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

Beyond the readOnlyHint/openWorldHint annotations, it discloses the return shape (mapped/unmapped status plus target sequence), an explicit data-scope disclaimer, and crucially the commercial/auth behavior: $0.01 USDC per successful call on Base and an x402-capable wallet requirement. These are exactly the non-obvious operational traits an agent needs before invoking.

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?

Four tight sentences, front-loaded with what it does, then return shape, then scope limits, then payment/auth terms. No filler and nothing repeated from the schema.

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 still names the returned fields, and it covers the two things an agent would otherwise get wrong: input scope (single scalar, no personal text) and the paid x402 wallet prerequisite. Nothing material is left unspecified for a 2-parameter lookup.

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 both parameters (codepoint prefix/surrogate rules, unicodeVersion default and response version echo) are already fully documented in the schema. The description restates the single-scalar constraint but adds no syntax or format detail beyond the schema, which is the expected baseline 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?

States a precise verb (look up) and resource (one hexadecimal Unicode scalar) plus the authoritative data source (official UTS #39 confusables). The sibling tools are all unrelated data services, so no sibling differentiation is needed and none is missing.

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 makes the intended input scope explicit ('one hexadecimal Unicode scalar') and draws negative boundaries ('accepts no personal text', 'makes no identifier-safety judgment'), which tells the agent when this tool is not appropriate. It stops short of naming an alternative tool or an explicit 'use this when...' trigger.

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