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

No annotations are provided, so the description carries the burden. It discloses the pricing (3¢ per call), the mode behavior switch at SAL level, and the requirement to pass fields. It doesn't mention return format or error conditions, but the output schema exists and the description adds meaningful behavioral context beyond the schema.

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 compact and front-loaded: it states the core purpose in the first sentence, then adds the mode detail and pricing. Every sentence earns its place, and the formatting with bold for pricing is clear.

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?

Given the tool has an output schema and 100% schema coverage, the description is complete enough for an agent to call it correctly. It covers the key behavioral nuance (mode=sa1) and pricing. It could mention that region is case-insensitive or that SA1 uses codes, but those are in the schema. Overall, adequate.

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 schema already documents all parameters. The description adds the constraint that fields must be 1-5 codes from /fields, which is already in the schema. It doesn't add much beyond the schema, but the baseline 3 applies because the schema is complete.

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 states a specific verb ('fetch') and resource ('statistical field values for one region'), and clarifies the fields parameter. It distinguishes the SAL mode behavior, but doesn't explicitly differentiate from sibling tools like area_stats_fields or area_stats_regions, though the name and context make it reasonably clear.

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 explains when to use the tool: pass fields (1-5) from /fields to fetch. It also gives a specific usage condition for SAL level with mode=sa1. It doesn't explicitly state when not to use it or name alternatives, but the context is clear enough for an agent to select it appropriately.

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