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

With no annotations, the description carries the full burden and does real work: it discloses the preprocessing step ('stripping non-digits first'), the short-input edge case ('four digits or fewer come back unchanged'), and a rate-limit/pricing constraint ('5 free trial calls per signed wallet'). What is missing is whether the input is validated (e.g. Luhn) and what the masked output looks like.

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?

Two compact sentences, with the core transformation front-loaded ahead of the edge-case caveat and pricing note. The leading 'Mask Card:' echo of the tool name is minor redundancy but not wasteful.

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 simple string-transform tool with no output schema, the behavioral rules are adequately covered, but the complete absence of parameter documentation for two similarly named string params leaves a real gap an agent can trip on.

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 0% and both parameters ('text' and 'input') are undocumented in the schema, so the description must compensate. Instead it never ties the input to either parameter, leaving an agent unable to tell whether to send the card number in 'text' or 'input' or why two string parameters exist.

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 ('Mask Card: Mask all but the last four digits of a card number') plus the transformation rule, so an agent can distinguish it from generic masking siblings like x402-mask or from x402-mask-email/x402-mask-email. It does not explicitly name those siblings, so it stops short of a 5.

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 implied by the name and the 'all but last four digits' rule, and the brief pricing sentence hints at the paid-tool context. There is no explicit when-to-use/when-not guidance or pointer to the generic x402-mask or x402-redact alternatives that an agent could otherwise confuse with this one.

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