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

Only openWorldHint is annotated, so the description carries the disclosure burden and does well: it warns the API key is shown once (non-retrievable), that the HM-XXXXX code must appear on every envelope, and that an operator-less mailbox is capped at 250 €/month with letters only and no parcels. It omits whether registration itself costs money, whether the call is idempotent, and what happens on failure, which keeps it short of a 5.

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?

Front-loaded with the action and the return values, then the operator/address_name semantics, then the legal prerequisite. It is a dense single paragraph using 'operator_*' shorthand that is efficient but slightly harder to scan than a short bulleted breakdown; no sentence is wasted.

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 12-parameter, open-world mutation with no output schema, the description supplies the essentials an agent needs: what is created, what comes back, the operator trade-off and its limits, and the terms prerequisite. It is not fully complete — pricing/charging behavior and idempotency/error semantics are unstated.

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 only 50%, so the description must compensate, and it targets the highest-stakes group: operator_* is framed collectively as 'the responsible human or company (the contract partner)' with the explicit trade-off for omitting it, and address_name is constrained (not a company, brand, authority or famous person). The remaining parameters (purpose, found_via, agent_model, webhook_url, open_policy) are left to schema text, so coverage of the 12-param surface is partial.

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?

Opens with a specific verb and resource plus scope: 'Create your own mailbox at a physical street address in Germany.' No sibling tool registers a mailbox, so the agent can route here without opening any schema, and the description even states what the call produces (API key shown once, HM-XXXXX mailbox code).

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?

It gives one prerequisite ('Read https://humanmailbox.com/legal/terms first') and explains the two operating modes (with vs. without operator_*), which is genuine decision guidance. But it never positions itself against siblings like start_demo, get_prices or buy_bundle — an agent cannot tell whether to try a demo, check pricing, or register directly.

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