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

Annotations already declare readOnlyHint=true, so the safety profile is known. The description adds useful behavioral context: it clarifies the meaning of 'current' (active in the last 5 minutes) and specifies the return type as a single integer. It also provides the exact API endpoint, going beyond the annotation baseline without contradicting it.

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 concise and front-loaded with the core purpose in the first sentence. It contains exactly three short sentences: the function, the return type, and the API reference. Every sentence adds value without unnecessary fluff, making it exceptionally efficient.

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 simple read-only tool with a single parameter and no output schema, the description fully covers what an agent needs: the metric definition, the parameter (via schema), the return type, and the endpoint. There is no missing information about pagination or complex behaviors. The combination of description, annotations, and schema is 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 has 100% coverage for the only parameter site_id, with a clear description including an example ('example.com'). The tool description does not add further parameter details, but since the schema already provides complete semantics, a baseline score of 3 is appropriate. No additional guidance is needed.

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 tool's function: 'Get the number of current visitors on a site (active in the last 5 minutes).' It specifies the resource (a site) and the exact metric (realtime visitor count), which distinguishes it from sibling analytics tools like plausible_timeseries or plausible_breakdown. The mention of the API endpoint further reinforces the scope.

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 description provides clear context for when to use this tool: it's for current visitors active in the last 5 minutes, implying realtime rather than historical analysis. However, it does not explicitly name alternatives or state when not to use it, so it falls short of a 5. Still, the context is sufficient for an agent to differentiate it from the broader stats tools.

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