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Substack MCP Server and API by Cotlet

by dev.cotlet
OverviewDiscussionsAdmin

List subscribers in Substack

substack_list_subscribers
Read-onlyIdempotent

List subscribers in the user's own Substack account from ChatGPT, Claude or any AI agent. Returns how to do it in Substack step by step (screens, buttons, fields) and opens nothing by itself; then do it with the cotlet_browser_* tools in the user's Substack Cloud Browser. Substack's official API: Substack has only a limited API (checked 11 Oct 2026).

Tool call template

This tool declares no required parameters. Add optional parameters from the input schema as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {},
    "name": "substack_list_subscribers"
  }
}

Input Schema

NameRequiredDescriptionDefault

No arguments

Schema Changelog

Changes observed during successful MCP inspections.

  1. Added

TDQS

B3.4/5.0
Behavior4/5

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

Beyond the annotations (readOnlyHint, idempotentHint, destructiveHint=false, openWorldHint=false), it adds two useful behavioral facts: the tool 'opens nothing by itself' and it returns screens/buttons/fields instructions. The closing note that Substack's official API is limited adds real context about why a browser-based path exists, though the sentence is truncated and vague.

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

Conciseness3/5

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

The core behavior is front-loaded, which is good, but 'from ChatGPT, Claude or any AI agent' is filler and the trailing 'Substack's official API: Substack has only a limited API (checked 11 Oct 2026).' is an incomplete thought dangling after a colon. Tightening would sharpen it without losing meaning.

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

Completeness3/5

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

For a zero-parameter, no-output-schema guidance tool, the description covers what it returns and how to act on it. It is incomplete on the key sibling disambiguation (data-returning 'list_subscribers' vs. this instruction-giving tool), which matters given the crowded subscriber-management toolset.

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

Parameters4/5

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

The tool takes zero parameters, so there is nothing for the description to disambiguate and the baseline is 4. The description correctly implies no inputs are needed.

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

Purpose3/5

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

The description states a specific verb and resource ('List subscribers in the user's own Substack account') and then reveals that the tool actually returns step-by-step instructions rather than data, which is important. However, it never distinguishes itself from the sibling tools 'list_subscribers' and 'export_subscribers', so an agent cannot tell which one actually returns subscriber data.

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

Usage Guidelines3/5

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

It gives forward guidance ('then do it with the cotlet_browser_* tools in the user's Substack Cloud Browser'), which implies a two-step workflow. It gives no explicit condition for choosing this tool over 'list_subscribers', 'export_subscribers', or the browser tools directly, so the routing decision is left to inference.

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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