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thenavidm

substack-mcp

by thenavidm
OverviewSchemaSourceRelated ServersDiscussionsAdmin
TypeScript
Hybrid

Comment on a post

comment_on_post
Destructive

Publish a comment on a Substack post under your name. No draft or preview, requires confirm: true to avoid accidental publishing.

Instructions

Post a comment on one of your posts. This is published immediately under your name and is visible to everyone. There is no draft and no preview, so it refuses to run without confirm: true.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
bodyYesComment text.
confirmNoSet true only when the user asked for exactly this action.
post_idYesNumeric post id.
parent_idNoReply to another comment by its id, rather than commenting on the post.
publicationNoWhich connected publication to act on, matched loosely against its hostname (for example 'example.substack.com' or just 'example'). Defaults to the first connected publication.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed3 schema fields changedv3.0.1
    • removedInput schema / $schema
      Removed value: -"http://json-schema.org/draft-07/schema#"
    • removedInput schema / additionalProperties
      Removed value: -false
    • changedInput schema / properties / confirm / description
      Previous value: -"Must be true for this to run. This action cannot be undone, so it is refused without an explicit confirmation."New value: +"Set true only when the user asked for exactly this action."
  2. First observedv2.1.1

TDQS

A4.2/5.0
Behavior4/5

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

Annotations already declare readOnlyHint=false, destructiveHint=true and idempotentHint=false, so the write/irreversible profile is covered. The description adds genuinely new context beyond that: the comment is published immediately, publicly attributed to the user, has no draft or preview, and is gated behind a confirm flag that is enforced server-side.

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

Conciseness5/5

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

Three short sentences, front-loaded with the action and its public consequence, then the confirm gate. No filler and no restatement of the title.

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?

For a destructive public write with no output schema and full schema coverage, the description covers the risk profile an agent needs: immediate publication, public visibility, no undo path, and the confirm requirement. It omits only secondary details such as error behaviour or rate limits, which are minor here.

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%, so post_id, body, parent_id and publication are already documented in the schema; baseline is 3. The description only reinforces the confirm parameter's enforcement ("it refuses to run") without adding syntax, defaults, or constraints beyond what the schema lists.

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?

States a specific verb and resource ("Post a comment on one of your posts") and clearly scopes it to the user's own posts. An agent can distinguish it from the read-side siblings get_post_comments, get_comment_thread and delete_comment without inspecting any schema.

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?

Gives a concrete precondition: no draft and no preview, so the call refuses to run without confirm: true. That tells the agent when the call will succeed or fail, but it names no alternative tool (e.g. for replying to a comment via parent_id, or for reading comments first), so it stops short of explicit when/when-not routing.

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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