get_substack_subscriber_countsHow the user's Substack publication is growing: daily subscriber totals by plan over the last 7, 30, 90 or 365 days, with the change over the window. Reads the last sync. Use it for 'how many subscribers', 'how is my Substack growing' and 'paid subscribers this month'.
| Name | Required | Description | Default |
|---|---|---|---|
| days | No | The window, default 30. |
Changes observed during successful MCP inspections.
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
'Reads the last sync' reveals a data-source behavior (stale/periodic data rather than live) not visible in readOnlyHint. The description also discloses what the tool returns: daily counts by plan and deltas over the window. These add behavioral context beyond the annotation.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.
Is the description appropriately sized, front-loaded, and free of redundancy?
The description is in ~45 words across three sentences, front-loading the core result, then staleness, then usage mapping. There is slight redundancy ('how growing' in sentence 1 vs 'how is my Substack growing' in sentence 3), so not a 5.
Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.
Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?
With a single optional parameter, no output schema, and a readOnly annotation, the description connects the returned data (totals by plan, window delta), the data source (last sync), and example user queries. It does not detail exact plan list but is sufficient for invoking the tool correctly.
Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.
Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?
The only parameter, days, has 100% schema coverage (enum + default). The description restates the windows ('over the last 7, 30, 90 or 365 days') and implies this is the metric window, but it does not add materially new meaning beyond the schema. Baseline 3 applies.
Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.
Does the description clearly state what the tool does and how it differs from similar tools?
The description states a specific resource (Substack subscriber counts), scope (daily totals by plan), and windowing semantics (7/30/90/365 days with change). This clearly differentiates from siblings like get_substack_posts and get_substack_post_stats, which are about content rather than audience counts.
Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.
Does the description explain when to use this tool, when not to, or what alternatives exist?
Explicitly gives when-to-use intent through natural-language examples ('how many subscribers', 'how is my Substack growing', 'paid subscribers this month'). It does not mention when-not-to-use or alternatives, but the guidance is clear enough for routing without siblings.
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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