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

Annotations already declare readOnly, idempotent, and closed-world behavior, so the bar is lower. The description still adds real value: data provenance (Section 8 extract, 40th percentile) and an explicit failure contract — when the ZIP is absent or multiple town rents exist, it reports this rather than guessing. That edge-case behavior is not in the 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?

Three sentences, each earning its place: what it returns, what the number means, and what happens on failure. Front-loaded with the core purpose and the FY scope.

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 two-parameter read-only lookup with no output schema, the description covers purpose, data source, and failure semantics sufficiently to call it correctly. It could note the return shape (single rent value plus town context), but nothing essential 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 both parameters are already fully documented. The description's '(studio through 4)' merely restates what the schema says (0 is studio, 4 is the max stored), adding no syntax or format detail beyond it. Baseline 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?

States a specific verb (lookup) and resource (HUD Fair Market Rent / Small Area FMR) scoped to a ZIP and bedroom count. It also explicitly distinguishes itself from estimate_rent by clarifying it is 'not a market-rent estimate,' so an agent can tell it apart from siblings without opening a schema.

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?

Gives clear context (published FY2026 figures, 40th-percentile voucher rent in the Section 8 extract) and the condition that selects it over market-rent tools. It stops short of explicitly naming an alternative to use instead, but the negative framing steers the agent well.

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