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

With no annotations provided, the description must carry the full burden of behavioral disclosure. It implies a read-only check but does not explicitly state that it is non-destructive, whether authentication is required beyond the API key, or what happens on invalid keys. The behavior is under-disclosed, leaving the agent to infer safety and side effects.

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 description is a single, clear sentence with no wasted words. It is front-loaded with the primary action and resource, making it easy to scan. However, it is so brief that it sacrifices useful detail, so it earns a 4 rather than 5.

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?

Given the tool's simplicity (one parameter, no output schema), the description is still incomplete. It does not describe what the response contains (e.g., a number of credits, a formatted string) nor any error cases. An agent lacks information about the return value and the exact usage of the api_key parameter, making it hard to invoke correctly without additional context.

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 schema has a single api_key parameter with 0% description coverage, and the description does not explain its role beyond the phrase 'your STYX key'. It does not clarify that the api_key parameter is the STYX key, nor does it provide format, requirements, or default behavior. The description adds negligible value to the parameter semantics.

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 clearly states the verb 'Check' and the resource 'remaining credits on your STYX key'. It is specific and distinct from siblings like styx_renew (renewing credits) or styx_order (ordering credits). The scope is unambiguous, so an agent can easily tell what this tool does.

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 on when to use this tool versus alternatives. It does not mention any prerequisites, such as needing a valid STYX key, nor does it note that it is a read-only operation compared to mutating tools like styx_renew or styx_order. The description only states what it does, not when to choose it.

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