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

No annotations are supplied, so the description carries the full burden. It does disclose the core behavior (deduplicated merge), but says nothing about output ordering, how duplicate inputs are collapsed, or what the return shape looks like — gaps that matter for a set-operation tool with no readOnly/destructive hints to lean on.

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 clauses, front-loaded with the operation and then the required inputs. No filler, though the final 'Provide a and b arrays' sentence is close to redundant with the schema.

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?

No output schema and no annotations, so the description is the only source of information, and it omits input format (the schema types say string while the description says arrays) and result ordering. Adequate for a trivial set operation but leaves the actual call format ambiguous.

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 100% but the schema descriptions are placeholders ('A to process'), so they add no meaning. The description at least tells the agent that a and b are arrays, which mildly contradicts the schema's string typing; the baseline 3 applies since the schema nominally documents both parameters.

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 the parenthetical 'unique elements from both' pins down the semantics, which meaningfully separates it from near-siblings like x402-intersect and x402-symmetric-difference. It never names those siblings explicitly, so differentiation is via definition rather than routing.

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?

The phrasing implies the use case (combine two arrays keeping unique elements) but there is no explicit when-to-use, when-not-to-use, or pointer to the intersect/difference alternatives that appear in the sibling list. Usage is inferable but not guided.

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