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

Going well beyond the readOnly/idempotent annotations, it discloses the return payload (both forms, per-label breakdown, first rule violated), the specific validation rules applied, that strict mode may reject names, and the cost and payment path ($0.05 per successful call via x402/USDC on Base). This is exactly the extra context annotations cannot carry.

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 and each sentence carries information (direction, validation, mode semantics, price). The dense enumeration of IDNA2008 rules and the garbled fragment 'it may reject names /v1/code/url displays' cost it a point on polish.

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 tells the agent what comes back (both forms, per-label breakdown, first rule violated) and covers failure and cost. For a 3-parameter, single-purpose converter this is sufficient to call correctly.

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% and both enums are already fully documented with defaults and the ß→ss example. The description restates the strict-mode semantics rather than adding new parameter meaning, so the baseline 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?

States a specific verb and resource: convert internationalised domain names between Unicode and ASCII punycode in either direction, label by label. The scope plus the embedded IDNA2008 validation makes it clearly distinct from neighbours like cyrillic_transliterate or url_parse without opening any schema.

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 explains the strict-vs-non-strict choice (IDNA2008 vs UTS 46 transitional) and warns that output can differ from browsers and URL displays, which implies when each mode applies. However it never says when to pick this tool over siblings such as url_parse or unicode_confusables_check, so tool-selection guidance is left to inference.

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