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

Annotations already declare readOnlyHint=true, so the read-only nature is covered. The description adds useful behavioral context beyond annotations: results include a packageCode in brackets, which is the key output needed for purchase_phone_plan, and it clarifies coverage details (US, local, global).

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 front-loaded with the core purpose and includes only relevant supporting details about coverage and packageCode usage. It is slightly longer than strictly necessary, but each sentence contributes useful selection or output-routing information.

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 read-only search tool with optional parameters and no output schema, the description provides enough context to choose it correctly and understand what results will contain. It could be more complete by naming the data-only alternative explicitly, but the current information is sufficient for correct invocation.

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?

The schema already documents all three parameters with 100% coverage, so the baseline is 3. The description adds some context about what results contain (packageCode) and ties country to real phone number regions, but it does not materially deepen the meaning of query, region, or country beyond the schema.

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 a specific verb ('Search'), a specific resource ('PikaSim PHONE-NUMBER eSIMs'), and clarifies the core distinction: real carrier phone numbers with voice, SMS, and data, not VoIP. It also differentiates from data-only eSIM products and explains regional coverage variants.

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 explicitly says 'Use this when a user wants to call or text, not just data,' giving a clear condition for selecting this tool. It does not name a specific sibling alternative such as search_esim_packages, but the 'not just data' exclusion makes the intended context reasonably clear.

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