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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description adds the auth requirement (Cotlet API key with app access) and the causal link to upgrade operations. It doesn't state rate limits or freshness of credits, but that is a minor gap for a read tool.

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?

Every sentence earns its place: the first defines the resource and what is returned, the second states the return contract, the third gives the pre-flight check rationale and auth requirement. No filler, front-loaded with the core purpose.

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-param read tool with no output schema, the description covers the what (credits + expiry + listings), the why (pre-upgrade check), and the auth requirement. 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 tool takes zero parameters, so parameter semantics are trivially satisfied. The description does not need to explain parameters, and the baseline for zero params is 4.

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?

States a specific verb (get) and resource (upgrade credits) and details exactly what is returned: credit counts, expiry, and the listings using them. This is far more specific than the sibling get_* tools like zoopla_get_featured_properties_report, which it can be distinguished from on content alone.

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?

Explicitly names the two sibling tools it gates (upgrade_to_premium, feature_listing) and states the exact condition for using it: each upgrade consumes one credit. This is textbook when-to-use guidance that prevents wasted upgrade attempts.

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