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

The annotations already mark the tool as read-only, idempotent, and non-destructive. The description adds useful behavioral context beyond that: the HTTP method, a fixed provider value, and a pacing constraint ('Space catalog calls ~1s apart (agent catalog_gap)'). This adds value without contradicting the annotations.

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?

The description is compact and front-loaded, with five short fragments that each carry useful information. No filler is present, and the endpoint, domain, provider, optional filter, follow-up action, and pacing guidance all earn their 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 simple listing tool with read-only annotations, no required parameters, and no output schema, the description covers the endpoint, provider, optional country filter, downstream purchase action, and rate limiting. An explicit return-shape note is absent, but the title and 'plan id' reference make the response concept predictable enough.

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 description coverage is 0%, so the prose must carry the semantic weight. The description does this by explaining 'esim_provider=yesim' and 'Optional country ISO2', giving both parameters meaning and format guidance that the bare schema lacks.

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 names the exact endpoint and resource ('POST /api/esim/plans'), and the title states 'List eSIM plans' clearly. It also distinguishes this listing step from the purchase step via 'Use plan id with purchase_esim', so an agent can separate catalog discovery from purchasing.

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 (travel/data eSIM), specifies the provider value, marks country as optional, and directs the plan id to purchase_esim. It does not explicitly state when-not-to-use this tool or list all alternatives, but the purchase_esim reference gives enough routing for the common workflow.

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