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

With no annotations, the description carries the full behavioral burden. It usefully discloses that only passed fields change and that the operation costs no credits, but it omits permissions, error behavior when a contact is missing, validation behavior, and return/response details.

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?

The description is two compact clauses and is front-loaded with the action and scope. The partial-update note and the 'No credits' note both add useful information without waste.

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 17-parameter mutation tool with no annotations, no output schema, and very low schema description coverage, the description is too sparse. It covers partial-update semantics and cost, but leaves parameter meanings, permissions, and failure behavior undocumented.

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 only 12%, with just the two required UUID fields documented. The description mentions partial updates but does not explain the other 15 optional contact fields or add meaning beyond their property names, so it does not compensate for the low schema coverage.

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?

The description states a specific verb and resource: editing one contact in a custom audience. It also clarifies partial-update behavior, which helps distinguish it from bulk-add or delete siblings, though it does not explicitly name those 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?

The phrase 'Edit one contact' implies this is for updating an existing custom-audience contact, and 'only the fields you pass change' implies partial updates. However, it gives no explicit when-to-use guidance against siblings like add_custom_audience_contacts, delete_custom_audience_contact, or get_custom_audience_contacts.

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