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

Annotations already declare readOnly, idempotent, non-destructive, closed-world behavior, so the bar is lower. The description still adds value with the auth requirement ('requires a key') and the explicit boundary that it does not reconcile balances, which the annotations cannot express.

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?

Two short sentences with the core action front-loaded and no filler. The trailing 'does not render videos' clause is slightly tangential but does serve as scope disambiguation.

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 zero-parameter, read-only tool whose safety profile is fully covered by annotations and with no output schema to explain, the description is nearly complete. Only minor gaps remain, such as the balance's unit or format.

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?

There are zero parameters and 100% schema coverage, so parameter semantics are trivially complete; the baseline of 4 applies since no parameter meaning needs explaining.

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 ('Read your spendable Stivio credit balance') with the scope qualifier 'spendable', which distinguishes it from cost-estimation siblings like estimate_video_cost.

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 gives exclusions ('does not reconcile or render videos') and a prerequisite ('requires a key'), but never names the actual alternative for related needs, such as estimate_video_cost or list_video_models. Usage is implied rather than explicitly routed.

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