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Substack MCP Server and API by Cotlet

by dev.cotlet
OverviewDiscussionsAdmin

Cotlet: how to work in an app

cotlet_app_guide
Read-onlyIdempotent

How to work in an app by clicking in the user's Cloud Browser with Cotlet's browser tools: which browser to use for this workspace and the sign-in link, the method step by step, the app's screens, menus and actions, notes, and the app's dedicated tools when Cotlet has them. Call it before the first browser step in an app. Works for any web app; one outside Cotlet's catalog gets the general method. Free, no browser session. For Substack it covers these actions: Create post, Schedule post, Publish Note, Reply to Notes, Send direct message, Moderate comments, List subscribers, Get post stats.

Tool call template

This tool declares no required parameters. Add optional parameters from the input schema as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {},
    "name": "cotlet_app_guide"
  }
}

Input Schema

NameRequiredDescriptionDefault
appNoThe app's name or Cotlet slug. Default: Substack.
taskNoWhat the user wants done in the app, in their words.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Added

TDQS

A4.5/5.0
Behavior4/5

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

Annotations already cover safety (readOnly, idempotent, non-destructive, closed-world), and the description adds context the annotations cannot: it is free and consumes no browser session. It also discloses the return payload in outline (browser choice, sign-in link, method, screens/menus/actions, notes, dedicated tools), though it does not say how the content is formatted or how long it is.

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?

Purpose is front-loaded in the first clause and the call-timing rule follows immediately; the closing Substack action list is long but serves routing by telling the agent which tasks this guide actually covers. Slightly dense and list-heavy, but no sentence is filler.

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 read-only guidance tool with no output schema and two optional parameters, the definition covers trigger, scope, fallback behavior, cost, and returned content. The only gap is how the returned guidance is presented (structure/length), which is minor given the tool's advisory nature.

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 fields themselves are documented; the description still adds behavioral meaning by noting that an app outside the catalog falls back to a general method, which tells the agent how an unknown `app` value will be handled. It also scopes the Substack-specific action list, giving the `task` parameter concrete grounding.

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: it returns guidance on working in an app via the Cloud Browser and enumerates exactly what the guidance contains (browser choice, sign-in link, step-by-step method, screens, menus, actions, notes, dedicated tools). This clearly separates it from the cotlet_browser_* action siblings, which perform clicks/navigation rather than explain them.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives an explicit trigger ("Call it before the first browser step in an app") plus a coverage rule for apps outside the catalog, and points to the app's dedicated tools as the alternative where they exist. The agent knows both when to call this and when a specialized sibling would be better.

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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