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

Annotations already declare readOnlyHint and openWorldHint, so safety is covered; the description adds genuinely useful behavioral context beyond them — the 2-credit cost, JSON response format, and the fact that free-tier results differ from paid-tier results (breakdown by state/county/city/district and origin-vs-destination taxing). It does not mention error behavior for invalid ZIP/state codes, which is the main remaining gap.

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 body is dense and front-loaded, but it opens with the redundant 'Sales Tax Lookup: Sales Tax Lookup is...' restatement of the title, which wastes the most salient position. The paid-plan sentence is long but carries real information, so the main deduction is the duplicated opening.

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 an output schema present, the description needn't explain return values, and it still covers tier-dependent behavior and cost. For a two-parameter read-only lookup it is nearly complete; the missing pieces are sibling differentiation and failure behavior on invalid codes.

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 are already well documented, so the baseline is 3. The description adds one real piece of meaning — that ZIP and state are alternatives ('or')' — which the schema does not express since there is no oneOf and zero required parameters; however it does not state what happens if both are supplied or if neither is, leaving the key ambiguity unresolved.

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 names a specific verb and resource ('US sales tax rate API'), states accepted inputs (ZIP, ZIP+4, or two-letter state), and describes the returned fields, so an agent knows exactly what it does. It loses a point because it never distinguishes itself from the directly adjacent SalesTaxCalculator sibling, leaving a plausible ambiguity about which to pick.

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?

Usage is only implied by the purpose statement — the inputs tell the agent roughly when it applies, but there is no explicit when-to-use, no when-not to use, and no mention of SalesTaxCalculator as the alternative for computing a tax amount rather than a rate. The cost note ('2 credits') gives some invocation guidance but not routing guidance.

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