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

Annotations only say it is a non-destructive, non-open-world write; the description adds real context beyond that: it requires the settings:write scope, is unavailable to site-restricted keys, and demands expected_version read from get_account (optimistic concurrency). It stops short of saying what happens on a version mismatch or whether the change is reversible.

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?

The functional sentence is front-loaded and dense with useful detail, and the bracketed [When to use] header aids scanning. The second paragraph is generic safety boilerplate that could be shorter, but it is clearly separated rather than buried in the core sentence.

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 3-parameter mutation with no output schema, the description covers scope/auth, concurrency versioning, the at-least-one-field rule, and safe-use policy. The remaining gap is failure behavior on a stale expected_version, which an agent would need to handle errors correctly.

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 the baseline is 3, but the description adds a cross-parameter constraint the schema cannot express ('pass at least one of locale / timezone') and explains the provenance of expected_version ('the settings_version from get_account'). That is genuine meaning beyond the field descriptions.

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 ('Set the account's language and time zone') and names the exact fields affected, plus the key scope required. It does not explicitly contrast with the near-siblings update_settings or set_site_locale, so the agent must infer that 'account's' means account-level rather than site-level.

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 [When to use] block gives an explicit precondition (only when the user explicitly asks) and a prompt-injection exclusion, and the body states the key-scope availability constraint and the 'pass at least one of locale / timezone' rule. No named alternatives are offered, so routing versus update_settings/set_site_locale is left implicit.

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