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Substack MCP Server

Substack MCP Server

by cotletdev
OverviewSchemaSourceRelated ServersDiscussionsAdmin
Python
Hybrid

Update draft

update_draft

Edit a draft or scheduled post's title, subtitle, body, or audience while leaving other fields unchanged; published posts return an error.

Instructions

Change the title, subtitle, body or audience of a draft.

Changes the fields you pass and leaves the others as they are. body_markdown replaces the whole body. Drafts and scheduled posts only: a published post returns an error. Nothing reaches your subscribers, so it runs without asking.

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": {
      "post_id": "<post_id>",
      "fields": "<fields>"
    },
    "name": "update_draft"
  }
}

Input Schema

NameRequiredDescriptionDefault
fieldsYesJSON object with any of title, subtitle, body_markdown, audience, e.g. {"title": "New title"}.
post_idYesPost ID from list_posts.

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.1.0

TDQS

A4.4/5.0
Behavior5/5

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

Annotations only declare the safety profile (readOnlyHint=false, destructiveHint=false), and the description adds real behavioral context beyond them: partial-update merge semantics, that body_markdown replaces the entire body, the error condition for published posts, and that no subscriber notification is triggered so confirmation can be skipped.

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?

Four short sentences, each carrying distinct information — scope, merge semantics, applicability, and safety — with the core action front-loaded. No filler or restatement of the title.

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 two-parameter mutation with no output schema, the description covers what changes, how partial updates behave, when the call fails, and whether confirmation is needed. Nothing essential for correct invocation is missing.

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%, so the baseline is 3, but the description adds meaning the schema lacks: the fields object is a patch (only passed fields change), and body_markdown is a full replacement rather than an append. It does not elaborate on the audience field's accepted values.

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 (change) and resource (draft) and enumerates the mutable fields: title, subtitle, body, audience. It is clear what the tool does, though it never names a sibling like create_draft or publish_post to sharpen the boundary.

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 applicability boundary — 'Drafts and scheduled posts only: a published post returns an error' — which tells the agent when not to call it. It stops short of pointing to an alternative tool for the published-post case.

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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