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

Annotations already cover read-only, idempotent, non-destructive and closed-world, yet the description adds real behavioral context: only published regions with active displayed regional plans are returned, and no account or API key is required. The country-membership caveat (membership describes the region, not per-plan coverage) is a genuine semantic warning beyond structured data. Minor redundancy in restating 'Read-only' keeps it from 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?

Four sentences, all front-loaded with the core purpose first, then filtering behavior, then cross-tool routing, then the caveat. Dense but each sentence carries distinct information; only 'Read-only; no account or key is needed' partly duplicates the 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?

For a simple filtered-list tool with full schema coverage, an output schema, and rich annotations, the description covers purpose, filtering behavior, auth requirements, and how to consume the returned slugs downstream. Nothing an agent needs to call it 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 description coverage is 100% and all three parameters (limit, search, currency) are fully documented in the schema, so the baseline of 3 applies. The description adds no parameter-level syntax or format detail beyond what the schema already provides.

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 and resource ('Discover available eSIM regions') and enumerates the returned fields (canonical slugs, display names, member countries, starting prices). It clearly distinguishes itself from list-destinations and list-plans by scoping to regions and even names the sibling it feeds into.

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 routes the agent: pass the returned region slug to list-plans as 'region', not 'destination', and use get-plan-coverage when per-plan country coverage matters. Both the when-to-use and the when-to-use-something-else conditions are stated.

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