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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false, so the safety profile is covered and the description's 'Read-only' line is largely redundant. It does add real behavioral context beyond the annotations: what each result contains (data allowance, validity, tax-included JPY price, order page) and the linkage rule that trips over 90 days return monthly plans. No auth, rate-limit, or pagination detail, hence not a 5.

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?

Front-loaded with the triggering condition and packed into essentially two sentences with no filler; every clause maps to a real selection axis or exclusion. It is dense and clause-heavy rather than crisp, which keeps it just short of a 5.

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?

An output schema exists, so return values need not be enumerated, and the description still flags the key fields and currency. With all parameters enumerated, an explicit exclusion list, and an off-ramp for eSIM compatibility, an agent has everything needed to call this correctly against its siblings.

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% and all three enums are documented in-schema, so the baseline is 3. The description restates the same semantics in user-intent language ('which travel eSIM covers a trip of a given length', 'unlimited versus 3GB per day', 'monthly eSIM for living longer than 90 days'), but adds no mapping detail beyond what the schema already says — including the 90-day travel/monthly default, which the schema states verbatim.

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 and resource (finds travel/monthly eSIM plans for Japan) and enumerates the exact decision axes it resolves: unlimited vs 3GB/day, trip length, monthly vs travel, cost. It also carves itself away from adjacent siblings — physical SIMs, pocket Wi-Fi, roaming in other countries, phone repair — so an agent can separate it from search_travel_products without opening a schema.

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?

Explicit when-to-use ('user wants mobile data in Japan on an eSIM') plus explicit when-not-to-use ('Do not use it for physical SIM cards, pocket Wi-Fi rental, roaming in other countries, or phone repair'), and it routes the out-of-scope eSIM-compatibility question to the compatibility page in the result. That is the full when/when-not/alternative pattern.

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