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

Annotations already cover the read-only safety profile, and the description adds genuinely useful behavior beyond that: pagination (pages of up to 500, cursor round-trip), the semantics of the activity rating, and the auth requirement (Cotlet API key with access to this app). It does not state rate limits or error behavior, but the additions exceed what annotations and schema carry.

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?

Compact and front-loaded, with the returned fields and ordering leading. The opening is a slightly awkward fragment ('Substack: Subscribers with plan...'), but every sentence carries information and there is no padding.

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 usefully names the returned fields (plan, start date, activity rating), defines the rating scale, and covers pagination and auth. Only minor gaps remain, such as default page size behavior relative to the schema's limit=100.

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%, so all four parameters are already documented, and the baseline is 3. The description reinforces the cursor round-trip ('pass next_cursor back as cursor') but adds no new syntax or format detail for plan/since/limit beyond what the schema states.

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 and resource ('Lists subscribers') plus the fields returned (plan, start date, activity rating) and ordering (newest first). It is distinguishable from substack_add_subscriber by the 'Lists' verb, though it does not explicitly contrast with the similarly read-only substack_export_subscribers.

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?

Implies usage through filter hints ('Filter by plan or by the date they joined'), which tells the agent how to narrow results, but offers no explicit when-to-use guidance, prerequisites, or routing to alternatives like export_subscribers for bulk pulls.

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