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

The annotations already declare readOnlyHint=true, and the description adds useful behavioral context beyond that: no API key needed, the response includes variants/delivery options/pricing, and the Nano total should be carried into purchase to guard against exchange-rate drift. This gives the agent actionable expectations about auth, return scope, and downstream use.

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?

Three short sentences, all informative and none redundant. The core purpose is front-loaded, followed by auth context, return contents, and a specific downstream instruction—every sentence earns its place.

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 one-parameter read-only tool with no output schema, the description covers purpose, auth requirements, return content, and how to use the result in the purchase flow. Nothing critical is missing for an agent to call and use this tool correctly.

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 input schema has 100% coverage for the single parameter listingId, including the note that it comes from search_listings. The description adds no additional meaning to this parameter, so the baseline of 3 applies.

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 opens with a specific verb+resource: 'Fetch a single listing by ID.' It is clearly differentiated from sibling search_listings (which searches rather than fetches by ID) and from order/receipt tools by naming the listing resource and the returned detail scope.

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?

It provides clear context for when to call this tool: to retrieve a single listing's full details, with no API key required. It also connects to the purchase flow by instructing the agent to use the returned Nano total as quotedTotalNano, though it does not explicitly state exclusions or name alternative tools for the same task.

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