substack-mcp

Safe creator operations for Substack, via MCP. Prepare rich drafts, publish Notes, inspect analytics, and manage explicitly consented free subscribers across publications. Review and publish long-form posts in Substack.

The demo runs actual MCP handlers against offline sample data. No live API calls or publication occur. Follow the draft workflow to create, find, export and review a post.

This server imposes no Bestseller-status check. Use an authenticated account with permission to manage the publication; individual operations depend on your Substack access. Connect through local stdio or self-hosted HTTP.

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. The split is proportionate review, the piece of trust infrastructure for agents this server cares most about: the high-stakes surface gets a human gate, and the exception is stated loudly.

For 1.0 setup coverage and account eligibility, see compatibility. Maintainers can use the release checklist and distribution inventory.

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. Draft updates replace existing fields and carry destructiveHint: true; additive 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

| Tool | Description | |------|-------------| | get_subscriber_count | Get your publication's current subscriber count | | list_subscribers | Read a bounded page of private subscriber records | | get_subscriber | Look up membership by exact email; reconcile pending additions | | list_published_posts | List published posts with pagination | | get_publication | Read projected publication identity/settings, verify the configured host, and report missing fields; does not verify account identity or role | | list_publication_tags | Read tag definitions, including hidden tags by default, with bounded local pagination | | get_post_tags | Resolve post tag associations; preserves unresolved IDs and reports empty-result identity uncertainty. Draft coverage is currently live-verified only for empty responses | | search_posts | Search a publication archive by query and status; bounded pages with continuation metadata | | plan_draft_update | Review proposed changes, preflight and a receipt for best-effort stale detection; no writes | | export_draft | Editable Markdown, exact original body, conversion diagnostics, preflight and editor link | | preflight_draft | Read-only checks for title, audience, body structure, images, and paywalls, with an editor link | | list_drafts | List draft posts | | get_post | Get full content of a published post by ID | | get_draft | Get full content of a draft by ID | | get_post_comments | Get comments on a published post | | get_sections | List your publication's sections (categories) with their IDs | | get_post_analytics | Get a published post's stats (views, opens, signups, subscribes, reactions) by ID | | list_scheduled_posts | List posts scheduled for future publication (read-only; scheduling stays in Substack's editor) |

Archive search and draft review

search_posts accepts query (1–500 characters), status (published, drafts, or scheduled, default published), offset (default 0), and limit (1–50, default 25). It makes one authenticated archive request and returns projected metadata, returned, total, has_more, and next_offset. Continue with next_offset and the same query/status. When Substack omits the total and a page is full, has_more is null (unknown); another page may be empty. Substack controls matching and indexing: this is not a guaranteed full-text scan. Use get_post or get_draft to retrieve full content. Pagination is not a snapshot; concurrent edits can move results between pages.

preflight_draft accepts draft_id, reads it once, and returns checks_passed, findings with severity/code/message, content counts and an editor link. It checks title, audience, JSON/body shape, image wrappers and HTTPS sources, and paywall count and edge placement. Unknown nodes and external images produce review warnings. Bodies over two million characters, 10,000 nodes, or depth 100 are not fully checked; counts.complete is false and aggregate checks are skipped after a scan limit. Unknown-node warnings name up to five types for editor review. This is a focused static check, not full ProseMirror validation or publish approval. It does not fetch links/images, verify access settings, or prove final rendering. Review the draft in Substack; no content is modified.

Both tools require publication when multiple publications are configured.

Operator diagnostics

The CLI also exposes read-only workflows through the same MCP handlers:

substack-mcp status --json
substack-mcp drafts list --limit 10 --offset 0
substack-mcp drafts get 42
substack-mcp drafts export 42 --format json
substack-mcp analytics post 42
substack-mcp subscribers count
substack-mcp subscribers get [email protected]

Add --publication key when multiple publications are configured. Read commands return {format_version: 1, ok, command, publication, data} as JSON; --json is accepted explicitly. Exit 0 means a completed read, 1 means a configuration, upstream or output failure, and 2 means invalid arguments or selection. A missing analytics result or subscriber is still a completed read; inspect data. Counts retain their exact/approximate/unavailable precision. Output is capped at 4 MiB without partial printing. Draft and subscriber output is private.

status reports installed version, Node/platform and the same offline configuration diagnostics as doctor. It never opens a browser or claims a verified user identity. drafts export is an alias of the existing export command and retains its bundle/overwrite contract. Draft plan/apply also retain their existing output and exit codes; these aliases do not rewrap saved plans.

substack-mcp doctor --json
substack-mcp doctor --json --check-auth

doctor uses the same environment/stored-session resolution as the server. Both validate an HTTPS publication origin, a positive safe-integer user ID containing only digits, and an unquoted cookie value without whitespace or cookie separators. Invalid configuration stops server startup before requests. Percent-encoded cookie values are preserved exactly. Configuration checks do not prove that the supplied host belongs to Substack or that credentials work.

Upgrading: missing or malformed credentials now stop startup instead of starting tools that fail later. Correct all configured publications; invalid entries are never silently dropped. doctor and --help still run without a working session. Use substack-mcp-login to set up a first session.

By default, doctor checks configuration without network requests. --check-auth adds one draft-list GET per valid publication, with a five-second request deadline and redirects disabled. Use an HTTPS publication origin (no path, query, credentials, or custom port); a redirect or custom-domain block may require switching to the publication's canonical name.substack.com origin. All API requests reject redirects, including same-origin redirects, without following the destination or replaying a write. The publish-page Referer is retained for direct custom-domain requests. Configure the origin that serves the API directly; a redirect response is reported as redirect_rejected.

Each request has one deadline through headers and body consumption (30 seconds by default, five seconds for doctor --check-auth). Streamed response limits are 10 MiB for API JSON, 1 MiB for doctor, 2 MiB for the public-page count fallback, and 64 KiB for error bodies. Limits apply to bytes delivered by fetch, including decompressed bytes. Oversized responses fail without partial results. For an oversized HTTP error body, the HTTP error classification is preserved and diagnostic details are discarded. JSON parsing is synchronous and bounded by input bytes, so it cannot be interrupted mid-parse; a complete parsed result is retained if parsing finishes after the I/O deadline. The public count fallback permits at most three HTTPS redirects within the same deadline. It sends no cookies or authorization and rejects destinations with URL credentials or custom ports. It returns unavailable when its read fails.

Doctor distinguishes unexpected_html, malformed_json, response_too_large, redirect_rejected, timeout, rate_limited, and unauthorized_or_blocked. HTTP 401/403/429 bodies are discarded without waiting; rate-limit errors retain valid delta-seconds or standard HTTP-date Retry-After guidance. Error details are capped at 500 characters (plus an ellipsis) and matching cookie values are redacted before truncation. Requests are never automatically retried. A failed or timed-out write may have succeeded upstream; reconcile its state before trying again.

Output includes publication key, origin, credential source, configuration and authentication status. It omits session tokens, user IDs and upstream error bodies. A successful read does not establish user-ID binding or write access.

Exit codes: 0 = requested checks passed; 1 = configuration/authentication check failed; 2 = invalid command arguments. Without --check-auth, a 0 exit code does not mean the session is unexpired. Bare substack-mcp still starts the MCP server; substack-mcp serve is an explicit alias. --help does not connect to Substack. Browser login is substack-mcp login; substack-mcp-login remains an alias.

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

| Tool | Description | |------|-------------| | create_draft | Create a new draft from markdown (private) | | update_draft | Apply a reviewed change receipt; recheck unpublished state and report readback outcomes | | upload_image | Upload an image to Substack's CDN — returns a publicly-fetchable (unlisted) URL |

Publish (Notes — public immediately)

| Tool | Description | |------|-------------| | create_note | Publish a Substack Note (short-form, publishes immediately) | | create_note_with_link | Publish a Note with a link card attachment (publishes immediately) |

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

Subscriber management

add_free_subscriber adds one consenting reader to the free newsletter. It is a distribution change: that reader may receive future newsletter emails. It can request a welcome email with send_welcome_email: true (off by default). It never grants paid access or overrides Substack's suppression of previously unsubscribed addresses. Its MCP annotations identify it as an external write (readOnlyHint: false, openWorldHint: true).

{"email":"[email protected]","consent_confirmed":true,"consent_evidence":{"source":"booking:message-id","recorded_at":"2026-09-01T00:00:00Z"},"dry_run":true}

Dry-run is the default. After checking actual newsletter consent, set dry_run: false to execute. Live adds require the source reference and timestamp in consent_evidence; this attestation is echoed with the publication key for auditing and does not replace checking the underlying consent record. Multi-publication configurations also require the publication selector, just like every other tool.

Results distinguish existing, dry_run, verified, blocked, and unverified, busy, and retryable. busy performs no write; wait for the other operation. retryable means authentication or rate limiting refused the request; resolve that condition before explicitly retrying. An empty API acknowledgement is not proof of addition. verified means an exact membership lookup succeeded after the request; it does not prove that this request originally created the membership. Dashboard data can lag. A missing reader may also have previously unsubscribed.

For unverified, recheck with get_subscriber; never automatically repeat the add. For blocked, review in Substack without bypassing suppression. Automated callers must persist an attempt ledger before sending each live request. The client's in-memory duplicate guard does not survive restarts or separate HTTP sessions. Keep subscriber identities and consent evidence out of shared repositories, prompts to unapproved public services, and routine logs.

Implementation and live verification notes: subscriber API.

For Google Calendar booking opt-ins, the calendar sync helper provides a bounded Gmail scan, latest-answer selection, a private durable attempt ledger, and read-only reconciliation after uncertain writes. Scheduling is an explicit local setup step; installing the MCP does not start a background job.

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

Requires Node.js 22 or newer (CI covers Node 22 and 24). Browser login additionally requires Playwright.

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)

Install the server and optional Playwright dependency together in a local tools directory, then sign in:

npm install @conorbronsdon/substack-mcp playwright
npx playwright install chromium
npx substack-mcp login https://yourblog.substack.com --user-id 12345

substack-mcp-login remains a supported alias. Missing publication URL and user ID are prompted. Supply your own account's user ID; a post author's byline does not verify your identity. The browser opens for sign-in, including any CAPTCHA. Only a cookie applicable to the publication API is captured, and a bounded authenticated read must succeed before saving. This verifies read access, not the configured user ID or permission to write.

Without --profile, login saves ~/.substack-mcp/session.json (directory override: SUBSTACK_MCP_HOME). The server uses this legacy session when publication credential environment variables and SUBSTACK_PROFILES are unset.

Storage: sessions use AES-256-GCM with a key derived from the OS account and machine. File permissions request 0600; Windows access also depends on directory ACLs. This is a machine-bound file, not an OS keychain or secret vault. Code running as your OS user can derive the key. Use environment credentials if your MCP client manages secrets for you.

Named profiles and migration

npx substack-mcp login https://yourblog.substack.com --user-id 12345 --profile work
npx substack-mcp profiles list
# Copy an existing legacy session without changing its file:
npx substack-mcp profiles migrate --name personal

Keys start with a lowercase ASCII letter and contain only lowercase letters, digits and hyphens, up to 64 characters. Existing profiles require explicit --force to replace. List output contains keys, readability status, publication origins and file save times; it excludes cookies and user IDs. Unreadable profiles remain visible but cannot be selected. Save time records local persistence, including migration; it is not token issuance or expiration time. Listing is bounded to 32 profiles.

Profile storage requires a local filesystem supporting hard links (such as NTFS or a typical Linux filesystem), so creation can install a complete encrypted file without overwriting an existing name. FAT/exFAT and some network mounts are not supported: set SUBSTACK_MCP_HOME to a suitable local directory. Do not use --force to work around an unsupported filesystem.

Set SUBSTACK_PROFILES=work,personal in your MCP client's environment to select up to 32 distinct profiles. Remove all publication credential variables first: combining profile selection with legacy or named credential variables is an error, including empty variables. Missing, corrupt or invalid selected profiles stop startup; they never fall back to another account. Profiles on disk are never activated by discovery. With multiple profiles, tools require an explicit publication key and CLI reads require --publication.

To roll back, unset SUBSTACK_PROFILES and restore your previous environment configuration. Migration preserves the legacy session byte-for-byte. These files use the existing encryption format; an OS keychain is not currently supported. Run substack-mcp status --json for offline configuration diagnostics or substack-mcp doctor --check-auth --json for a bounded read per selected account.

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: Use the numeric ID of your signed-in Substack account from your authenticated account data. Do not use a publication post's byline ID: publications can have multiple authors. This server does not independently verify the supplied 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 and Codex plugin setup is available alongside manual MCP configuration. The repository marketplace and local plugin are separate from curated-directory acceptance or hosted ChatGPT support.

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?"

Tool output compatibility, response limits and versioning are documented in the tool contract.

Multiple publications

Running more than one publication behind a single server? Set a SUBSTACK_PUB_<KEY>_* triplet per publication instead of the plain SUBSTACK_* vars. <KEY> is any name you choose (letters, digits, underscores) — it becomes the publication's lowercase, hyphenated key, e.g. KEVIN_MULDOON → kevin-muldoon.

"env": {
  "SUBSTACK_PUB_KEVIN_MULDOON_PUBLICATION_URL": "https://kevinmuldoon.substack.com",
  "SUBSTACK_PUB_KEVIN_MULDOON_SESSION_TOKEN": "token-1",
  "SUBSTACK_PUB_KEVIN_MULDOON_USER_ID": "111",
  "SUBSTACK_PUB_SAPERE_PUBLICATION_URL": "https://sapere.substack.com",
  "SUBSTACK_PUB_SAPERE_SESSION_TOKEN": "token-2",
  "SUBSTACK_PUB_SAPERE_USER_ID": "222"
}

Each triplet is independent, and setting any SUBSTACK_PUB_<KEY>_* variable declares that publication. An incomplete triplet — a missing variable, an empty value, or a whitespace-only value — fails startup with an error naming the key, rather than silently dropping that publication. That matters because a dropped publication is not "one fewer publication": drop the only one and the server falls back to your stored browser-login session; drop one of two and every tool loses its publication parameter, so a call meant for the dropped publication routes silently to the surviving one.

Keys are compared case-insensitively, with _ folded to -. Two names that resolve to the same key (SUBSTACK_PUB_ALPHA_* and SUBSTACK_PUB_Alpha_*) are a startup error too — merging them silently would let one publication's URL pair with another's session token.

<KEY> accepts ASCII letters, digits, and underscores; the three suffixes must be uppercase and the whole name must have no stray whitespace. Anything that begins with SUBSTACK_PUB_ but does not fit that shape — a hyphen in the key, a lowercase suffix, an accented character, a trailing space — is a startup error naming the variable, not a variable that gets quietly ignored. For the same reason as above: an ignored publication is not one fewer publication, it is a silent reroute to a different one.

With two or more publications configured, every tool gains a required publication parameter — one of your configured keys (e.g. kevin-muldoon, sapere above). The calling model must specify one on every call; an unrecognized value is rejected before any Substack API call is made, so a stray write can't land on the wrong publication. With exactly one publication configured — the common case, whether via plain SUBSTACK_* vars or a single SUBSTACK_PUB_<KEY>_* triplet — no publication parameter is added at all; every tool's schema is unchanged from single-publication mode.

Don't mix the two styles: if any SUBSTACK_PUB_<KEY>_* var is set, the plain SUBSTACK_* vars are ignored (with a startup warning) rather than treated as an unnamed extra publication.

SUBSTACK_USER_AGENT and SUBSTACK_REQUEST_TIMEOUT_MS apply to every configured publication — they are not per-publication. Browser login also supports explicit named profiles; see Named profiles and migration.

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 ..."
}

Request timeout

Every request to Substack is bounded by a 30-second deadline. Node applies no request timeout of its own — only a 10-second connect timeout — so a host that accepts the connection and then goes silent (a proxy that drops packets rather than refusing them) would otherwise hang a tool call indefinitely. A request that hits the deadline fails with a TimeoutError naming the endpoint and the limit.

Raise or lower it with SUBSTACK_REQUEST_TIMEOUT_MS (milliseconds; a non-numeric or non-positive value is ignored with a warning and the default is used):

"env": {
  "SUBSTACK_REQUEST_TIMEOUT_MS": "60000"
}

Transports

By default the server speaks MCP over stdio — the client spawns it as a subprocess per session, which is what the Claude Desktop/Code configs above assume.

For a persistent, network-reachable deployment (e.g. one server shared by multiple machines, connected to via mcp-remote), set MCP_TRANSPORT=http. This starts a stateless Streamable HTTP server instead:

• POST /mcp — the MCP endpoint
• GET /health — returns {"status":"ok"} for container healthchecks

docker run -d --restart unless-stopped -p 127.0.0.1:8080:8080 \
  -e MCP_TRANSPORT=http \
  -e MCP_HTTP_ALLOWED_HOSTS=localhost:8080,127.0.0.1:8080 \
  -e MCP_HTTP_TOKEN="$(openssl rand -hex 32)" \
  -e SUBSTACK_PUBLICATION_URL=https://yourblog.substack.com \
  -e SUBSTACK_SESSION_TOKEN=your-session-token \
  -e SUBSTACK_USER_ID=your-user-id \
  substack-mcp

MCP_HTTP_PORT (default 8080) and MCP_HTTP_HOST (default 0.0.0.0, which is what makes a container reachable through -p) configure the listener. Each request gets its own server instance — there's no session state kept between requests, so nothing to lose if the container restarts.

What this listener will accept

Over stdio the trust boundary is your user account. Over HTTP it is whatever can open a socket to the port — and every request that gets through carries your Substack session cookie, including create_note, which publishes immediately with no undo. The listener therefore starts closed and is opened deliberately:

| Variable | Default | Effect | |---|---|---| | MCP_HTTP_ALLOWED_HOSTS | loopback names for the bound port | Comma-separated Host allowlist. A request whose Host is not listed gets 403. * disables the check. | | MCP_HTTP_ALLOWED_ORIGINS | loopback origins for the bound port | Comma-separated Origin allowlist; 403 otherwise. A request with no Origin is always allowed — non-browser MCP clients don't send one. * disables the check. | | MCP_HTTP_TOKEN | unset | When set, requires Authorization: Bearer <token>; 401 otherwise. | | MCP_HTTP_MAX_BODY_BYTES | 10485760 (10 MiB) | Hard cap enforced while the body streams. Over-limit requests get 413. |

Every one of these is checked before the request is handed to an MCP server, so a rejected request never reaches the Substack API.

Only origin-form request targets are served (POST /mcp, GET /health). An absolute-form target (POST http://elsewhere/mcp), a scheme-relative one (POST //elsewhere/mcp), or a malformed one all get 400 — none of them are routed, and none can take the process down.

Reaching the server under any name other than loopback requires setting MCP_HTTP_ALLOWED_HOSTS yourself. That is the DNS-rebinding defence: without it a page in your browser can resolve an attacker-controlled name to 127.0.0.1 and drive this server as you.

Host and Origin checks are not authentication. They stop a browser being used as a confused deputy; they do nothing about a process running on the same host, which can set any Host it likes and send no Origin at all. On a machine where anything else runs — another MCP server, a dev container, a shared box — set MCP_HTTP_TOKEN. Publish the port to 127.0.0.1 rather than every interface (-p 127.0.0.1:8080:8080), and put the service behind a VPN or private network as you would any other credentialed internal service.

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.

| Class | Status | Triggered by | |---|---|---| | AuthenticationError | 401/403 | Expired/invalid session token, or a Cloudflare error code: 1010 block (see above) | | RateLimitError | 429 | Too many requests against the Substack API in a short window | | ValidationError | 400 | Malformed or invalid arguments passed to a tool (e.g. a missing required field) | | NotFoundError | 404 | The referenced draft, post, or note doesn't exist | | ServerError | 5xx | Failure on Substack's side | | TimeoutError | 408 (synthetic) | The request hit the client's own deadline — no response arrived, so there is no real status to report (see Request timeout) | | SubstackAPIError | any other status | Fallback for unmapped status codes |

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.

Draft export

Use export_draft for a read-only Markdown/JSON bundle, or run:

substack-mcp export 42 --output draft-export.json
substack-mcp export 42 --format markdown --output draft.md

Markdown exports retain the exact original body in a .source.json sidecar. Inspect unsupported_nodes before reuse. Existing files require --force. See export and CLI behavior for publication selection, limits, partial exports and file recovery.

Markdown support

Drafts accept CommonMark/GFM Markdown: headings, nested bold/italic/strikethrough, links and reference links, images with captions and linked destinations, nested lists with starting numbers, code, blockquotes, rules and hard breaks. A standalone <!-- paywall --> block adds one paywall to a long-form draft.

Unsupported content returns unsupported_nodes before a write. After reviewing those diagnostics, draft callers can explicitly set allow_unsupported: true to retain literal fallbacks. Tables remain Markdown inside code blocks; native tables, footnotes, callouts and arbitrary embeds are not advertised as supported. Notes reject unsupported conversion before either publication or attachment creation and have no fallback override.

See Markdown authoring for mappings, limits, compatibility changes and the distinction between offline fixtures and live editor checks.

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 checks your credentials on startup, after the MCP handshake completes, and only warns — it never blocks startup on a network call. Tools still error individually if the token is expired, which is where the failure is actionable.
• SIGTERM and SIGINT are handled: the server closes its transport and exits 0, so docker stop returns promptly instead of waiting out the grace period.

Development

For opt-in live read checks, see live contract evidence. The probe is disabled in ordinary CI and never publishes or writes.

Before releasing, run npm run test:package. It installs the built tarball with production dependencies in a clean temporary directory, checks both executable entrypoints, and verifies the MCP version and the complete registered tool catalog without real credentials.

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.

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.

Companion tools:

• Transistor-MCP: manage podcast episodes, analytics, and transcripts on Transistor.fm
• podcastindex-mcp: search the Podcast Index and track guest appearances
• op3-mcp: report downloads, listener geography, and apps from OP3
• apple-podcasts-mcp: pull plays, followers, and per-episode listening from Apple Podcasts Connect
• gsc-mcp: query search performance, keywords, and sitemaps in Google Search Console
• podcast-benchmark: benchmark a show against its peers using only public data

Disclaimer

This is an independent personal project, not affiliated with, sponsored by, or endorsed by any company. All views expressed are my own.

Codex plugin

The repository includes a Codex manifest at .codex-plugin/plugin.json and an MCP configuration at .mcp.json. It runs the published npm package over stdio using npx; Node.js and npm must be available. The package version is pinned in .mcp.json, so upgrading the plugin's server is an explicit change.

Configure your Substack credentials outside the plugin using the environment variables or browser-login session described above. Never commit a session token. Installation does not authenticate an account or grant approval to post. Long-form posts remain drafts; Notes publish immediately.

License

MIT

For an always-on scheduler with durable cloud state and weekly email reports, see Cloud Calendar sync.

Review before changing a draft

In 0.9, call plan_draft_update, review its output, then call update_draft with the same fields and returned receipt. Published or known stale drafts are rejected. The read/write race remains; check readback outcomes and review in Substack. The CLI shares this flow through drafts plan and drafts apply. See draft changes and migration for examples and limits.

Versioned GHCR images and transport verification: container distribution.