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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and a closed world, so the safety profile is covered. The description adds substantive domain context beyond that by describing exactly what the returned definition contains, including negative scope ('what it excludes and what it does not claim'), which tells the agent to expect disclaimers and limits rather than raw data.

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?

Two sentences, no filler, and the content of the returned definition is front-loaded ahead of the usage instruction. Every clause carries information.

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 single-parameter, read-only, closed-world lookup with annotations covering safety and no output schema, the description covers what the tool is, what it returns conceptually, and where ids come from. It does not say what happens with an unknown or malformed id, which is the only notable gap.

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 coverage is 100%: the single id parameter is fully documented with type and an example format ('pay.usd.total.v1'). The description only adds that ids originate from other tools, a sourcing note rather than added syntax or constraint meaning, so baseline 3 applies.

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 the resource precisely ('the frozen definition of a metric') and enumerates its content (what it counts, method, exclusions, non-claims), which tells an agent this is a lookup of metric semantics, not a time series like metric_history. The verb is implicit ('returns') rather than stated outright, and no sibling is named explicitly, so it falls short of a 5.

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?

'Use the definition ids returned by the other tools' gives concrete guidance on how to obtain the required id, which is the main usage question for this tool. There is no statement of when-not to use it or how it differs from metric_history, so it is clear context without alternatives.

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