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

Annotations declare readOnlyHint=true and openWorldHint=true, which are consistent with the search behavior. The description adds meaningful behavioral details beyond annotations: real vs VoIP, AT&T/T-Mobile coverage for US, country list, and the packageCode return format. It does not mention pagination or rate limits, but for a search tool this is sufficient.

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 three sentences: a clear definition, coverage/network details, and usage guidance. Every sentence adds distinct value without redundancy. Information is front-loaded with the core purpose, making it easy for an agent to quickly grasp the tool's role.

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 no output schema, the description compensates by mentioning that results show packageCode in brackets for purchase_phone_plan. It also gives coverage scope and use-case context. It could be more complete by listing additional return fields, but for a search tool with openWorldHint, this is adequate for invocation and interpretation.

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 100%, with each parameter (query, region, country) having descriptive examples. The description itself doesn't add parameter-specific syntax or formats, but it does supply context like 'US for a real +1 number' indirectly through examples. Per the baseline rule, this is a 3.

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 clearly states the tool searches for PikaSim phone-number eSIMs with real carrier numbers, differentiating them from data-only or VoIP plans. It explicitly distinguishes from generic search tools by specifying 'REAL carrier phone number (not VoIP)' and gives concrete regional examples.

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 provides explicit when-to-use guidance: 'Use this when a user wants to call or text, not just data.' It also gives a when-not context ('not just data') and tips for the subsequent purchase step (packageCode). However, it does not explicitly name alternative tools for data-only plans, so it stops short of full alternative differentiation.

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