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

Annotations already declare readOnly and openWorld, so the bar is lower. The description adds meaningful context beyond them: per-page billing ($0.003), page size (12 items), newest-first ordering, and the return shape (items tagged note/post/comment, Notes carrying text, engagement, and quoted content). It stops short of auth requirements.

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 core purpose is front-loaded and the following sentences each carry information (scope, return tagging, identifiers, billing). It is dense and slightly run-on but has no truly wasted sentence.

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 must carry the return-value burden, and it does: item tagging, note contents, and the one-of identifier rule. Pagination and billing are covered, leaving only minor gaps like ordering/pagination limits detail.

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 the schema already documents all five parameters thoroughly. The description only restates the one-of username/user_id/url constraint and the type values, adding little beyond the schema, so 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 opens with a specific verb+resource ('Get an author's public activity feed') and enumerates the activity types covered, which differentiates it from narrower siblings like substack_user_posts. It does not explicitly name a sibling to distinguish against, but the stated scope is unambiguous.

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?

It implies usage through the type filter ('only their notes, posts, replies, restacks or likes') and the one-of identifier requirement, but never states when to prefer this over substack_user_posts or substack_note_details. No explicit when/when-not guidance is given.

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