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substack-mcp

substack-mcp

by nanameru
OverviewSchemaSourceRelated ServersDiscussionsAdmin
Python
Hybrid

upload_image

Upload an image to Substack's CDN to obtain a public URL, image ID, and dimensions for embedding in posts or setting as cover image.

Instructions

Upload an image to Substack's CDN and return its public URL.

Args: image_path: Local file path (e.g., /Users/foo/cover.png) or remote URL.

Returns: {url, id, width, height} — pass url to set_cover_image or embed in Markdown as alt.

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": {
      "image_path": "<image_path>"
    },
    "name": "upload_image"
  }
}

Input Schema

NameRequiredDescriptionDefault
image_pathYes

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.1.0

TDQS

A3.9/5.0
Behavior3/5

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

Without annotations, the description carries full burden. It discloses input (local/remote), return structure, and usage of result. However, it does not mention side effects, idempotency, file size limits, or permanence of URL, leaving gaps.

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?

The description is structured with Args and Returns sections, each sentence is relevant. It could be slightly more concise, but it's clear and well-organized.

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?

Given one parameter and no output schema, the description covers input format, output fields, and usage context. Missing file format/size constraints, but overall complete for a straightforward upload tool.

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?

The sole parameter image_path has 0% schema description coverage, but the description adds critical semantics: it accepts both local file paths and remote URLs with examples. This goes beyond the bare string type.

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 clearly states the tool uploads an image to Substack's CDN and returns a public URL, specifying the verb and resource. It distinguishes from sibling set_cover_image by indicating the URL can be passed there.

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?

The description implies usage before set_cover_image or Markdown embedding, but lacks explicit when-not or alternative tools beyond set_cover_image. No exclusions or context signals for when to avoid.

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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