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

With no annotations, the description carries the burden well: it discloses the return values, the irreversible trait that 'the account_key is shown once,' and the top-up dependency chain. It stops short of stating whether accounts expire or what happens on repeat calls, so not a full 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?

Three short sentences, front-loaded with what it does and what it returns, then the conditional usage, then the one-time-key warning. No filler and every clause carries actionable information.

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?

No output schema exists, so the description usefully supplies the return values itself, plus the critical one-time-visibility warning. Minor gaps remain around cost/expiry of the prepaid account, which an agent might reasonably want before creating one.

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?

The tool takes zero parameters, so the baseline is 4 and there is nothing for the description to disambiguate. The description instead spends its words on post-call semantics, which is the right allocation.

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 ('Create a prepaid account') and adds the concrete outcome ('get an account_key plus its account_id'), which immediately separates it from create_inbox and create_deposit. An agent can identify the tool without opening the 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?

Explicitly scopes necessity ('Only needed for longer-lived inboxes or heavy use'), which implies the default path is create_inbox without an account. It also names the follow-up sequence with siblings: create_deposit to top up, then pass account_key to create_inbox.

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