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

Annotations only declare readOnlyHint and openWorldHint; the description goes far beyond them by disclosing the $0.002 USDC per-call cost via x402 on Base/Solana/Arc, that failed calls are free, monthly refresh cadence, and the data's derivation (smoothed, seasonally adjusted, mid-tier, single-family + condo). These are decision-relevant facts an agent cannot get from structured fields.

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?

One dense, front-loaded sentence starting with the core offering, followed by a short exclusion and pricing note; nothing is wasted. The parenthetical disclosure chain is long enough to slightly tax scanning, but each element carries information.

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?

An output schema exists, so return shape need not be described, and annotations cover the read-only profile. The description nonetheless supplies the pricing model, payment networks, refresh cadence, match cap, and the listing-vs-index distinction — everything needed to call it correctly and predict cost.

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 the baseline is 3; the description earns extra credit by adding a constraint absent from the schema — results are capped at 10 matching regions — and by explaining that history runs back to 2000 and that metro includes a United States row. It does not restate the months range or level values, relying on the schema for those.

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 and resource: returns US home values from the Zillow Home Value Index for states or metro areas, with exact fields (latest monthly value, 12-month change, monthly history). It explicitly excludes listings and Zestimates, which separates it from the many listing-oriented siblings like realestate_zillow_rentals or realestate_redfin_property_listings.

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?

Clearly bounds the scope to state/metro level index data and warns that individual listing/Zestimate lookups are blocked on zillow.com, so an agent knows when this tool is not the right pick. It stops short of naming a concrete alternative for listings or for rental data, leaving that inference to the agent.

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