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

Annotations provide only a title and destructiveHint=true, so the description carries most of the behavioral burden. It says nothing about authentication requirements, how duplicate contacts are handled, whether the call is idempotent, or what side effects occur on failure.

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?

Two short sentences, front-loaded with the action and scope; the endpoint reference is modestly useful for agents mapping to the HTTP API. Nothing is redundant or padded, though 'Missive: POST /contacts' is of marginal value.

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 full schema coverage and only one required parameter, the mechanics of the call are covered. However, with no output schema the description should ideally indicate what is returned (e.g. the created contact id) and what errors look like when the contact book is invalid, which it omits.

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 every parameter (infos, starred, first_name, last_name, memberships, contact_book) is already documented in the schema. The description adds only the fact that contact_book is mandatory, which the schema's required list already states. Baseline 3 is appropriate.

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+resource ('Create a contact') plus the scoping constraint ('in the given contact book'), and the endpoint reference (POST /contacts) confirms the operation. It is clearly distinguishable from the read-oriented siblings (missive_get_contact, missive_list_contacts), though it doesn't explicitly name them as alternatives.

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?

No when-to-use or when-not-to-use guidance, and no alternative named. The requirement that a contact book id be obtained first (e.g. via missive_list_contact_books) is only implied by 'in the given contact book', so the agent must infer the prerequisite.

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