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

With no annotations, the description carries the full burden and falls short: it does not say how duplicates are collapsed or ordered, whether the result is returned as an array or a string, or what the response looks like. It also advertises the pricing model (5 free trial calls per signed wallet), which is useful economic context but not behavioral detail about the operation itself.

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 short sentences, front-loaded with the operation and its definition, followed by the input hint. The trailing pricing/emoji line is marketing but conveys the cost model, so it earns marginal keep; overall there is very little wasted text.

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 two-parameter tool with no annotations and no output schema, the description still leaves the essential contract undefined: the string-vs-array encoding of inputs, whether parameters are required, and the shape of the result. An agent could not reliably construct a valid call from this definition alone.

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 0%, so the description must compensate, and what it adds is actively risky: it calls a and b "arrays" while the schema types both as strings, leaving the actual encoding (JSON-encoded array? comma-separated list?) unspecified. It names both parameters but not how to serialize them, and neither is marked required despite both being necessary for a union.

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 ("Calculate the union of two arrays") and parenthetically defines the semantics ("unique elements from both"), so the operation is unambiguous. It does not distinguish itself from close siblings like x402-intersect, x402-difference, or x402-symmetric-difference, but the name plus parenthetical is specific enough to identify the operation.

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 guidance on when to choose union over the many set-operation siblings (intersect, difference, symmetric-difference, unique-array). The only usage instruction is "Provide a and b arrays," which is about inputs, not about selection between alternatives.

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