substack-mcp

A Model Context Protocol (MCP) server for Substack. Lets Claude Code create drafts, upload images, set cover thumbnails, schedule, and publish posts on your Substack publication.

Built on top of python-substack. Uses Substack's internal API (no public posting API exists). Not affiliated with Substack Inc.

Tools

Required

• create_draft(title, content_markdown, subtitle?, audience?) — Create a new draft from Markdown.

• update_draft(post_id, title?, subtitle?, content_markdown?, audience?) — Edit an existing draft.

• upload_image(image_path) — Upload a local file or remote URL to Substack's CDN, returning the URL.

• publish_draft(post_id, send_email?, share_automatically?) — Publish immediately. send_email toggles email delivery.

Recommended

• schedule_draft(post_id, iso_datetime) — Schedule a publish for a future date/time (ISO 8601).

• unschedule_draft(post_id) — Cancel a scheduled publish.

• set_cover_image(post_id, image_url) — Set the cover thumbnail (from upload_image URL).

Utility

• list_drafts(limit?) — List recent drafts.

• get_draft(post_id) — Get a draft's full body.

• delete_draft(post_id) — Permanent deletion.

Setup

# 1. Install dependencies
uv pip install -e .

# 2. Make sure you're logged in to Substack in Chrome (or Brave/Edge) — that's it.

# 3. Save credentials — auto-detects your existing browser session
substack-mcp-setup

# 4. Register with Claude Code
claude mcp add substack-mcp --scope user -- /Users/$USER/substack/.venv/bin/substack-mcp

Restart Claude Code, then /mcp should show substack-mcp as connected.

How auth works

By default substack-mcp-setup reads the substack.sid cookie directly from your existing Chrome session via pycookiecheat. Substack can't tell anything was automated because nothing was: it's the same session you're already using.

macOS will prompt once for Keychain access ("Chrome Safe Storage"). Click "Always Allow" so it doesn't ask again next time.

Supports: Chrome, Brave, Edge, Chromium, Vivaldi, Opera.

Fallback modes

# Specific browser
substack-mcp-setup --from-browser brave

# Playwright-based (often blocked by Substack — use --chrome instead)
substack-mcp-setup --browser

# Manual paste from DevTools
substack-mcp-setup --manual

Tokens are stored at ~/Library/Application Support/substack-mcp/config.json with 0600 permissions.

Security

The substack.sid cookie is equivalent to a password — anyone with it has full account access (publish posts, edit billing, etc.). Treat it as such.

Where the token lives

• macOS: ~/Library/Application Support/substack-mcp/config.json (mode 0600)

• Linux: ~/.config/substack-mcp/config.json (mode 0600)

• Or via env vars: SUBSTACK_PUBLICATION_URL + SUBSTACK_SESSION_TOKEN (env vars are inherited by child processes — be aware when spawning subprocesses)

The .gitignore excludes config.json; never commit it. The MCP also writes a temporary cookie file via tempfile.mkstemp (mode 0600) and deletes it in a finally block — see auth.py:write_cookie_file.

If a token leaks

1. Sign out of all sessions: Substack → Settings → Security → "Sign out of all sessions". This invalidates every existing substack.sid immediately.

2. Log back in to Substack in your browser.

3. Re-run substack-mcp-setup to capture the new cookie.

Image upload safety

upload_image only accepts:

• HTTP(S) URLs, or

• Local files with image extensions (.png, .jpg, .jpeg, .gif, .webp, .heic, .heif) that are not under sensitive system paths (/etc, /System, ~/.ssh, ~/.aws, ~/Library/Keychains, etc.)

This guards against an assistant being tricked (via prompt injection in fetched content) into uploading e.g. an SSH private key to Substack's CDN.

Known limitation: Markdown image syntax ![alt](path) inside create_draft is processed by python-substack and bypasses this validation. If you pass untrusted Markdown, sanitize image paths first.

Dependencies

Versions are pinned with ~= (compatible release, no major bumps). Bumping python-substack in particular should be reviewed — it talks to Substack's private API and lives outside Substack's official surface.

Notes

• audience accepts: everyone (default), only_paid, founding, only_free.

• Markdown image syntax ![alt](path/or/url) auto-uploads local files when you call create_draft.

• The cover image (set via set_cover_image) is what appears on your publication homepage and in social shares. If you don't set one explicitly, Substack typically uses the first image in the body.