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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety is covered; the description adds value by disclosing the shape of each returned record and the fact that the set is a fixed, supported list rather than arbitrary data. Since no output schema exists, that return-field disclosure is genuinely load-bearing. Minor gap: no indication of ordering, size of the list, or how the marketplace value should be keyed downstream.

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, zero waste: the resource and its returned fields come first, then the actionable directive. Every clause 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?

With no output schema, the description compensates by naming the four fields returned, and the no-param schema leaves nothing else to explain. It is nearly complete; a short note on how many storefronts exist or what 'tagConfigured' implies for the caller's affiliate tagging would close the last gap.

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 tool takes zero parameters, so semantics are baseline-neutral and the description correctly spends no words on arguments. It does hint at the value's eventual use ('pass it on search/catalog/resolve'), which is the only parameter-adjacent concept that matters here.

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?

Specific verb 'List' plus a precise resource 'supported Amazon storefronts', with the returned fields (domain, label, currencyHint, tagConfigured) named inline. No sibling tool enumerates marketplaces, so it is trivially distinguishable from add_item, search_gifts, get_catalog, and the rest.

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?

'Agents MUST pick the user's marketplace and pass it on search/catalog/resolve' tells the agent both the trigger (need a marketplace before other calls) and the downstream consumption, which is strong routing guidance. It stops short of naming the actual sibling tools (search_gifts, get_catalog, resolve_product) and gives no explicit when-not-to-call condition, keeping it below a 5.

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