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

Annotations already declare readOnly, idempotent, non-destructive, closed-world, so the safety profile is covered. The description adds real value beyond that: the scoping semantics under API-key vs OAuth auth (key spend vs whole app-connection spend, not just the current hourly token) and the fact that calls are free. It stops short of rate limits or result shape, but the output schema exists.

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 short paragraphs, front-loaded with what is returned, then when to use it, then the cost/safety note. Every sentence carries distinct information (return shape, auth scoping, use cases, sibling routing, free to call) with no repetition.

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, return values need no description, and the remaining gaps an agent would care about (auth-dependent spending scope, cost of the call, budget/account alternatives) are all covered. Nothing needed to call it correctly 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?

Schema coverage is 100% and the single period enum is fully documented in the schema, including the '24h' default and UTC note. The description adds no syntax or format detail beyond the schema, so the baseline 3 applies.

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 (what this connection has spent over a period) and enumerates the returned components: requests, tokens, cache savings, total in rubles. It also names the sibling it is not by pointing to balance_get for remaining balance, so the agent can distinguish it without opening either schema.

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 use cases (report what a run cost, check against a budget, price tools that do not return their own cost such as image_generate and speech_create) and routes the balance question to balance_get. Nothing about when to select this tool is left to inference.

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