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

Annotations already cover safety (readOnlyHint) and scope (openWorldHint), and the description adds genuinely new behavioral context: this is a paid call at $0.02, settled in USDC on Base via x402, and the data is 'live' rather than cached. It does not mention rate limits, latency or failure modes, which keeps it short of a 5.

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?

A single dense sentence with zero filler: the capability, the site scope, the returned metrics and the pricing model all appear in order of importance. Nothing is repeated from the schema or annotations.

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?

With no output schema, the description correctly compensates by enumerating return fields, and it discloses the payment requirement that an agent must handle before calling. The remaining gap is default/fallback behavior for the optional marketplace parameter.

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 only 33% (only cost is documented), so the description has to carry weight. It partially does: 'for one keyword' clarifies keyword as a single search term, 'on 8 eBay sites' frames the marketplace enum, and 'optional profit from your cost' explains the cost parameter. It never states the default marketplace when the parameter is omitted, leaving a real gap.

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?

Names the resource (live eBay market data for one keyword across 8 eBay sites) and enumerates the concrete outputs: price range with postage, seller concentration, China share, cheapest listings and optional profit. That is far more specific than a restatement of the name, though the verb is implicit (fetch/analyze) rather than stated.

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 mention of 'optional profit from your cost' implicitly tells the agent when to supply cost, and 'on 8 eBay sites' implies the marketplace selector. However there is no explicit when-to-use or when-not-to-use guidance, and no statement about what happens if marketplace is omitted.

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