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

With no annotations, the description carries the behavioral burden. It discloses that values are ignored ('Values discarded'), which is useful, but it does not state whether nested keys are counted, how invalid JSON is handled, or what the output value is.

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, front-loaded sentences with no filler. The second sentence earns its place by adding the key behavioral caveat that values are discarded.

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?

The tool has nine optional parameters and no output schema, but the description does not say which parameter carries the JSON object or describe the return value/edge cases. The `json` parameter description even mentions validation rather than counting, leaving an agent guessing about how to invoke the tool correctly.

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 description coverage is 100%, so the schema already documents every parameter and this is the baseline. The description does not explicitly map 'a JSON object' to the `json` parameter, so it adds little parameter-level meaning on its own.

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 gives a specific verb and object: 'Count keys in a JSON object.' The second sentence, 'Values discarded,' clarifies what the count ignores, which separates its behavior from validation or search siblings. It is clear, though it does not explicitly name any sibling to distinguish itself from.

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?

Usage is implied rather than stated: an agent can infer to use this when it needs a key count and not validation. The description does not name alternatives or give when-not-to-use conditions, so it stops short of explicit routing guidance.

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