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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), but the description adds real behavioral context beyond them: the error shape on failure and the readback asymmetry (region-qualified current language vs. settable base codes). It stops short of describing pagination or the exact response shape.

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 sentences, front-loaded with the primary action, then the precondition, then the non-obvious asymmetry. No filler; every clause carries decision-relevant information.

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 exists, so the description must characterize the return values, and it does (accepted languages/locales plus the current setting, with the region-qualified caveat). Nothing needed 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?

Single parameter (udid) with 100% schema description coverage, so the schema already documents the input. The description's detail concerns output semantics rather than the parameter, so it adds nothing for udid; baseline 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 and resource ('List the languages and locales this iOS device accepts, plus what it is set to now'), which an agent can immediately distinguish from the mutation sibling ios_set_device_language.

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: 'Read this before ios_set_device_language rather than guessing a code', and explains the cost of skipping it (a bare 'SetProhibited' with no reason). Both the when and the why are given.

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