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

With readOnlyHint=true, the agent already knows this is a safe read operation. The description adds useful behavioral context by disclosing the data source (EPA SDWA), the return fields (percentile rank, band, violation rate), and the granularity (per public water system). This goes beyond the annotations and helps the agent anticipate the response structure.

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 tightly written with three sentences, each providing distinct value: what the tool returns, how to pass the parameter, and coverage. It is front-loaded with the core purpose and avoids redundant or filler content. Every sentence earns its place.

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 single-parameter read-only tool with no output schema, the description gives a good overview of inputs and outputs. However, the phrase 'per public water system' creates slight ambiguity about whether the response contains multiple rows or an aggregated county-level result. Given the county-oriented title, this could confuse an agent. A bit more clarity on the return granularity would make it complete.

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?

The input schema already provides 100% coverage for the only parameter 'county', including examples of both accepted formats. The description repeats these examples but adds no new semantic details about the parameter other than coverage count. Since the schema does the heavy lifting, a baseline score 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 that the tool returns a drinking-water contamination index from EPA SDWA health-based violations, and specifies the exact outputs (national percentile rank, band, violation rate). This distinguishes it from sibling county-level tools like county_cancer, county_mortality, and county_sdoh, each of which addresses different topics.

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 provides clear context on what the tool does and how to pass the county parameter (FIPS or name/ST format). It does not explicitly name alternatives or exclusions, but the purpose is specific enough that an agent can infer when to use it. The mention of coverage (3,160 counties) also helps scope expectations.

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