Substack MCP Server

create_draft

Create a new Substack draft post from markdown. Converts content to Substack format without publishing.

Instructions

Create a new draft post. Accepts markdown body which is converted to Substack's format. Does NOT publish — creates a draft only.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`body`	No	Post body in markdown format
`title`	Yes	Post title
`audience`	No	Who can see this post	everyone
`subtitle`	No	Post subtitle
`allow_unsupported`	No	Acknowledge conversion diagnostics and retain unsupported Markdown literally in this private draft

Output Schema

TableJSON Schema

Name	Required	Description	Default
`id`	Yes
`title`	No
`message`	Yes
`unsupported_nodes`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

Changed2 schema fields changedSep 8, 2026

addedInput schema / properties / allow_unsupported

Added value: +{
+  "default": false,
+  "description": "Acknowledge conversion diagnostics and retain unsupported Markdown literally in this private draft",
+  "type": "boolean"
+}

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "additionalProperties": false,
+  "properties": {
+    "id": {
+      "exclusiveMinimum": 0,
+      "maximum": 9007199254740991,
+      "type": "integer"
+    },
+    "message": {
+      "type": "string"
+    },
+    "title": {
+      "type": [
+        "string",
+        "null"
+      ]
+    },
+    "unsupported_nodes": {
+      "items": {
+        "additionalProperties": true,
+        "properties": {},
+        "type": "object"
+      },
+      "type": "array"
+    }
+  },
+  "required": [
+    "id",
+    "unsupported_nodes",
+    "message"
+  ],
+  "type": "object"
+}

First observedv1.0.0Mar 13, 2026

TDQS

A4.4/5.0

Behavior4/5

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

Annotations only indicate readOnlyHint=false, destructiveHint=false, and openWorldHint=false. The description adds meaningful behavioral context beyond that: it accepts markdown and converts it to Substack's format, and it creates a draft without publishing. This helps the agent understand side effects and scope without contradicting the annotations.

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 tight sentences with no filler. The primary action and the most important caveat ('Does NOT publish') are front-loaded, and the markdown conversion detail earns its place.

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 create tool with five parameters, a full output schema, and annotations covering the safety profile, the description is sufficient: it names the resource, the format conversion, and the non-publishing behavior. It could be slightly more complete by explicitly distinguishing from update_draft, but that gap is minor.

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. The description adds extra meaning to the body parameter ('converted to Substack's format'), which enriches what the schema says. It doesn't detail every parameter, but the schema already covers those well.

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?

The description states a specific action ('Create a new draft post') on a specific resource, and explicitly distinguishes it from publishing by saying 'Does NOT publish — creates a draft only.' The word 'new' also differentiates it from sibling tools like update_draft and export_draft.

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?

The description clearly implies when to use the tool: to create a draft rather than publish. It also states an explicit exclusion ('Does NOT publish'). However, it does not explicitly name alternatives such as update_draft for existing drafts or create_note for notes, so it stops short of full routing guidance.

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