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

Annotations already cover read-only/idempotent/no-side-effect semantics, yet the description adds real context: pricing is net per §19 UStG (no VAT), the pricing model is Pay per Fax, and the call performs a permission check on the number. It stops short of describing rate limits or what the validation result looks like.

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?

A single tight sentence that front-loads the primary outcome (price) and appends the constraint (sends nothing). Dense but every clause carries information; the tax parenthetical is verbose but legally relevant.

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 no-side-effect price lookup with no output schema, the description covers both outputs qualitatively (a net price and a validity check on the number), including the tax treatment. It does not specify the exact return shape or units, which is the only remaining gap.

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 both parameters are already documented, and the description's reference to checking the fax number adds only marginal semantic value. The optional pages parameter, which clearly drives the price, is not discussed in the description at all.

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+resource: it returns the price of a fax and validates whether the fax number is permitted. The clause "Sendet nichts" implicitly separates it from send_fax, but no sibling is named explicitly, so differentiation is left partly to inference.

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 mention that it checks whether the fax number is allowed implies this is a pre-send lookup, but there is no explicit "use this before send_fax" guidance or any contrast with get_fax_status. Usage is inferable rather than stated.

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