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

Beyond the readOnlyHint=true annotation, the description adds key behavioral context: the tool reports rate limits, not affordability; the old marketplace-credit wallet is retired; the returned marketplace_credits_cents is always 0 and must not be interpreted as a funds signal. This goes well beyond the structured annotations.

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 primary purpose is front-loaded in the first sentence, and each subsequent sentence earns its place by preventing a specific misreading: purchase gating, legacy naming, and the always-zero funds field. Despite its length, none of the content is redundant.

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-only status tool with no output schema, the description covers what the tool reports, what it doesn't mean, when to use an alternative, and how to interpret legacy fields. Nothing an agent needs 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 input schema is empty (0 parameters) and schema coverage is 100%, so there are no parameter semantics to add. Per the baseline for zero-parameter tools, 4 is appropriate; no parameter documentation is needed.

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 names a specific verb and resource: 'Report this API key's tier and daily rate limit,' and clarifies what it is not ('not what you can afford'). This differentiates it from payment/purchase tools like pay_with_x402 and from the legacy get_credit_balance semantics.

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?

It explicitly gives a when-not case: 'Nothing here gates a purchase,' and routes purchases to the sibling pay_with_x402, noting that alternative needs no key or balance. It also warns against reading marketplace_credits_cents as 'out of funds,' preventing misuse.

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