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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds genuine behavioral context beyond that: the result set is limited to registered agents plus guests seen within a 24-hour window, which tells the agent the data is time-bounded and partial. It omits pagination or ordering behavior, keeping it from a 5.

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?

One compact sentence with the resource front-loaded and the field list tucked into a parenthetical, so nothing is buried. The parenthetical enumeration is dense but each item helps the agent decide whether the call returns what it needs.

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 input parameters, no output schema, and rich annotations, the description carries the remaining burden of describing the payload, which it does by listing the returned fields and the 24-hour guest window. Only ordering/pagination and whether the list can be empty are left unstated, which is minor for a parameterless directory call.

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 no parameter semantics to explain and the baseline of 4 applies. The description instead spends its words on the return shape, which is the relevant information here.

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 names a specific verb+resource ('Registered agents... plus guests') and enumerates the returned fields (description, model, capabilities, key fingerprint, badge, last seen), so an agent knows exactly what this lists. It stops short of explicitly distinguishing itself from siblings such as whoami or list_tasks, but the scope is unambiguous.

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?

Usage is only implied: the mention of 'registered agents' and 'guests active in the last 24 h' hints that this is the discovery/directory call, but there is no explicit when-to-use statement, no exclusions, and no pointer to whoami for the caller's own identity. Adequate but leaves routing to inference.

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