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

Annotations already declare readOnlyHint=false, destructiveHint=false, idempotentHint=false, and openWorldHint=true, so the bar is lower. The description adds value beyond them by disclosing the auth requirement (manage token) and the observable effect (the milestone appears on the site's reports), though it says nothing about duplicate-name behavior or error conditions.

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 compact sentences: purpose first, then constraint, then endpoint. Every sentence carries distinct information with no repetition or filler.

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 three-required-param create tool with no output schema, the definition covers purpose, auth prerequisite, visible effect, and endpoint. It is nearly complete, though it omits return value expectations and any uniqueness constraints on the milestone name.

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 name, site_id, and milestone_date (including format, length limit, and the fathom_list_sites cross-reference). The description adds no parameter-level meaning beyond that, so the 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?

Specific verb ('Add') plus resource ('a milestone'), with a parenthetical that defines what a milestone actually is (a dated annotation shown on reports, e.g. a launch). This distinguishes it cleanly from fathom_update_milestone, fathom_get_milestone, and fathom_delete_milestone without the agent needing to open another 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 states a prerequisite ('Needs a manage token for the site'), which is useful gating information, but gives no when-to-use vs. when-to-use-an-alternative guidance (e.g. update vs. create). Usage is only implied by the verb.

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