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

No annotations are provided, so the description carries the behavioral disclosure burden. It does disclose a meaningful trait: 'Values discarded', indicating input values are ignored and not persisted. However, it does not describe the return type, whether only top-level keys are counted, or any error behavior, so coverage is only partial.

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 short sentences with no filler, front-loading the action and target. Both sentences earn their place: the first states what the tool does, the second states a key behavioral constraint. Conciseness is excellent even though completeness suffers elsewhere.

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 nine optional parameters, no annotations, and no output schema, this definition is not complete enough for reliable invocation. The description never explains which parameter should carry the JSON object, whether the object is the tool-call argument itself, or why all parameters are optional and rejected if unknown. An agent is left unsure whether to pass `json`, or any subset of the other keys, to obtain the count.

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 documents all nine parameters (100% coverage), so the baseline is 3. The top-level statement 'Values discarded' adds useful global parameter semantics: it tells the agent that the actual values supplied for fields like url, city, zone, and query are irrelevant, and only the presence of keys matters. This is not stated consistently for every parameter in the schema.

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 a specific operation ('Count keys') and resource ('a JSON object'), and 'Values discarded' clarifies that values do not affect the result. It is clear enough on its own, but it does not explicitly differentiate from siblings such as validate-json or search-query-len, so it stops short of full sibling differentiation.

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?

No guidance is given about when this tool should be used versus alternatives, nor are any exclusions or prerequisites mentioned. The only usage signal is implicit in the name and the verb, which is not enough for an agent deciding between similar-looking utilities.

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