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

Annotations already establish readOnlyHint, openWorldHint, and idempotentHint. The description adds useful context by specifying the source ('as Zillow lists them') and that Zillow's tag is included, but it does not disclose response shape, aggregation behavior, or freshness caveats. There is 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?

The description is a single, tightly written sentence with no filler. It front-loads the core purpose ('Today's mortgage rates') and then lists the exact data elements returned, making every word informative.

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 low-complexity tool with no required parameters, the description covers the main return contents: product, rate, APR, and tag. The absence of an output schema is partially mitigated by this enumeration. Minor ambiguity remains around what 'Zillow's tag' concretely looks like, but not enough to prevent correct invocation.

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%, so the single optional 'expand' parameter is fully documented in the schema. The description adds no parameter-level detail beyond that, but none is needed; the baseline of 3 is appropriate.

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?

The description clearly states the tool returns today's mortgage rates with product type (30-year fixed, FHA, VA, 15-year, ARMs), rate, APR, and Zillow's tag. It is easily distinguished from all sibling tools, which concern properties, agents, schools, and market statistics rather than mortgage rates.

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 phrase 'Today's mortgage rates' clearly frames when this tool is relevant: when current mortgage rate information is needed. It does not explicitly name alternatives or exclusions, but among the siblings this is the only rates-focused tool, so the intended usage context is clear.

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