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

With no annotations, the description carries the full burden, and it delivers: it discloses persistence, system-wide effect, SpringBoard relaunch, a 5–10s flicker, delayed read-back, and iOS's refusal to accept region-qualified language codes with automatic splitting. This is rich behavioral context well beyond the bare operation.

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, each earning its place: purpose/scope, side effects, parameter rule, and alternative routing. The most important information is front-loaded, and there is no filler or repetition of schema details.

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 state-changing tool with no annotations and no output schema, the description covers purpose, side effects, timing, parameter constraints, and alternatives. It is complete enough for an agent to select and invoke the tool correctly without further investigation.

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%, but the description adds critical nuance: use a BASE language code ('fr', not 'fr-CA'), and if a regional code is passed, it is split into base language plus locale automatically. This clarifies the interplay between language and locale in a way the schema alone does not.

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?

The description opens with a specific verb and resource: 'Change the iOS device's system language and / or locale'. It immediately adds scope ('persistent, affects every app') and explicitly distinguishes itself from the per-app alternative, ios_launch_app_in_language, making its purpose unmistakable.

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?

The description gives an explicit when-not: 'For per-app testing without changing the whole device, prefer ios_launch_app_in_language.' It also makes clear this tool is for whole-device, persistent changes, which tells an agent exactly when to choose it over the sibling per-app tool.

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