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

Annotations already declare readOnlyHint=true, destructiveHint=false, openWorldHint=false, so the safety profile is covered. The description adds genuinely useful context beyond them: 'A quote grants no entitlement and performs no execution,' and it names the pricing components returned. It doesn't mention quote validity/expiry or refresh behavior, which would matter for a pricing artifact.

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?

Front-loads the scope in the first clause and is dense with routing information, but the 'Do NOT use ... use cost.estimate' pattern appears twice with the same alternative, which is mildly redundant. Slightly longer than needed but every remaining sentence carries routing value.

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 an output schema covering return values, full annotations, and a single fully documented parameter, the description covers everything an agent needs: the prerequisite (active capabilityId), the safety profile, the boundaries, and the next step for execution.

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?

Only one parameter and schema description coverage is 100%, so the schema already documents capabilityId fully. The description's 'active marketplace capabilityId' phrasing adds no syntax or constraint detail beyond the schema's 'Exact active public marketplace capability identifier.' Baseline 3 is appropriate.

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 ('return authoritative gross price, CrossingKey fee, provider share, and x402 v1/v2 payment requirements' for a marketplace capabilityId). It explicitly distinguishes itself from cost.estimate and capability.purchase, so an agent can route without opening any schema.

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?

Gives explicit when-to-use ('after selecting an active marketplace capabilityId'), two explicit when-not cases (first-party paid capabilities; generic price lookup) each paired with the alternative to use instead, plus the follow-on tool for execution. Nothing is left to inference.

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