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

The readOnlyHint annotation already establishes that the tool is non-mutating, so the description does not need to repeat safety claims. The description adds useful behavioral context by showing the exact return shape, including key, event, count, and first_seen, and clarifies the operation is purely discoverable listing rather than a query or aggregation.

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 a single, information-dense sentence followed by a compact return example. The purpose is front-loaded, the query_stats connection is included, and the output shape is shown without 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?

For a simple read-only listing tool with two optional, fully documented parameters and no output schema, the description provides the key behavioral context: what is listed, how it relates to query_stats, and the exact response structure. Nothing essential for invoking 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 input schema already provides 100% coverage of both parameters, including detailed descriptions for site and period. The tool description does not add parameter-level meaning beyond the schema, so a baseline score of 3 is appropriate: the schema carries the semantic load.

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 uses a specific verb and resource: 'List the custom-property keys a site has recorded,' and clarifies it returns the associated event for each key. It also explicitly positions the tool as discovery for the event:props:<key> breakdown in query_stats, which distinguishes its purpose from the sibling query_stats tool.

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 gives clear context by stating this is a discovery step for the event:props:<key> breakdown in query_stats, so an agent knows when it is relevant. It does not explicitly name alternatives or exclusions, but the intended use case is strongly implied and specific enough for selecting this tool.

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