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Substack MCP Server

Substack MCP Server

by cotletdev
OverviewSchemaSourceRelated ServersDiscussionsAdmin
Python
Hybrid

Reply to Note

reply_to_note
Destructive

Post a reply under any Substack Note or an existing reply, visible to everyone who sees the Note. Sends the author a notification.

Instructions

Reply as you under any Note or Note reply.

Posts a reply under a Note (yours or anyone's) or under a reply to one, visible to everyone who sees the Note. Text is 1 to 1,000 characters; Markdown bold, italics and links work. The author gets Substack's usual notification.

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": {
      "note_id": "<note_id>",
      "body": "<body>"
    },
    "name": "reply_to_note"
  }
}

Input Schema

NameRequiredDescriptionDefault
bodyYesReply text, up to 1,000 characters.
note_idYesNote ID from list_notes, list_note_replies or search_substack.

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.1.0

TDQS

A3.8/5.0
Behavior4/5

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

Annotations already flag this as a non-idempotent, open-world, destructive write; the description adds real context beyond them: audience ('visible to everyone who sees the Note'), the author notification, supported Markdown, and the 1–1,000 character bound. It does not explain why the operation is flagged destructive (irreversibility, inability to edit/delete) — the one remaining gap.

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?

Front-loaded with the action and target in the first sentence; the second sentence packs visibility, limits, formatting, and notification without filler. Slightly dense but every clause carries information.

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

Completeness4/5

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

With only two required parameters, full schema coverage, and annotations covering the safety profile, the description supplies the remaining behavioral essentials. The only omission is what a successful call returns (reply ID/object), which matters somewhat since no output schema exists.

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?

Schema coverage is 100% and the schema already documents note_id sources and the 1,000-char limit. The description adds formatting semantics for body (bold, italics, links render) and reinforces the 1-character minimum, which the schema omits.

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

Purpose4/5

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

States a specific verb ('reply') and resource ('Note or Note reply') plus the acting identity ('as you'), which lets an agent distinguish it from publish_note and reply_to_comment. It stops short of naming those siblings explicitly, so the differentiation is inferential rather than stated.

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?

Specifies that the target may be a top-level Note or a reply to one ('yours or anyone's'), which resolves the main targeting ambiguity. However, it never names an alternative tool or says when not to use this one — e.g. that replies to comments belong to reply_to_comment.

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

Deploy Server

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