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

Annotations already declare readOnlyHint=false, destructiveHint=false, idempotentHint=false, and openWorldHint=false, so safety and idempotency are covered structurally. The description adds a genuinely useful behavioral fact not in the annotations: 'Contacts are subscribed by default'. It does not say what happens when the email already exists, which is the main remaining unknown for a non-idempotent create.

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 action and scope, then the sibling routing rule. The inline example is short and directly disambiguates the tool's intent rather than padding the text.

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?

Covers purpose, scope limit, subscription default, and bulk alternative, which is most of what an agent needs for a 7-parameter create. The notable gap is return behavior (e.g. the created contact ID) since no output schema exists, and handling of duplicate emails is unstated.

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% and all 7 parameters are documented in the schema (including tag case-insensitivity, audienceIds referencing list_audiences, and the apiKey override). The description only restates the audience/tag capability at a high level, adding no syntax or format detail beyond the schema, so the baseline of 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?

States a specific verb and resource with scope ('Add a single new contact'), and immediately narrows it against the bulk sibling by name ('use import_contacts instead'). An agent can distinguish this from import_contacts, add_contacts_to_audience, and update_contact without opening a 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?

Explicit routing guidance for the bulk case ('For more than a handful of people, use import_contacts instead') plus a concrete usage example. It does not address the existing-contact case (update_contact) or when to prefer add_contacts_to_audience over the audienceIds parameter, but the primary boundary is clearly drawn.

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