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

Annotations only declare readOnlyHint, but the description adds two substantive behavioral traits: no credit consumption and no verification record created. Those billing and audit side-effect disclosures are exactly the context an agent needs before choosing between this and a verification call.

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, with the identifying behavior and the no-credits/no-audit guarantee front-loaded. The trailing 'Unkey: POST /v2/keys.whoami' is terse endpoint metadata rather than prose padding.

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?

There is no output schema, and the description covers the return at a summary level ('which key it is and its metadata'), which is enough to set expectations. It could say more about the shape of the returned metadata, but nothing needed to invoke the tool 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?

With one parameter at 100% schema description coverage, the schema already documents 'key' as the plaintext API key to identify. The description reinforces the input form but adds no format, syntax, or validation detail beyond what the schema provides, so the 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?

States a precise verb and resource: 'Given a plaintext API key, return which key it is and its metadata.' The qualifier 'plaintext API key' immediately separates it from ID-based lookups like unkey_get_key, so an agent can route correctly without opening either schema.

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 usage context, and the contrast with 'recording a verification' implies the verification path is the alternative when credits/audit records are desired. It stops short of an explicit when-not or naming a sibling tool.

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