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

Annotations already declare readOnly=false, idempotent=true, destructive=false and openWorld=false, so the safety profile is covered. The description adds real value on top: it discloses the return behavior ('Returns all preferences') and imposes a consent prerequisite (only after the user asks), which the annotations do not express.

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 short sentences, front-loaded with the usage trigger, and the closing pointer to get_account is well placed. There is mild redundancy: the first sentence's 'pages and e-mails in English or German' is restated almost verbatim by the second sentence's field enumeration, so one sentence could be trimmed.

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 two-parameter, zero-required mutation tool with an output schema and full annotation coverage, the description is nearly complete: it states the mutation target, the consent rule, and the read alternative. The only material gap is whether a partial update (one field supplied) is permitted and what happens to the unspecified preference, which matters given both parameters are optional.

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 both properties carry enum descriptions ('en: English. de: German.' / 'true: the user gets an e-mail when a letter goes to the post...'), so the description's restatement of 'en or de' and 'on or off' adds nothing beyond structured data. The one thing it could clarify and does not is whether omitting a field leaves that preference untouched. 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?

Names a specific verb and resource ('Changes the preferences of the account') and enumerates exactly which two preferences it touches: page/e-mail language (en/de) and letter-status e-mails (on/off). It also distinguishes itself from the read path by noting 'get_account also shows them', so an agent can route between writing and reading preferences without opening either schema.

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 opening sentence gives concrete user-intent triggers ('user wants pages and e-mails in English or German, or wants e-mails about letters on or off') and adds a meaningful constraint: 'Change a preference only after the user asks for it.' The alternative for viewing preferences (get_account) is named, though the description never states a 'do not use when' condition or that omitted parameters remain unchanged.

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