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

Beyond the readOnly and idempotent hints, the description discloses the paid nature ($0.10 per call), the error condition when no credit (PAYMENT_REQUIRED), the single upstream call guarantee, and the sample behavior. These are behavioral traits not covered by 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 description is somewhat long but well-structured: it leads with the main purpose, then explains kinds, alternative use, return fields, cost, and sample mode in logical order. Every sentence contributes actionable information; only minor redundancy with the schema prevents a 5.

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?

Given the moderate complexity of three modes, an alternative tool, and a payment model, this description covers all relevant aspects: purpose, each mode's result, the alternative tool, the return structure, cost, and the free sample. Nothing an agent needs to decide when and how to 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?

The input schema already describes each parameter in detail (including the kind enum descriptions, address format, and sample default). The description essentially repeats this information without adding significant new meaning, so the baseline of 3 applies.

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 the specific verb 'Fetch' and the resource 'RentCast record for a US street address'. Clearly distinguishes the three kinds (property, value, rent) and explicitly names the sibling tool for the combined use case, making the purpose unmistakable.

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 tells when to use this tool versus the sibling: 'use realestate_deal_memo when you need all three plus the underwriting'. Also explains the sample mode and cost implications, leaving no ambiguity about appropriate usage.

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