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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and a closed world, so safety is covered. The description adds one genuinely new trait not carried by structured fields: the call is free and consumes no credit, which matters for agent cost budgeting. It stops short of describing output shape or rate limits.

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, no waste, with the payload (remaining credits) front-loaded before the secondary note on purchasing and the cost disclaimer. Ideal size for a zero-parameter tool.

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 no input parameters, no output schema and fully covering annotations, the description supplies the essentials: what is returned, both keyed and keyless modes, and a pointer to purchasing. It could have sketched the response fields a little more explicitly, but nothing needed to call it correctly is missing.

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 tool takes zero parameters, so per the baseline there is nothing to document and no gap to compensate for. The description correctly implies the key/identity is taken from ambient authentication rather than an argument.

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 states exactly what the tool returns: credits remaining on the API key, or free calls left today when no key is supplied, plus how to purchase more. That distinguishes it from the data-oriented siblings (contracts, stocks, IPO radar), though the name 'account' alone would not; the title and description together make the resource unambiguous.

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?

Implicit guidance is present: an agent learns it can call this without spending a credit and that the response explains purchasing, which suggests pre-flight balance checks. However, it never explicitly says when to call this versus just proceeding, and gives no conditions or exclusions.

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