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

Annotations already declare readOnly/openWorld/non-destructive, so the safety profile is covered. The description adds real value beyond them: the API-key/access prerequisite and the important behavioral trait that metrics keep growing after a send, plus the scope note that email numbers cover the send while views include the web.

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?

Front-loads the metric list before the explanatory sentences, and the freshness caveat and auth prerequisite each earn their place. The opening reads as a fragment list rather than a full sentence, but it is compact and non-redundant.

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 no output schema, the description carries the burden of describing return values and does so by enumerating the exact metrics. Combined with the prerequisite and freshness note, an agent has everything needed to call and interpret this read-only tool.

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?

Schema description coverage is 100% for the single post_id parameter ('Post ID from list_posts.'), so the schema already carries the semantics. The description adds nothing about post_id, so the baseline of 3 for high-coverage schemas 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?

States a specific verb (returns/get) and resource (engagement numbers of one published post), and enumerates the exact metrics returned (recipients, opens, open rate, views, likes, restacks, subscribers gained). The phrase 'one published post' distinguishes it from substack_get_publication_stats without needing the schema.

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?

Gives clear context: what the tool corresponds to ('its stats page in the dashboard') and a freshness caveat ('numbers keep growing for days after a send, so check again later'). It does not explicitly name an alternative or exclusion versus substack_get_publication_stats, so it stops short of full routing guidance.

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