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

Annotations already declare the write/mutation profile (readOnlyHint=false, destructiveHint=false, idempotentHint=false), so the description's added value is the behavioral detail: the new balance starts at the amount, and auth/sandbox constraints that produce a 403. It does not disclose the failure modes for invalid amounts, duplicate grants, or whether the grant can be reversed with delete_credit.

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 sentences, front-loaded with the action, the required parameters and the resulting state, then the two authorization gates. Nothing is redundant with the schema and no sentence could be dropped without losing 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?

For a mutating tool with five parameters, nested metadata and no output schema, the description covers action, required inputs, resulting state and authorization prerequisites. It is only short on what the call returns (the created credit record and its id) and on any rate-limit or duplicate-grant behavior, which matter because there is no output schema to fall back on.

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 the baseline is 3, but the description adds real meaning beyond the schema: it establishes that amount is the starting balance value, confirms currency defaulting to USD, and marks expires_at and metadata as optional. The meaning of 'amount' as a unit (major units vs. minor units) remains ambiguous, matching the schema's bare 'The amount.'

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 opens with a specific verb and resource ('Grant a customer a prepaid credit bundle'), which is far more precise than the tool name alone and clearly differentiates it as a per-customer credit issuance rather than a wallet top-up. It does not, however, name the adjacent operations (top_up_balance, create_card_topup, create_invoice) it should be distinguished from.

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 states concrete preconditions for use: a claimed Last Price credential with billing write scope, and a claimed workspace, with the explicit note that a self-registration sandbox key gets a 403. That is strong 'when this will work' guidance, but there is no explicit 'when not to use this / use X instead' routing among the many sibling credit and billing tools.

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