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

No annotations exist, so the description carries the full burden, and it discloses little: no side-effect profile (presumably a pure function, but unstated), no error behavior for invalid ISO codes, and no indication of the output format. The '5 free trial calls per signed wallet' line is genuine behavioral context (a rate/pricing constraint) and is the only thing lifting this above a 1.

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 core sentence is front-loaded and wastes nothing, and the trial note is short. The marketing emoji/credits clause is slightly off-task but not enough to derail the definition.

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 4-parameter tool with zero schema descriptions, no annotations, and no output schema, the agent needs to know which parameters are required (none are marked required), how value and amount differ, and what comes back. None of this is covered, leaving the call under-specified.

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% across 4 parameters, so the description must compensate, and it only partially does: 'amount', 'ISO code' (currency) and 'locale' are implied, but the 'value' parameter is never mentioned. Worse, value and amount look redundant, and nothing explains which one the tool actually reads.

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 ('Formats an amount as currency') and even names the mechanism (Intl.NumberFormat), so the operation is unambiguous. However, it does not distinguish itself from the near-identical sibling x402-format-currency, nor from x402-accounting-format, which is exactly the gap that keeps it below 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?

There is no when-to-use guidance, no prerequisites, and no named alternative despite a sibling (x402-format-currency) that appears to do the same thing. The agent is left to guess which of the two formatting tools to call.

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