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

Annotations cover the safety profile (readOnlyHint, openWorldHint), and the description adds the key behavioral fact that results are scoped to the caller — identity comes from auth, not arguments. It also names the three data classes returned, which helps the agent know a single call answers both quota and throttling questions.

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?

One sentence, front-loaded with the verb, and every element (credits, usage, rate limit) carries information. Nothing is redundant with the annotations or schema.

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 tool with an output schema and read-only annotations, the description supplies what's needed: what it returns and that scope is caller-derived. Only minor gaps remain, such as whether values are live or cached and how rate-limit data is represented.

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. The description's use of 'the caller's' correctly signals that scoping is implicit rather than argument-driven.

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 ('Return the caller's MP4.now API credits, usage, and rate limit'), which clearly separates it from the conversion-oriented siblings. It could not be confused with convert_image_url or list_converters, though it doesn't name them explicitly.

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?

Usage is implied rather than stated: an agent can infer this is the tool to call when it needs credit balance or rate-limit state before issuing conversions. There are no explicit when-to-use/when-not conditions or references to alternatives.

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