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substack-mcp

by thenavidm
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TypeScript
Hybrid

Preview how a body will render

preview_draft_body
Read-onlyIdempotent

Convert a post body to Substack's document format and preview the output without creating anything, so you can verify embeds, paywalls and nested lists render as intended.

Instructions

Convert a body to Substack's document format and show what it produced, without creating or touching anything. Use this to check that an embed, a paywall or a nested list came out the way you meant before you write it to a draft.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
bodyYesPost body. Markdown by default. Supported: headings, bold, italic, inline code, strikethrough, links, images, nested lists (any depth, ordered and unordered mixed), fenced code blocks with a language, blockquotes and horizontal rules. Two things worth knowing: - A line containing only a YouTube, X, Spotify or Vimeo URL becomes a real embedded player, not a link. Put the URL inside a sentence if you want a plain link. - Write <paywall> on its own line to mark where the paywall starts. Everything after it is for paying subscribers only. One per post. Markdown tables have no equivalent in Substack's editor, so they are preserved verbatim in a code block rather than being mangled.
body_formatNoHow to read `body`. 'auto' detects it and is almost always right. Use 'prosemirror' to send a Substack document directly.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed2 schema fields changedv3.0.1
    • removedInput schema / $schema
      Removed value: -"http://json-schema.org/draft-07/schema#"
    • removedInput schema / additionalProperties
      Removed value: -false
  2. First observedv2.1.1

TDQS

A4/5.0
Behavior3/5

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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false, so 'without creating or touching anything' largely restates structured data. The genuinely additive behavior note is that it performs a format conversion into Substack's document format, but the shape of the preview result (rendered HTML, warnings, error list?) is left unstated despite there being no output schema.

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?

Two sentences, zero padding, with the non-mutating guarantee and the conversion stated first and the recommended use case second. Nothing here could be cut without losing routing 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?

Annotations cover the safety profile and the schema covers inputs thoroughly, so the description only needs to round out intent — which it does. The one remaining gap is what the preview actually returns, which matters for a tool whose entire value is its output and which has no output schema.

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 body parameter's own description already documents markdown support, embeds, and the <paywall> marker, so the description adds nothing parameter-specific. Baseline 3 applies when the schema carries the full parameter burden.

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?

Specific verb+resource: converts a body to Substack's document format and returns the rendered result. It also distinguishes itself from the write-side siblings by stating it does so 'without creating or touching anything', so an agent can separate it from set_draft_body or create_draft at a glance.

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 gives a clear use case and the moment to use it: verify embeds, paywalls, or nested lists render correctly 'before you write it to a draft'. The implicit alternative (writing the body to a draft) is named as the thing that follows, though no sibling is called out by name and no explicit 'do not use this for X' exclusion is given.

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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