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

Annotations provide only readOnlyHint, so the description carries real weight here and delivers: it discloses two non-obvious side effects (no credit consumption, no verification recorded) that an agent would otherwise have to guess. It also sketches the return shape, which matters because there is no output schema.

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?

Three tight sentences: the core behavior leads, the safety/context claim follows, and the endpoint mapping is a compact trailing reference. No filler or restatement of the title.

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 one-parameter read tool with no output schema, the description gives enough: what goes in, what comes back at a high level, and the notable absence of side effects. Only the exact response fields remain unspecified, which is a minor gap for a lookup.

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?

Single parameter at 100% schema description coverage, so the schema already documents it fully ('The plaintext API key to identify'). The description restates 'plaintext API key' without adding format, prefix, or validation semantics. Baseline 3.

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?

States a specific verb+resource+input shape: 'Given a plaintext API key, return which key it is and its metadata.' The plaintext-secret input implicitly separates it from unkey_get_key, but no sibling is named, so the differentiation is left to inference.

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?

'This is the safe way to identify a key a customer sent you' gives a concrete use context, and 'without spending credits or recording a verification' implies the verify path is the alternative. It never names that alternative tool or states an explicit when-not condition, so it stops short of a 5.

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