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

Annotations already declare readOnlyHint=true, destructiveHint=false, openWorldHint=true, so safety is covered. The description adds genuinely useful context beyond them: newest-first ordering, the 50-posts-per-page cap, and per-page billing ($0.003), which matters for cost-aware callers. It stops short of describing failure modes or edge cases.

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?

Three tight sentences, front-loaded with the resource and scope, then return fields, then pagination/identity constraint and cost. No filler; every clause carries information.

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?

No output schema exists, and the description compensates by enumerating the returned fields (title, publication, date, engagement, paywall status, word count, tags). Combined with annotations covering the safety profile and the schema covering inputs, an agent has enough to invoke correctly; only error/empty-result behavior is unaddressed.

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 baseline is 3. The description's parameter content ('provide exactly one of username, user_id, url' and the page cap/billing) largely repeats what the schema already states for each field, adding only slight emphasis on the mutual-exclusivity requirement.

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 ('Get an author's posts across all their publications, newest first'), which precisely distinguishes it from the sibling substack_publication_posts (posts within one publication) and substack_user_activity. An agent can select it without opening 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?

The description gives a clear input constraint ('provide exactly one of username, user_id, url') and the scope (across all publications), so usage is implied. However, it never states when to prefer this over substack_publication_posts, substack_user_activity, or search_substack, leaving the sibling choice to inference.

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