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

Annotations already declare readOnlyHint and openWorldHint, but the description adds genuinely non-structured traits: the upstream source and residential-proxy access, the exact billing model ($0.002 USDC per successful call via x402, failed calls not charged), and the hard pagination ceiling (1-28). That pricing and failure-charging detail is exactly the kind of context annotations cannot convey.

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 dense sentences that lead with the resource and field list, then finish with source/proxy and pricing. The long field enumeration is information-bearing rather than padding, though it slightly duplicates the output schema.

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 an output schema present, the return-value field list is partly redundant, but the description still closes the gaps an agent actually needs: pagination limits, per-page size, US-city scope, and cost model. Nothing essential for a correct call 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?

Schema coverage is 100%, so baseline is 3, but the description adds the per-page count (40 per page), which the schema omits and which directly informs how many pages an agent must walk. It also restates the page 1-28 bound and ties rent to USD/month, though both are largely echoed elsewhere.

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?

Names a specific resource (Apartments.com rental listings for a US city) and enumerates the exact fields returned, so an agent can tell it apart from generic real-estate tools. It stops short of naming the closest siblings (realestate_zillow_rentals, realestate_hotpads_rentals) it competes with, so sibling differentiation is only implicit via the source domain.

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?

The 'for a US city' framing implies the scope boundary (US-only rental search and the required city/state), which is useful against the many non-US siblings. However, there is no explicit when-to-use, when-not-to-use, or named alternative, so the agent must infer routing from the source name alone.

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