Publish Substack Article

Publish Markdown content to Substack post editor, converting Markdown to HTML and pasting as rich text. Saves as draft for user review before publishing.

Prerequisites

• Browser automation MCP (either one):
  • Chrome DevTools MCP (mcp__chrome-devtools__*)
  • Playwright MCP (mcp__playwright__*)
• User logged into Substack
• Python 3 with markdown package (pip install markdown)
• copy_to_clipboard.py script (shared from publish-zsxq-article skill)

Browser MCP Tool Mapping

This skill works with both Chrome DevTools MCP and Playwright MCP:

Priority: Default to Playwright MCP. Use Chrome DevTools MCP only when Playwright MCP is unavailable.

Detection: At runtime, prefer mcp__playwright__browser_navigate. Fall back to mcp__chrome-devtools__navigate_page only if Playwright tools are not available.

Key URLs

• Substack dashboard: https://{publication}.substack.com/publish
• Post editor: https://{publication}.substack.com/publish/post/{postId}

Publication Resolution

The Substack publication subdomain (e.g., verysmallwoods in verysmallwoods.substack.com) is required to navigate to the dashboard. Do not hardcode a default.

Before any navigation, resolve the publication in this order:

1. Skill argument — if the user invoked the skill with a publication subdomain, use it.
2. Environment variable — check SUBSTACK_PUBLICATION (optional, for users who publish repeatedly to the same publication).
3. Prompt the user if neither is available:

   请提供 Substack publication subdomain（例如 verysmallwoods.substack.com 里的 verysmallwoods）。

Do not proceed to Step 3 (Navigate) without a resolved publication subdomain.

Editor Interface

The Substack post editor uses Tiptap (ProseMirror-based WYSIWYG editor).

Key Elements

• Title input: textbox "title" (placeholder: "Title")
• Subtitle input: textbox "Add a subtitle…"
• Content area: .ProseMirror (Tiptap editor, "Start writing...")
• Save status: button "Saved" (auto-saves)
• Preview button: button "Preview"
• Continue button: button "Continue" (publish flow - DO NOT USE)
• Settings sidebar: button "Settings" (title, description, thumbnail)

Settings Sidebar (left panel)

When "Settings" or "File Settings" is open:

• Title: textbox "Add a title..."
• Description: textbox "Add a description..."
• Thumbnail: Upload button (3:2 aspect ratio)

Toolbar

Bold, Italic, Strikethrough, Code, Link, Image, Audio, Video, Quote, Lists (bullet/ordered), Button, More (Code block, Divider, Footnote, LaTeX, etc.)

Content Insertion Method

CRITICAL: Use clipboard paste with HTML content, NOT direct fill or plain Markdown paste.

The Tiptap editor handles HTML paste natively and renders it as rich content. The workflow is:

1. Convert Markdown to HTML using Python's markdown library
2. Copy HTML to system clipboard using copy_to_clipboard.py html
3. Focus the editor content area
4. Press Cmd+V (macOS) or Ctrl+V (Windows/Linux) to paste

Why HTML paste?

• fill tool → Content treated as plain text, no formatting
• Plain Markdown paste → Tiptap does NOT parse Markdown on paste
• HTML paste → Tiptap renders HTML as rich content (headings, code blocks, links, bold, etc.)

Known limitation: Substack's editor does NOT support HTML tables. Tables will be collapsed into plain text. See Step 0: Pre-Processing for converting tables to images.

Main Workflow

Step 0: Pre-Processing — Convert Tables to Images

Substack does NOT render HTML tables. They collapse into plain text. Any Markdown table must be converted to a PNG image and uploaded separately.

Workflow:

1. Detect tables in the Markdown file (lines with | forming table structure)

2. Convert each table to PNG using the diagram-to-image skill:

# Extract table to temp file
cat > /tmp/table1.md << 'TABLE_EOF'
| Column 1 | Column 2 | Column 3 |
|----------|----------|----------|
| Data 1   | Data 2   | Data 3   |
TABLE_EOF

# Convert via diagramless.xyz API (auto-detects as table)
node ~/.claude/skills/diagram-to-image/scripts/diagram-to-image.mjs /tmp/table1.md -o /tmp/table1.png

3. Note the position of each table in the article for later insertion (after which heading/paragraph)

4. Remove table Markdown from the content before HTML conversion (so it won't appear as plain text in the pasted content)

Image upload happens after pasting the main content — see Step 7.

Step 1: Prepare Content

Read the Markdown file and extract:

• Title: from YAML frontmatter title field, or H1 header # Title, or filename
• Subtitle: from YAML frontmatter excerpt or description field
• Content: full Markdown body (strip YAML frontmatter and any cross-reference links)

Step 2: Convert Markdown to HTML

Use Python's markdown library with tables and fenced_code extensions:

import markdown
import re

with open('/path/to/article.md', 'r') as f:
    content = f.read()

# Strip YAML frontmatter
content = re.sub(r'^---\n.*?\n---\n', '', content, flags=re.DOTALL)

# Strip cross-reference links (e.g., English version link)
# Adjust pattern as needed for your articles
content = re.sub(r'^> .* available at.*\n\n?', '', content, flags=re.MULTILINE)

# Convert to HTML
html = markdown.markdown(content, extensions=['tables', 'fenced_code'])

# Write to temp file
with open('/tmp/substack_article.html', 'w') as f:
    f.write(html)

IMPORTANT: Do NOT use nl2br extension - it converts single newlines to <br> tags, causing extra line breaks in the editor.

Step 3: Navigate to Substack

Navigate to the Substack dashboard and create a new post:

# Navigate to Substack dashboard
navigate to: https://{publication}.substack.com/publish

If not logged in, prompt user to log in:

请先登录 Substack，登录完成后告诉我。
Please log in to Substack first, then let me know.

Step 4: Create New Post

From the dashboard, create a new text post:

1. Click "Create new" in the sidebar
2. Select "Text post" (or navigate directly to a new post URL)

Alternatively, if the editor is already open with an empty post, proceed directly.

Step 5: Fill Title and Subtitle

1. Click the title textbox (textbox "title")
2. Type the article title
3. Click the subtitle textbox (textbox "Add a subtitle…")
4. Type the subtitle/excerpt

click: title textbox
fill/type: article title

click: subtitle textbox
fill/type: article subtitle

Step 6: Insert HTML Content (via Clipboard Paste)

CRITICAL: Do NOT use fill tool - it inserts plain text without formatting.

1. Copy HTML to system clipboard:

python3 /path/to/copy_to_clipboard.py html --file /tmp/substack_article.html

2. Click the editor content area (.ProseMirror or paragraph element inside it)

3. Press Cmd+V to paste:

press_key: Meta+v  (macOS)
press_key: Control+v  (Windows/Linux)

This triggers Tiptap's HTML paste handler, which renders the content as rich text with proper formatting.

Step 7: Insert Table Images

If the article had tables converted to images in Step 0, insert them now:

1. Navigate to the correct position in the editor — click on the paragraph or empty line where the table should appear (after the relevant heading/text)

2. Click the Image toolbar button (button "Image") — a dropdown menu appears with options: Image, Gallery, Stock photos, Generate image

3. Click "Image" menuitem from the dropdown — a file chooser dialog opens

4. Upload the image via file chooser:

   • Playwright MCP: browser_file_upload with the image path
   • Chrome DevTools MCP: upload_file with the image path

Important notes:

• File path restriction: Playwright MCP only allows file uploads from within allowed roots (project directories). If your image is in /tmp/, copy it to the project directory first
• Repeat for each table: Position cursor at the correct location, then upload each table image
• Delete residual text: If table content was pasted as plain text (because it wasn't removed in pre-processing), select it (triple-click to select paragraph) and delete before inserting the image

Step 8: Verify Draft

After pasting:

1. Check the "Saved" status indicator (green dot + "Saved" text)
2. Take a snapshot to verify content structure
3. Optionally take a screenshot for visual verification

The editor auto-saves, so no explicit save action is needed.

Step 9: Report Completion

草稿已保存到 Substack。请在 Substack 中预览并手动发布。
Draft saved to Substack. Please preview and publish manually.

Post URL: https://{publication}.substack.com/publish/post/{postId}

Complete Example Flow

User: "把 /path/to/my-article.md 发布到 Substack"

0. Pre-process tables (if any)
   - Detect Markdown tables
   - Create styled HTML for each table
   - Render to screenshots (open in browser, screenshot, close tab)
   - Remove table Markdown from content
   - Note insertion positions

1. Read /path/to/my-article.md
   - Extract title from frontmatter or H1
   - Extract subtitle from frontmatter excerpt
   - Get full Markdown content (with tables removed)

2. Convert Markdown to HTML
   - Strip frontmatter
   - Use markdown.markdown() with ['tables', 'fenced_code']
   - Write to /tmp/substack_article.html

3. Navigate to Substack dashboard or new post

4. Check if logged in
   - If not, prompt user to login

5. Fill title and subtitle

6. Copy HTML to clipboard + Paste
   - python3 copy_to_clipboard.py html --file /tmp/substack_article.html
   - Click editor content area
   - Press Cmd+V

7. Insert table images at correct positions
   - For each table: click position → Image button → Image menuitem → file upload

8. Verify draft saved
   - Check "Saved" status

9. Report success
   - "草稿已保存，请手动预览并发布"

Critical Rules

1. NEVER click "Continue" - This starts the publish flow. Only save as draft (auto-save handles this)
2. Always convert to HTML first - Plain Markdown will not be parsed by the Tiptap editor
3. Use clipboard paste - The only reliable way to insert formatted content
4. Check login status - Prompt user to login if needed
5. Preserve original file - Never modify the source Markdown file
6. Report completion - Tell user the draft is saved and needs manual review
7. No nl2br extension - Causes double line breaks
8. Tables → images - Pre-process tables before pasting content; upload images after paste
9. Playwright file paths - Playwright MCP restricts file uploads to allowed roots; copy temp files to project directory before uploading
10. Resolve publication first - Never hardcode a publication subdomain. Resolve via skill argument, SUBSTACK_PUBLICATION env var, or user prompt before any navigation (see Publication Resolution)
11. Prefer Playwright MCP - Default to Playwright MCP; only use Chrome DevTools MCP when Playwright is unavailable

Troubleshooting

Content Shows as Plain Text (No Formatting)

If you see raw HTML tags or unformatted text:

• Cause: Content was inserted using fill tool instead of clipboard paste
• Solution: Use the copy_to_clipboard.py + Cmd+V method (see Step 6)

Tables Not Rendering (Shows Plain Text)

Substack's Tiptap editor does not support HTML tables. They collapse into inline plain text.

• Solution: Convert tables to PNG via diagram-to-image skill → upload as images (see Step 0 and Step 7)
• Alternative: Restructure simple tables as formatted lists
• If plain text already pasted: Triple-click the plain text paragraph to select it, press Backspace to delete, then insert the table image at that position

Login Required

If page shows login prompt:

请先登录 Substack: https://{publication}.substack.com
登录完成后告诉我。

Editor Not Loading

If editor elements are not visible:

1. Wait for page to fully load
2. Take a new snapshot
3. If still not loading, refresh the page

Clipboard Copy Fails

If copy_to_clipboard.py fails:

• Ensure dependencies: pip install pyobjc-framework-Cocoa (macOS)
• Check the HTML file exists and is readable
• Try copying a smaller test string first

Element Reference

Technical Details

Editor Stack

• Tiptap: A headless, framework-agnostic rich-text editor built on ProseMirror
• ProseMirror: The underlying rich-text editing framework
• Paste handling: Tiptap natively parses HTML from clipboard and converts to its internal document model

Content Conversion Pipeline

Markdown file
    ↓ (Python markdown library)
HTML string
    ↓ (copy_to_clipboard.py)
System clipboard (text/html + text/plain)
    ↓ (Cmd+V keyboard shortcut)
Tiptap ProseMirror editor
    ↓ (auto-save)
Substack draft

Supported Formatting

The following Markdown elements are correctly rendered after HTML conversion and paste: