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

Beyond the safe read operation indicated by annotations, the description provides critical constraints: the contacts are derived only from 'recent Sent and Inbox messages,' and the result includes '(name, address, message count).' This adds important scoping and output behavior information.

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 highly efficient, using a single, well-structured sentence to convey the purpose, the constraints, and the output fields, minimizing wasted words.

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?

Given that the safety profile is clear via annotations and the parameter definition is complete, the description successfully defines the scope, data source, and content of the contact list, making the tool definition highly complete.

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?

The input schema is comprehensive (100% coverage), defining 'mailbox' as the 'Full mailbox address.' The description reinforces the parameter's role as the source of the contact list but does not add substantial semantic information beyond the schema.

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 clearly states the specific function: listing correspondents for a given mailbox. It distinguishes this function by defining the source of the contacts (people in recent Sent and Inbox messages), which separates it from simple listing tools.

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?

It provides clear context regarding when to use the tool by restricting the results to 'everyone it has written to, from recent Sent and Inbox messages.' This helps guide the agent to use it when message history derivation is needed, without explicitly naming alternatives.

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