substack-mcp

An MCP server for Substack. Read your publication data and manage drafts from your AI agent. Long-form posts are draft-only by design — no publish, no delete. Short-form Notes publish immediately.

An MCP server for Substack that lets AI assistants read your publication data and manage drafts. The draft list shown in the demo above is sample data, not real account values.

Safe by design — with one loud exception: This server cannot publish or delete long-form posts. Post tools create and edit drafts only; you review and publish manually through Substack's editor. The exception is Substack Notes: create_note and create_note_with_link publish short-form Notes immediately, because Notes have no draft state on Substack. Treat the Note tools as public-publish actions — there is no preview step and no undo from this server.

About

Built and maintained by Conor Bronsdon for the Chain of Thought podcast production workflow, where it drafts and reviews newsletter posts before a human hits publish. Conor hosts Chain of Thought, a show about AI infrastructure and how practitioners actually build with it. More tools for creators live in ai-tools-for-creators. Find Conor on X at @ConorBronsdon.

Sibling MCP servers:

• Transistor-MCP: manage podcast episodes, analytics, and transcripts on Transistor.fm

• podcastindex-mcp: search the Podcast Index and track guest appearances

Related MCP server: Substack-OPS

Tools

Every tool declares MCP tool annotations, set explicitly rather than left to MCP's defaults (an omitted destructiveHint or openWorldHint defaults to true). Reads carry readOnlyHint: true. Every write is additive, so all writes carry destructiveHint: false. Draft writes are private (openWorldHint: false); upload_image carries openWorldHint: true because it returns a publicly-fetchable CDN URL; and the Note tools carry openWorldHint: true for immediate public publish. Annotations are untrusted hints, so the authoritative wording lives in each tool's description.

Read

Write (private drafts; image upload returns a public URL)

Publish (Notes — public immediately)

Notes have no draft state on Substack, so there is no draft-first option for these two tools.

Intentionally excluded

• Publish posts — Publishing long-form posts should be a deliberate human action (Notes are the documented exception above)

• Delete — Too destructive for an AI tool

• Schedule — Use Substack's editor for scheduling. (list_scheduled_posts reads what you've queued there, but this server never creates, edits, or cancels a schedule.)

Setup

You can supply credentials two ways: paste them as env vars (below), or run the optional browser login which captures and stores them for you.

Option A — Browser login (optional, no manual cookie copying)

Removes the DevTools cookie hunt and the ~90-day re-copy. Playwright is not bundled (it's large), so install it once, then sign in:

npm i -g playwright && npx playwright install chromium
npx --package @conorbronsdon/substack-mcp substack-mcp-login https://yourblog.substack.com

A browser opens; sign in to Substack (CAPTCHA included). The tool captures your session cookie, auto-resolves your user id, and writes them to ~/.substack-mcp/session.json (override the directory with SUBSTACK_MCP_HOME). The MCP server reads that file automatically whenever the SUBSTACK_* env vars are unset — so with browser login you can omit the env block entirely.

Storage & security: the file is written 0600 and encrypted with AES-256-GCM under a key derived from this OS account + machine (never stored). A copied file is useless elsewhere and casual disk/backup reads see only ciphertext. This is machine-binding + obfuscation, not a secret vault — code running as you on this machine can re-derive the key (the same caveat as the plaintext env-var path). If you prefer, use Option B and let your MCP client handle the secret.

Option B — Get your credentials manually

Open your Substack in a browser, then:

1. Session token: Navigate to your publication, open DevTools → Application → Cookies → copy the value of connect.sid (URL-encoded string starting with s%3A)

2. User ID: In DevTools Console, run: fetch('/api/v1/archive?sort=new&limit=1').then(r=>r.json()).then(d=>console.log(d[0]?.publishedBylines?.[0]?.id))

3. Publication URL: Your Substack URL, including custom domain if you have one (e.g., https://newsletter.yourdomain.com or https://yourblog.substack.com)

2. Configure your MCP client

Claude Desktop

Add to your claude_desktop_config.json:

{
  "mcpServers": {
    "substack": {
      "command": "npx",
      "args": ["-y", "@conorbronsdon/substack-mcp"],
      "env": {
        "SUBSTACK_PUBLICATION_URL": "https://yourblog.substack.com",
        "SUBSTACK_SESSION_TOKEN": "your-session-token",
        "SUBSTACK_USER_ID": "your-user-id"
      }
    }
  }
}

Claude Code

Add to your .mcp.json:

{
  "mcpServers": {
    "substack": {
      "command": "npx",
      "args": ["-y", "@conorbronsdon/substack-mcp"],
      "env": {
        "SUBSTACK_PUBLICATION_URL": "https://yourblog.substack.com",
        "SUBSTACK_SESSION_TOKEN": "your-session-token",
        "SUBSTACK_USER_ID": "your-user-id"
      }
    }
  }
}

3. Verify

Ask your AI assistant: "How many Substack subscribers do I have?"

Token expiration

Substack session tokens expire periodically (typically ~90 days). If you get authentication errors, grab a fresh connect.sid cookie from your browser and update the env var (make sure ad blockers are disabled when copying the cookie) — or, if you used the browser login, just re-run substack-mcp-login to refresh the stored session.

Custom domains & Cloudflare

Substack publications served on a custom domain (e.g. blog.example.com) sit behind Cloudflare, which can reject non-browser requests with 403 error code: 1010. To avoid this, the server sends a browser User-Agent and a Referer by default, and addresses the publication by its canonical *.substack.com host.

• Use the canonical host. Set SUBSTACK_PUBLICATION_URL to the publication's *.substack.com address rather than the custom domain. Calls to the canonical host are served directly; custom-domain calls may 301-redirect and then 401.

• Override the User-Agent (optional) via SUBSTACK_USER_AGENT if you need a different browser signature:

"env": {
  "SUBSTACK_PUBLICATION_URL": "https://yourblog.substack.com",
  "SUBSTACK_SESSION_TOKEN": "your-session-token",
  "SUBSTACK_USER_ID": "your-user-id",
  "SUBSTACK_USER_AGENT": "Mozilla/5.0 ..."
}

Typed errors

API failures are mapped to a typed error hierarchy (SubstackAPIError base, with AuthenticationError, RateLimitError, ValidationError, NotFoundError, and ServerError subclasses keyed off HTTP status) in src/utils/errors.ts. Every tool call still surfaces the same error response shape on failure — the typed hierarchy just makes the message specific to what went wrong instead of a single generic "Substack API error" string.

Substack error response bodies are inconsistent — sometimes JSON ({"error": "..."} or {"errors": [...]}), sometimes plain text, and sometimes a large Cloudflare HTML block page. extractErrorDetail handles all three: it tries JSON.parse first, falls back to the raw text (trimmed and capped at ~500 characters so a multi-KB HTML page doesn't become the whole error message), and only uses a generic fallback string if the body is empty.

Markdown support

The create_draft and update_draft tools accept markdown and convert it to Substack's native format. Supported:

• Paragraphs, headings (h1–h6)

• Bold, italic, inline code

• Links

• Images

• Bullet and numbered lists, including nested lists (arbitrary depth, mixed ordered/unordered)

• Code blocks (with language)

• Blockquotes

• Horizontal rules

Tables: Substack's post editor has no table node, so a markdown table cannot be rendered natively. Rather than mangle the pipes into a paragraph, a detected GFM table is preserved verbatim inside a code block — the content survives so you can reformat it (as an image or embed) in Substack's editor.

Important notes

• This server uses Substack's unofficial API. It may break if Substack changes their endpoints.

• Session tokens are sent as cookies. Keep your SUBSTACK_SESSION_TOKEN secure.

• The server validates authentication on startup and will fail fast if your token is expired.

Development

git clone https://github.com/conorbronsdon/substack-mcp.git
cd substack-mcp
npm install
npm run build

Run locally:

SUBSTACK_PUBLICATION_URL=https://yourblog.substack.com \
SUBSTACK_SESSION_TOKEN=your-token \
SUBSTACK_USER_ID=your-id \
npm start

Contributing

Issues and pull requests are welcome. Because this server uses Substack's unofficial API, the most useful contributions are fixes when an endpoint changes. If a tool stops working, open an issue with the tool name and the error. The safe-by-design boundary stays: no publish, no delete, no schedule for long-form posts. Notes publish immediately by design and must keep saying so loudly in their descriptions.

Disclaimer

All views, opinions, and statements expressed on this account are solely my own and are made in my personal capacity. They do not reflect, and should not be construed as reflecting, the views, positions, or policies of Modular. This account is not affiliated with, authorized by, or endorsed by Modular in any way.

License

MIT