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

Annotations already declare readOnly, idempotent, non-destructive, closed-world, so the safety profile is covered. The description adds real behavior beyond that: the authoritative source (manufacturers' official lists), the brand coverage, the regional exception handling for mainland China/Hong Kong/Macao, and the shape of the verdict (supports_esim / no_esim).

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 tight sentences, front-loaded with the purpose and the concrete example, followed by source scope and the parameter hint. The China/HK/Macao exception list is slightly granular but directly relevant to a common failure mode, so it 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?

Only three parameters, all documented in the schema, an output schema exists so return values need not be described, and annotations carry the safety profile. The description covers the remaining unknowns: source of truth, coverage limits, regional caveats, and when to supply the optional region.

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%, so the baseline would be 3. The description adds a genuine usage rule for purchase_region — supply it when the user mentions where the phone was bought — which the schema only defines syntactically as an ISO code, not as a when-to-supply signal.

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+resource (check whether a phone supports eSIM) and scopes the data sources to manufacturers' official lists for iPhone, Pixel and Galaxy. The example query ('does my iPhone 13 support eSIM?') makes it immediately distinguishable from siblings like search_plans or compare_networks.

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 a clear triggering context via the example user question and states the conditional rule 'pass purchase_region when the user says where the phone was bought.' It does not name an alternative tool or state when not to use it, but none of the listed siblings overlap, so the routing risk is low.

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