAIL=your_email@example.com
   SUBSTACK_PASSWORD=your_password
   SUBSTACK_PUBLICATION=myblog
   

3. Or export them in your shell.

If the env vars are not set, ask the user before proceeding. Do NOT hardcode credentials anywhere.

Inputs

Required:

• Blog file path — local path to a markdown blog file.

Optional:

• Title — defaults to the first # H1 line of the file, falling back to the filename without extension.
• Subtitle
• Action — draft (default) or publish.

Workflow

Step 1: Read credentials

echo $SUBSTACK_EMAIL
echo $SUBSTACK_PASSWORD
echo $SUBSTACK_PUBLICATION

If blank, check .socials:

grep -E "^SUBSTACK_(EMAIL|PASSWORD|PUBLICATION)=" .socials 2>/dev/null

If still missing, ask the user.

Step 2: Read and parse the blog file

1. Read the file from disk.
2. Detect and strip frontmatter. If the file begins with a YAML block (--- ... ---) or TOML block (+++ ... +++), parse it and remove it from the body. Handles Hugo, Jekyll, Zola, Hexo, Astro, Obsidian, etc. Files without frontmatter pass through unchanged.
3. Resolve title (first non-empty wins): explicit user input → frontmatter title → first # H1 in body (strip from body if used) → filename without extension. For generic filenames (index, README, post), use the parent directory name instead.
4. Resolve subtitle: explicit user input → frontmatter subtitle → description → summary → excerpt → blank.
5. Body = remaining markdown after frontmatter is stripped (and after the leading H1 is stripped if it was used as the title).

Step 3: Log in

1. Navigate to https://substack.com/sign-in.
2. Click the "Sign in with password" link — Substack defaults to magic-link, the password field is hidden until this is clicked.
3. Fill the email field with SUBSTACK_EMAIL and the password field with SUBSTACK_PASSWORD.
4. Click the sign-in button.
5. Verify login succeeded. If a captcha / device check appears, stop and ask the user to complete it in the same browser session.

Step 4: Open the editor

• If SUBSTACK_PUBLICATION is set: navigate to https://<publication>.substack.com/publish/post?type=newsletter.
• Otherwise: navigate to https://substack.com, then click "New post" on the dashboard.

Step 5: Fill the post

1. Type the title in the title field.
2. If a subtitle was given, type it in the subtitle field.
3. Click into the body editor. Do not paste the whole body as one blob — pasted ## lines come through as literal text. Substack only auto-styles a heading when # /## /### is typed at line start and the space key is pressed (the space triggers the conversion). Split the body on lines matching ^#{1,3} ; paste prose chunks between headings (so inline markdown still converts), and for each heading line type the prefix + space + heading text + Enter via the type/keypress tools. Code blocks, images, and tables still won't render — flag in the final report.

Step 6: Save as draft

Click "Save draft" or wait for autosave. Capture the draft URL.

Step 7: Confirm before publishing

Stop at draft by default. Report the draft URL and any markdown elements that didn't convert cleanly. Ask the user before clicking Publish.

If the user confirmed publish: Continue → "Send to everyone now" → submit. Verify and report the public URL.

Error Handling

• "Sign in with password" link missing: take a screenshot, report.
• Password login fails: user likely never set a password — tell them to set one at https://substack.com/account/login-options.
• 2FA / captcha: stop, hand off to the user, retry.
• File missing / unreadable: stop and report.
• Editor element not found: screenshot and report.
• Login rate limit: tell the user to wait a few minutes.

Important Notes

• Markdown auto-conversion is partial — code blocks, images, and tables don't survive a paste.
• Drafts are private until published; default behavior stops at draft.
• Magic-link-only accounts cannot use this skill.