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

Annotations already cover the safety profile (readOnlyHint=true, destructiveHint=false, openWorldHint=true). The description adds the auth requirement ('needs a key') and the UTC frame for 'today', which annotations do not convey. However, it discloses nothing about response shape, quota/rate behavior, or caching side effects.

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, front-loaded with the returned fields and ending with the only prerequisite. No filler, though the telegraphic 'Needs a key' could be slightly more explicit without adding bloat.

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 input parameters and no output schema, the description enumerates the returned quantities, which is largely sufficient for a read-only status tool. It stops short of specifying response format or units for budget/spend, leaving modest gaps.

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 the baseline is 4. No parameter meaning needs to be conveyed beyond the implicit key credential already noted in the description.

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 concretely what is returned: the key's balance, daily budget, today's UTC spend, cache hits and misses. This is specific and clearly distinct from the sibling search/keyword tools. The verb is only implied ('returns') rather than stated, but the resource and scope are unambiguous.

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?

'Needs a key' gives a prerequisite but no guidance on when this tool is the right choice versus siblings, nor any exclusions. Usage is implied by the self-descriptive purpose rather than explained, which is the minimum viable level.

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