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

Annotations already establish readOnly, idempotent, and closed-world behavior, so the bar is lower. The description adds genuinely new operational context: no cost, no API key required, and the FIPS-ordering/pagination default that shapes results. It does not describe the response envelope, but for a read-only reference tool this is solid.

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?

A single dense paragraph that front-loads the contents, then the filtering caveat. Every clause carries information, though the sentence is long and could be broken for faster scanning.

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?

With no output schema, the description carries the burden of describing return content, and it does: it lists the fields returned and warns about the default 25-row, FIPS-ordered result. An agent has everything needed to call it correctly and interpret the response.

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% and both parameters already document their defaults, including the state-vs-no-state limit behavior. The description largely restates those defaults, adding only the Texas-at-254 scale detail. Baseline 3 is correct when the schema does the heavy lifting.

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?

Names a specific resource (US counties and county equivalents, all 3,235) and enumerates the attributes returned (Census/USPS names, ANSI code, legal type, area, delivery footprint). This is clearly distinguishable from siblings like city_name_rules, metro_area_reference, and zip_facts, which cover different geographies.

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 explicit conditional guidance: filter by state to get every county, and warns that an unfiltered call returns only the first 25 in FIPS order, so it answers with Alabama rather than what was asked about. That is a real when-to-use/when-not distinction. It stops short of naming a sibling alternative for other geographic lookups.

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