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

With no annotations, the description carries the behavioral burden, and it does disclose the key behavior: deduplication across both arrays. The term 'Calculate' implies a pure, non-mutating operation, and no side effects are hidden; this is adequate for a simple mathematical function.

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 sentences, front-loaded with the purpose and no real fluff. The leading 'Union:' label is mildly redundant with the tool name, but it does not harm readability.

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 deterministic calculation, the description covers the operation, the deduplication behavior, and the two expected inputs. However, with no output schema and an input schema that lists no parameters, it leaves the exact invocation format and return representation to inference.

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?

The schema exposes no parameters, so the description is responsible for conveying the intended inputs. It explicitly names 'a and b arrays', which is meaningful semantic information above the empty schema, even though it does not specify element types or input encoding.

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?

The description gives a specific verb, 'Calculate', a resource, 'the union of two arrays', and clarifies that union means 'unique elements from both'. This mathematically separates it from sibling set operations like intersect or symmetric-difference, so an agent can tell what it does.

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 this tool over related siblings such as x402-intersect or x402-symmetric-difference, and no exclusions or prerequisites. The only usage clue is the instruction to provide a and b arrays, which is not a when-to-use statement.

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