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

Annotations already declare readOnly, idempotent, non-destructive, closed-world behavior, so the bar is lower. The description adds genuinely new context: results are informational only and do not reserve inventory or prices, which prevents an agent from treating output as a booking or price lock.

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?

Three tight sentences: purpose and payload first, routing guidance second, behavioral caveat last. No redundant restatement of the title or schema.

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?

No output schema, yet the description tells the agent what fields come back (names, identifiers, starting EUR prices) and what the results do not imply (no reservation). Combined with full annotation coverage, nothing needed to call this correctly is missing.

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%, so baseline is 3, but the description adds real semantics for `type` — that country and region are the two options and omitting it returns both — which the schema's bare 'Filter by type.' does not convey. `lang` is left to the schema, which fully documents it.

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?

Specific verb plus resource ('List currently offered travel eSIM destinations') with scope detail: countries and multi-country regions, localized names, identifiers, starting EUR prices. It is clearly distinguishable from search_plans, which the description names as the downstream step.

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 states when to use it ('when the travel destination is unknown or to discover valid destinations before search_plans') and how to apply the type filter ('type=country or type=region, or omit it for both'). The alternative tool and the condition that selects it are both named.

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