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

With only readOnlyHint available, the description carries real behavioral weight: it discloses nullability rules (lowest_cost and best_combined are null unless every site has a rate; best_* = null with *_tied lists on ties), data sources and vintages (eGRID, Green-e residual mix, EIA-861 annual, ZIP's own 2023 utility average), and the time-zone basis for the daily window.

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?

Front-loaded with the action and scope before the dense per-site output inventory. The single paragraph is information-dense with little filler, though the long chain of returned fields pushes it toward the verbose side.

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?

No output schema exists, so the description must describe returns, and it does so thoroughly: per-site metrics, rankings, gap percentages, tie handling, and the annual figures gated by annual_kwh. An agent has everything needed to invoke and interpret it.

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 3 baseline applies, but the description adds genuine meaning beyond the schema: it explains that $/kWh is the state-level latest full-year EIA-861 average with the ZIP's own 2023 utility average as reference, that duration maps to the cleanest daily window from a 7-day hourly profile, and that annual_kwh yields annual tCO2e and cost.

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 (Compare and rank) plus resource (candidate US sites / ZIP codes) with explicit scope (2-25, grid carbon and electricity cost). This clearly distinguishes it from siblings like lookup_batch and lookup_by_coordinates, which operate on single locations.

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 'in one call' and the target use cases (facility, data center, EV/flexible load) imply this is the batch siting tool, and the duration/annual_kwh params hint at when extra detail is wanted. However, no alternative sibling is named and no when-not-to-use condition is stated, leaving routing to inference.

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