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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the agent knows this is a safe read operation. The description adds context about the API key requirement and the data scope, but it does not disclose details like output format, latency, or potential errors. This is acceptable given the strong annotation coverage.

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?

A single compact sentence front-loads the return values and includes the key prerequisite without redundancy. Every phrase earns its place; no filler or repetition of schema annotations.

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 only one parameter and no output schema, the description adequately enumerates the returned metrics and their time windows. It is slightly ambiguous what 'ログイン成否' means precisely (e.g., counts vs. statuses), but overall a caller has enough context to invoke the tool and understand its result.

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?

Schema description coverage is 100% and already documents api_key with format 'sk_live_…'. The description adds meaningful provenance by explaining that the key is issued from the admin screen (管理画面で発行したキー), which goes beyond the schema and helps the agent understand where to obtain the required parameter.

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 the specific verb '返します' and enumerates the exact metrics returned: MAU (6 months), completed registrations, login success/failure in the last 30 days, user count, and key types. This clearly identifies the resource and content, and distinguishes get_stats from sibling list/get tools that return records rather than aggregate metrics.

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 implies usage: call this tool when you need these aggregated statistics. It provides a prerequisite ('api_key が必要: 管理画面で発行したキー'), but it does not explicitly state when to prefer this over alternatives such as list_users or get_app, nor give any when-not-to-use guidance.

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