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

Annotations already cover read-only/idempotent/non-destructive, and the description goes well beyond them: it discloses the tri-state return (supported=true/false/null), warns that null means unknown rather than incompatible, mentions 'close model matches and caveats', and flags regional-variant and EID/unlock caveats. That is exactly the probabilistic-result context an agent needs before relaying an answer to a user.

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?

Verb and resource are front-loaded in the first clause, followed by return semantics and then the safety advisory in priority order. No filler sentences; each clause adds information the agent will act on.

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 one fully documented parameter and no output schema, the description compensates well by explaining the return values and their interpretation. Minor gaps remain, such as data-source freshness and what the 'optional supplier check' entails, but nothing blocks a correct call.

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 100% and the property description already specifies the full commercial model name and region variant. The description's examples add a little illustrative value but largely restate the same format guidance, so the baseline 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?

States a specific verb and resource: 'Look up a phone model's probable eSIM compatibility by model name,' with concrete examples ('iPhone 12 mini', 'Galaxy S21'). It is clearly distinguishable from every sibling (card, wallet, plan and payment tools) without opening the 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 clear usage context and boundaries: use it for per-model compatibility lookup, and always advise users to confirm EID and carrier unlock status. It also rules out misuse ('this is not a purchase approval'). It stops short of naming an alternative or an explicit when-not-to-call condition, though no sibling overlaps this function.

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