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

With no annotations, the description carries the full behavioral burden, but it only discloses that values are discarded. It does not state whether nested keys are counted, whether invalid JSON is an error, what the return value looks like, or which of the nine parameters is the input; the schema's 'json' parameter says 'validate' rather than 'count', which adds ambiguity.

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?

The definition is only two sentences, front-loaded with the core operation and no filler. The second sentence adds a useful behavioral qualifier, though the overall terseness contributes to the ambiguity penalized 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?

For a tool with nine parameters, no annotations, and no output schema, this is incomplete: it does not identify which parameter holds the JSON object, what the count means, or what the tool returns. An agent cannot reliably construct a correct invocation from this description alone.

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?

Although the schema claims 100% parameter coverage, the parameter descriptions are largely unrelated to counting keys (e.g., git ref, URL, city, feed, host, path) and the 'json' parameter is described as text to validate, not as the object whose keys are counted. The tool description does not map 'JSON object' to any parameter, so the high schema coverage does not help an agent populate the call.

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 opens with a specific verb ('Count') and a concrete resource ('keys in a JSON object'), clearly distinguishing this from validation and shape-checking siblings. The additional phrase 'Values discarded' removes ambiguity about whether values are preserved.

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?

There is no guidance about when to use this tool instead of validate-json or the many shape-check siblings, nor any exclusions or context. The agent must infer usage from the name and one-line description.

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