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

Annotations already declare readOnlyHint=true, openWorldHint=false and destructiveHint=false, so the safety profile is covered. The description adds two useful behavioral facts beyond that: the call is free and the credential source is the Authorization header. It does not say what happens with a missing/invalid key or what form the response takes.

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, zero waste, with the key fact (free credit balance) and its unit front-loaded. Nothing is padded or repeated.

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 zero-parameter read tool, this covers the essentials, and the unit conversion helps interpret the result even absent an output schema. A note on the return shape (e.g., a numeric balance) or failure behavior for an invalid key would make it fully complete.

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?

Zero parameters, so semantics carry no burden and the baseline is 4. The description adds value by explaining that the implicit input is the Authorization header credential.

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 makes clear this returns the remaining credit balance tied to the API key in the Authorization header, plus the unit conversion (1 credit = US$0.001). No explicit verb is used and it partly restates the title, but the resource and scope are unambiguous. The sibling tools (read, extract, screenshot, delta, site, actions, preflight) operate on entirely different resources, so no sibling disambiguation is needed.

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?

'Free.' signals that calling it costs nothing, which implicitly encourages calling it before metered operations. However, it never states when to use this versus a preflight-style check, nor any preconditions (e.g., valid key required). Usage is only implied, not stated.

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