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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so safety is covered and the description only needs to add return context. It does explain what each balance pays for, which helps the agent decide relevance, but with no output schema it does not say whether values are remaining vs. allowance, in what units, or whether plan info includes tier/limits.

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 compact sentence, front-loaded with the primary output (plan and both balances) and immediately qualifying each balance by its purpose. No filler, no repetition of the title.

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 simple zero-parameter read tool this is close to sufficient, and the annotations cover the safety profile. However, since no output schema exists, the description should carry the return shape and it stops short of describing units, structure, or whether an empty/zero balance is signalled.

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; the baseline for a no-param tool applies. The description adds no misleading input expectations either.

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?

The description names the resource and its contents precisely: the account's plan plus the two distinct credit pools (build credits for creating/editing apps, agent credits for agent runs). That clearly separates it from any sibling tool, none of which deal with billing balances. It lacks an explicit verb ('Returns...') and leans on the title to convey the read action, 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 guidance, no stated preconditions, and no mention of alternatives. An agent must infer that this is the tool to call to check remaining quota before, say, invoking an agent run. The semantic mapping of credit types to actions hints at usage but never states it.

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