Substack MCP Server

get_subscriber

Read-only

Look up a subscriber by exact email to confirm membership and reconcile uncertain adds. Read-only verification for Substack subscriber lists.

Instructions

Look up a subscriber by exact email address. A listed free subscriber is a member even without paid access. Absence does not prove the address is eligible: Substack may suppress previous unsubscribes, and dashboard data can lag. Read-only; use to reconcile uncertain adds.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`email`	Yes

Output Schema

TableJSON Schema

Name	Required	Description	Default
`note`	Yes
`email`	Yes
`last_sync`	Yes
`subscriber`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

AddedSep 8, 2026

TDQS

A4.7/5.0

Behavior5/5

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

Annotations declare readOnlyHint, and the description adds material behavioral caveats: a listed free subscriber counts as a member, absence may be due to suppression or lag, and absence does not prove ineligibility. These go well beyond the annotation and help the agent interpret results correctly.

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 sentences, each earning its place: core lookup, membership semantics, data caveats, and usage. The main action is front-loaded; only the word 'Read-only' is redundant with the annotation, which is negligible.

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?

Complete for a one-parameter read-only lookup. Output schema covers return shape, and the description covers matching behavior, data reliability caveats, and the intended reconciliation use case. Nothing needed to call it correctly 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?

With 0% schema description coverage, the description compensates by specifying 'exact email address,' which clarifies matching semantics beyond the schema's email format and maxLength. For a single self-describing parameter, this is sufficient.

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 action ('Look up a subscriber') with an exact-match criterion on email address, which distinguishes it from sibling listing tools like list_subscribers and get_subscriber_count. The resource and lookup semantics are unambiguous.

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 clear usage context: 'use to reconcile uncertain adds.' It does not explicitly name alternatives or state when not to use it, but the exact-email lookup and reconciliation purpose are enough for an agent to select it appropriately.

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