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

Annotations already declare read-only, idempotent, non-destructive and closed-world, so safety is covered. The description adds real behavioral value beyond that: it is a static built-in table lookup, not a live carrier query, and most travel eSIMs auto-configure, which sets agent expectations about empty/limited results.

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, front-loaded with the use condition, and the caveat sentence earns its place by preventing unnecessary calls. No filler.

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 no explanation, and a single fully-documented parameter keeps the surface small. It could add what to do when a carrier is absent from the table, but the definition is adequate for a simple static 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?

Only one parameter, and the schema has 100% description coverage including richer examples (T-Mobile, UK, EU roaming) than the description's bare mention of 'a carrier'. Baseline 3 applies since the schema already carries the parameter meaning.

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 (looks up) and resource (APN settings for a carrier) plus the underlying mechanism ('built-in APN table'), which distinguishes it from live/network tools like find_best_network or check_roaming_status. It does not explicitly name a sibling, so it stops short of 5.

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?

'Use when the user needs APN settings for a carrier after installing an eSIM' gives a clear trigger condition, and the note that most travel eSIMs configure automatically implicitly tells the agent when this may be unnecessary. No explicit alternative (e.g. troubleshoot_esim) is named, so it is not a 5.

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