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

Annotations already declare readOnlyHint=true and idempotentHint=true, covering the safety profile. The description adds beyond that: it reveals the data source (manufacturer-sourced), the interpretation rule ('Unknown never means incompatible'), and the boundary of no plan recommendation/linking. This is meaningful behavioral context that annotations don't convey.

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 sentences, each earning its place: the trigger, the core mechanism, and the explicit exclusions. The most decision-relevant information is front-loaded, and there is no repetition of schema or annotation data. Exactly as concise as needed.

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 an output schema present, return values are already defined elsewhere. The description gives the agent everything needed to invoke correctly: the when, the what, the alternative to avoid, and the critical interpretation caveat about 'Unknown'. Nothing essential is missing given the tool's read-only, low-risk nature.

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 only 29%, so the description must compensate—and it does by naming the key check dimensions (regional variants, carrier locks, EID, Add eSIM settings) which map to region, carrierUnlocked, eidPresent, and addEsimSettingPresent. However, locale and manufacturer remain undocumented in both schema and description, leaving a small gap. The model parameter is also enriched by the schema's 'not a broad product family' note.

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 states a specific verb ('Check') plus a precise resource ('Skyalo's manufacturer-sourced compatibility catalog') and enumerates the factors considered (regional variants, carrier locks, EID, Add eSIM settings). The final sentence explicitly lists what the tool does not do, sharply distinguishing it from the sibling tool build_travel_connectivity_guide. A pure device question is unambiguously routed here.

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?

The description opens with an explicit when-to-use trigger ('whenever the user asks whether a phone supports eSIM or is ready for a travel eSIM') and directly instructs not to call the sibling for a pure device question. It also enumerates exclusions ('does not search, compare, recommend, price, sell, or link to connectivity plans'), giving the agent clear boundaries.

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