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

With annotations already declaring non-read-only, non-idempotent, non-destructive behavior, the description adds the real gaps: it requires an Admin token (authorization requirement), it returns the new site id used in the tracking code (a return-value behavior no output schema covers), and it names the endpoint POST /sites. It does not discuss rate limits or failure/duplicate-name behavior, keeping it short of a 5.

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: purpose and return value first, then prerequisites and conditionals, then the HTTP endpoint. Zero padding and 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?

For a six-parameter create tool with no output schema and no nested objects, the description covers what it does, the auth prerequisite, the conditional parameter couplings, and the return value. Nothing an agent needs in order to call it correctly is missing.

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 coverage is 100%, so the baseline is 3, but the description usefully consolidates the cross-field conditional dependencies (private sharing implies share_password; multi_domain implies multi_domain_option) into a single readable clause, saving the agent from auditing individual schema entries. It adds no format detail beyond the schema, so it stops short of 5.

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 ('Create a new site in the Fathom account') and immediately differentiates itself from siblings like fathom_update_site and fathom_get_site by noting it returns the new id used in the tracking code. An agent can identify the tool's role 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?

Gives concrete context for invoking it: an Admin token is required, and the conditional dependencies for share_password and multi_domain_option are spelled out. It doesn't give explicit when-not-to-use guidance or point at alternatives, but for a straightforward create op the context is clear.

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