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

Annotations already declare readOnlyHint=false and openWorldHint=true, so the description's real contribution is billing behavior ('trækkes fra den forudbetalte saldo') and irreversibility ('kan ikke annulleres efter afsendelse') – valuable context an agent cannot get from the schema. It stops short of describing dispatch timing or what happens on insufficient balance. The irreversibility note sits in mild tension with destructiveHint=false but does not directly contradict it, since that hint concerns data destruction rather than an external physical side effect.

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?

Two short sentences with zero filler; the physical-letter scope is front-loaded and the cost/irreversibility facts follow. It is efficient, though the extreme brevity leaves no room for the routing information an 11-parameter tool needs.

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 an 11-parameter, nested-object, enum-bearing write tool with no output schema, the description covers only cost and irreversibility. It omits the three-way choice among text/template/fileBase64, what a successful send returns, and how it relates to its closest sibling, so an agent could easily mis-invoke it.

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 64% and the description adds no parameter meaning whatsoever – it never explains the letterType enum ('almindelig' vs 'rekommanderet'), the mutual exclusivity of text/template/fileBase64, or the testOutcome test-key requirement. With moderate coverage the schema baseline of 3 applies, but the description does nothing to close the remaining gap.

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 names a specific verb and resource ('Sender et fysisk brev i Danmark'), which is far better than restating the title. However, it does not distinguish this tool from the sibling create_letter_order, and for an agent the difference between 'send' and 'create order' is exactly the decision that matters.

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?

No when-to-use guidance is given: nothing says when to pick send_letter over create_letter_order, nor whether a balance check (get_balance) or pricing lookup (get_pricing) should precede the call. The only implicit context is the prepaid-balance mention, which hints at a prerequisite but never states it as one.

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