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

Goes well beyond the annotations: it discloses that the saved card is charged, that the owner is emailed, that it costs money ('[SPENDS MONEY]'), the required permission scope (billing:charge), and hard prerequisites (account + MCP key). Annotations cover only readOnly/idempotent/openWorld, so this added operational context is genuinely valuable.

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 core action and the two-step flow, then bracket-tagged facts (spends money, permission, prerequisites). Efficient overall, though the trailing '[Requires a free GPU Cloud account and MCP key]' reads as standing boilerplate rather than task-specific guidance.

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 money-spending, non-idempotent mutation with no output schema, the description covers the full picture: two-step confirmation, what each step returns (amount + token), the side effects (card charged, owner emailed), caps, and auth prerequisites. Nothing an agent needs to invoke this safely is absent.

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 description coverage is 50%: confirmationToken is documented in the schema (validity window, single use), while amountCad has only constraint bounds (25–10000) and no prose. The description explains the token workflow and the key caps, but adds no currency or amount-capping semantics beyond what the schema and caps note imply, so it lands at the baseline.

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 ('Buys credits') with the funding source ('with the saved card') and a scope qualifier ('within the key caps'). It is clearly distinguishable from siblings like create_order, add_payment_method, or set_auto_reload, and the title reinforces the money-spending nature.

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 two-step protocol is spelled out — call without confirmationToken to get the amount and token, call with it to charge — which tells the agent exactly how to sequence invocations. It does not, however, contrast itself against adjacent siblings such as set_auto_reload/get_budget or add_payment_method, so the alternative-routing guidance is implied rather than explicit.

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