Substack MCP Server

get_subscriber_count

Read-only

Retrieve the current subscriber count for your Substack publication, with precision status indicating exact, approximate, or unavailable values.

Instructions

Get the current subscriber count for your Substack publication. Returns precision: 'exact' when the API reports a true count, 'approximate' when only Substack's rounded value is available (the real number is that or higher — render it hedged, e.g. '1,000+'), or 'unavailable' with count -1. Never treat an approximate value as exact.

Input Schema

TableJSON Schema

Name	Required	Description	Default
No arguments

Output Schema

TableJSON Schema

Name	Required	Description	Default
`note`	Yes
`count`	Yes
`precision`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedSep 8, 2026

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "additionalProperties": false,
+  "properties": {
+    "count": {
+      "maximum": 9007199254740991,
+      "minimum": -1,
+      "type": "integer"
+    },
+    "note": {
+      "type": "string"
+    },
+    "precision": {
+      "enum": [
+        "exact",
+        "approximate",
+        "unavailable"
+      ],
+      "type": "string"
+    }
+  },
+  "required": [
+    "count",
+    "precision",
+    "note"
+  ],
+  "type": "object"
+}

First observedv1.0.0Mar 13, 2026

TDQS

A4/5.0

Behavior4/5

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

The readOnlyHint annotation already indicates this is a read-only operation. The description adds valuable behavioral context by explaining the precision field (exact/approximate/unavailable) and the explicit warning 'Never treat an approximate value as exact.' This goes beyond the basic annotation.

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 concise and front-loaded with the primary purpose. It includes necessary detail about precision handling but is slightly verbose with the example and repeated caution. However, it remains focused and easy to parse.

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?

The description sufficiently covers the return value nuances, including all possible precision values and the meaning of count -1. While an output schema is indicated as present, the description alone provides enough context for typical usage. It does not mention error cases or edge conditions beyond the precision, but these are minor gaps.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters5/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The tool has zero parameters, and the input schema is an empty object. There are no parameter semantics to explain, so the description is fully sufficient. Schema coverage is 100% by definition.

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 action: 'Get the current subscriber count for your Substack publication.' It also explains the output precision semantics, making the purpose unambiguous. It distinguishes itself from sibling tools like list_subscribers or get_subscriber by focusing specifically on the aggregate count.

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

Usage Guidelines2/5

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

The description does not provide any guidance on when to use this tool versus alternatives. It does not mention any conditions or contrasts with sibling tools like list_subscribers or get_subscriber, leaving the decision to the agent to infer.

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