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Substack MCP Server

Substack MCP Server

by cotletdev
OverviewSchemaSourceRelated ServersDiscussionsAdmin
Python
Hybrid

Add subscriber

add_subscriber
Destructive

Adds one email as a free subscriber to a Substack list and sends the usual welcome email; use only for contacts who agreed to receive updates.

Instructions

Add one email to your list as a free subscriber.

Adds one email as a free subscriber, the same as Add subscribers in the dashboard, and Substack sends its usual welcome email. Substack adds the email in the background, so list_subscribers may show it a minute later. Add only people who agreed to hear from you.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "email": "<email>"
    },
    "name": "add_subscriber"
  }
}

Input Schema

NameRequiredDescriptionDefault
emailYesSubscriber email.

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.1.0

TDQS

A4.2/5.0
Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare the mutation and open-world nature. The description adds useful behavioral context beyond annotations: Substack sends a welcome email, processing happens in the background, and list_subscribers may lag by a minute. It does not fully explain why destructiveHint is true or whether the add can be undone, which keeps it from a 5.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The important information is front-loaded and the description is appropriately short for a one-parameter tool. However, the first two sentences are partially redundant, both restating that one email is added as a free subscriber.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a simple one-parameter tool with no output schema, the description covers the key operational facts: external welcome email, asynchronous visibility, dashboard equivalence, and consent requirement. There are no major gaps that would prevent correct use.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100% and the sole email parameter is already documented in the schema. The description reinforces that one email is added but does not add format or validation details beyond the schema.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific verb and resource: it adds one email as a free subscriber. It also distinguishes the scope from bulk/dashboard behavior and references the sibling list_subscribers for delayed visibility.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It provides clear usage context: this is the one-email add path, equivalent to Add subscribers in the dashboard, and should be used only for people who consented. It does not explicitly name alternatives for bulk adds, but it gives enough scenario guidance for correct invocation.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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