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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false and openWorldHint=false, so the safety profile is fully covered by structured data. The description adds two genuinely useful behavioral facts beyond that: the balance is denominated in EUR and the payload also reports consumed credit plus a top-up pointer. It stops short of richer context such as whether the api_key may be omitted or rate limits, so a 3 is appropriate.

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?

A single front-loaded sentence with no filler; the core action comes first and the secondary details (currency, usage, top-up) follow in the same sentence. Nothing is redundant or padded.

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?

With an output schema present, the description need not describe return values, and the rich annotations cover the safety profile, so the remaining burden is small. The one real gap is the optional/default behavior of api_key, which is left unaddressed in both description and schema.

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 0% for the single api_key parameter, so the schema alone gives no semantics. The description partially compensates by tying the call to 'the account of your API key', implying the key identifies whose credit is returned, but it never explains that api_key is optional (default "") or what happens when it is omitted.

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?

Specific verb ('Show') and resource ('the credit left on the account of your API key'), with added detail on currency (EUR) and the two companion outputs (what was used, where to top up). It is distinguishable in substance from get_account and get_price, but it never names or contrasts a sibling explicitly, which keeps it just short of a 5.

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?

There is no when-to-use statement, no prerequisite, and no alternative named. The agent must infer that this is the tool to call when it wants a balance check, and nothing tells it when to prefer get_account or get_price instead.

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