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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so safety is covered; the description adds real value by detailing the payload contents (throttled speed, hotspot/tethering rule, top-up eligibility, whether checkout is open) which matter for downstream decisions. It omits failure behavior for an unknown or stale plan_id and whether pricing varies by locale, so it is not complete.

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?

Two sentences: the capability and the enumerated facts first, then the usage rule. The long field list is dense but every item carries decision-relevant information given the absence of an output schema; nothing is redundant with the schema or annotations.

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 enumerating the returned fields, which is exactly what an agent needs to know before calling. The remaining gap is edge-case behavior (invalid/expired id, locale effect on currency), which is minor for a read-only single-record lookup.

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%: plan_id already says 'exactly as returned by search_plans' and locale documents its enum and default. The description only restates the id lookup ('by its id') and adds no new meaning about locale or id formatting, so the baseline of 3 applies.

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 (get) and resource (one travel eSIM plan) plus the lookup key (id), then enumerates the returned facts: price in USD, data, validity, daily limit/throttled speed, hotspot rule, networks, covered/not-covered countries, SMS, 5G, top-up and checkout status. This is clearly distinct from siblings like search_plans (discovery) and check_coverage (coverage-only lookup).

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?

Explicitly gives the sequencing condition: 'Use it after search_plans when someone asks about a specific plan,' which tells the agent both the trigger and the prerequisite tool. It does not state when NOT to use it (e.g., for browsing options or bulk comparison), so it stops short of full when/when-not guidance.

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