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

Annotations already declare a read-only, non-destructive, closed-world lookup, so the bar is lower. The description still adds meaningful behavior: the result is a three-way heuristic (likely / unlikely / check) and it explains how to confirm on the device itself when the answer is uncertain.

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 tight clauses with the capability stated first and the call-to-action second; nothing is padded. The parenthetical verdict list is compact, though the sentence is slightly dense.

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 single-parameter lookup with full schema coverage, annotations, and a declared output schema, the description supplies everything an agent needs: what it returns, how confident the answer is, and when to invoke it relative to buy_esim.

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 the schema documents it fully at 100% coverage with concrete examples ('iPhone 13', 'Galaxy S23'). The description's phrase 'from the model name' confirms scope but adds no format or edge-case detail (e.g. how to pass an unknown or ambiguous model).

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?

Names a specific verb and resource ('says whether a phone model is likely to take an eSIM'), states the three possible verdicts, and makes clear it works from the model name rather than a live device check. Clearly distinct from the plan/cart siblings.

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 sequencing and a condition: 'Call it before buy_esim when the customer is unsure.' The alternative and ordering are named, though there is no explicit when-not case (e.g. what to do when the customer already knows the model is supported).

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