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

Annotations already declare readOnlyHint, idempotentHint, and openWorldHint, so the safety profile is covered. The description adds genuinely useful behavior beyond annotations: it returns null where Zillow publishes no Zestimate, explicitly does not fabricate an estimate, and describes the monthly history cadence. No contradiction with 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?

Two sentences with no filler. The central resource is front-loaded, the returned data is organized in a clear list, and every clause adds either precision or behavioral context.

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?

There is no output schema, so the description carries the full burden of explaining return content. It enumerates the key fields, explains the null behavior, and clarifies the history span. Nothing an agent needs to decide whether to call the tool 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 description coverage is 100%, and both parameters (zpid and expand) are already well documented in the input schema. The description adds no parameter-level meaning, but it does not need to because the schema fully carries that burden.

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?

Although phrased as a noun phrase rather than an imperative, it unambiguously identifies the resource: the Zestimate for a given zpid, including value range, value 30 days ago, rent Zestimate, zestimate history, and price history. The phrase 'for any zpid, listed or not' clearly separates this tool from listing/search-oriented siblings.

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 description gives a clear usage condition: use this when you need Zestimate data for any zpid, whether or not the property is currently listed. It does not explicitly name sibling alternatives or when-not conditions, so it falls short of full routing guidance, but the scope is clear.

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