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

Annotations already declare readOnlyHint=true, openWorldHint=true and destructiveHint=false, so the safety profile is covered. The description adds genuinely new behavioral context by disclosing the per-request billing cost ($0.003), which the annotations do not. It stops short of auth, rate-limit or pagination behavior, but with annotations present this is useful added value.

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 verb and resource in the first clause, then folds the return-field list and the input constraint into a compact parenthetical. Every clause carries information; there is no filler despite the length.

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 in play, the description carries the return-value burden and does so thoroughly, listing the fields an agent will receive. Combined with the input-selection constraint and the billing note, nothing material is missing for a read-only profile lookup.

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 each parameter's format (URL shape, numeric ID, handle with optional leading @, length limits) is already fully documented. The description only restates the three identifiers and the 'exactly one' rule, adding marginal precision over the schema's 'Provide one of' wording. Baseline 3 applies when the schema does the heavy lifting.

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 ('Get') and resource ('a Substack author's full profile') and explicitly distinguishes the three accepted identifier forms. It enumerates the returned fields (bio, follower count, bestseller tier, publications written for, leaderboard placement), so an agent can tell it apart from siblings like substack_user_posts or search_substack_users without opening either 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?

Usage is implied by the profile-lookup framing and the parenthetical 'provide exactly one of username, user_id, url', which gives input-selection guidance. However, it never states when to choose this tool over adjacent siblings such as substack_user_activity or search_substack_users, and gives no exclusions or prerequisites.

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