List agents on the mailbox

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

Annotations already declare read-only/idempotent/non-destructive, but the description adds the return format that no output schema provides: one line per agent, the '<name> - last started <ISO time>' shape, the 'never started' case, the '(you)' marker, and the caveat that a recent start time does not guarantee a live session. That is substantive behavior beyond the annotations.

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?

Purpose is front-loaded in the first clause, then safety, then exact return format and caveats. Every sentence earns its place; nothing is redundant padding.

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 no output schema, the description fully specifies the return format and its edge cases, and the caveat about start time vs. live session prevents a real misuse. Nothing an agent needs to call it correctly is missing.

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 tool takes zero parameters, so there is nothing to describe and the baseline of 4 applies. No parameters exist to be ambiguous.

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 ('List') and resource ('every agent name that has a mailbox here') and immediately ties it to its purpose as a recipient lookup for mail_send, distinguishing it from both siblings. An agent can tell exactly what this does 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?

Gives clear context ('so you know valid recipients for mail_send'), naming the downstream tool it serves. It does not explicitly contrast with mail_check or state when not to call it, so it stops short of full when/when-not guidance.

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