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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), and the description adds substantial context beyond them: UCMR 5 entry-point samples are 'not compliance determinations or household tap results', the May 2026 rescission/deadline documents are only proposals, and systems with withheld names are shown by PWSID and city. This is exactly the caveat-level disclosure an agent needs for a regulatory dataset.

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?

Purpose is front-loaded in the first clause, but the second sentence is a very long multi-clause run-on enumerating every returned field and every caveat. Given there is no output schema some return detail is warranted, but the density and lack of sentence breaks hurt scanability.

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 return-value burden and does so thoroughly — it lists the match fields (PWSID, name, state, city, population served, owner type/label, activity status, derived flags, 2024 limits, ZIP match scope) plus source dates, limit statuses, and register caveats. Nothing material for correct invocation appears 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 description coverage is 100%, so the baseline is 3. The description largely restates what the schema already documents ('a service-area hint, not a confirmed supplier', 'one state file, or TRIBAL', name filters by system name or city), adding the mutual-exclusivity constraint and little else.

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 opening states a concrete verb and resource ('Looks up public water systems in the MORVS PFAS Drinking-Water Register') and names the underlying data sources, so an agent knows exactly what is being queried. It does not explicitly distinguish itself from near siblings like pfas_system_history or pfas_rule_impact, which is the only gap.

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?

It states the governing rule clearly: 'Give exactly one of pwsid (one system), zip (every system that reported serving that ZIP code) or state (one state file, or TRIBAL)', and explains that name only filters within a state. That is genuine when-to-use guidance for the parameter choices, though no alternative tool is named as a fallback.

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