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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so safety is covered. The description adds genuine context beyond that: that Unlimited is a total allowance rather than a guarantee of full speed, and that the phone must be unlocked and eSIM-capable. Those are useful caveats, though the framing is repetitive and partly self-contradictory in tone.

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 core action is front-loaded correctly, but the body repeats 'Unlimited' three times in different phrasings ('storefront data label: Unlimited', 'total dailyAllowance is Unlimited', 'Unlimited is the total data allowance'), which is confusing rather than clarifying. Several sentences do not earn their 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?

There is no output schema, so the description should indicate what comes back; it only gestures at fields (dailyAllowance, fairUsagePolicyUrl) without structure. With four of five parameters undocumented in the schema, the description leaves an agent guessing on the required planId format and the currency/locale effect.

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 20% (only 'days' is documented), so the description must compensate. It does explain includeTechnicalDetails semantics well, one of five params, but leaves planId format, currency and locale unexplained, so it only partly closes the gap.

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 opening sentence gives a clear verb and resource: read a selected plan and its coverage. It is distinguishable from siblings like list_esim_plans and compare_esim_prices. However, the purpose is muddied by the repeated and confusing 'Unlimited' statements that read more like marketing copy than a scope statement.

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 does give conditional guidance for includeTechnicalDetails (false for shopping, true for activation or network questions) and routes fair-usage questions to fairUsagePolicyUrl. But it never names a sibling tool or explains when to call this instead of list_esim_plans or compare_esim_prices.

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