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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds genuinely useful context beyond that: that the call is free, and that the auto-top-up state exists so the agent can distinguish a hard stop from a balance that refills on the next billable call. It does not describe rate limits or response envelope shape, so it stops 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 what is returned, and each clause carries information. The only waste is redundancy: 'Free to call' and the trailing 'Cost: free' say the same thing twice in a short description.

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 no output schema, the description must carry the return-value burden, and it does: it names the balance plus the autoTopup subfields so an agent knows what it will receive. Combined with the zero-parameter schema and full annotation coverage, nothing needed to call or interpret this tool is missing.

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 there is nothing for the description to disambiguate and the baseline is a 4. The description correctly spends no effort on parameter syntax and instead uses its space to describe the return payload.

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 ('Returns the current API credit balance') and goes on to enumerate the exact payload it returns (autoTopup: configured, enabled, pack, monthly charge cap and count, last error). That detail makes it unmistakable against siblings like get_pricing or create_topup_link.

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?

Gives explicit when-to-use guidance ('check the balance before starting large batch jobs') and an error-handling instruction ('surface "insufficient_credits" errors to the user with a link to top up'). It stops short of naming the alternative tools (create_topup_link, get_pricing) for the top-up path, so it is clear context rather than a full routing rule.

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