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

With no annotations provided, the description carries full responsibility for behavioral disclosure. It states the tool returns a constant value in meters and requires zero inputs, making the behavior transparent for a pure constant-lookup operation. It could add detail on return format or precision, but the core behavior is clear.

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 sentence delivers the purpose, the value, the unit context, and the input requirement with zero waste. It is front-loaded with the action and result.

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 constant-value tool, the description is complete. It specifies the return quantity, the exact value, the unit, and the lack of inputs. No output schema exists, but the description adequately fills that gap for this trivial call.

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?

With zero parameters, the baseline is 4. The description reinforces the empty schema by explicitly stating 'No input needed,' so there is no ambiguity about whether the agent must supply any arguments.

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 a specific verb ('Return'), a precise resource (the length of a parsec in meters), and the exact value (3.0857 x 10^16). It also identifies the domain ('astronomical distance unit'), which helps distinguish it from other constant-lookup tools in the large sibling list.

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?

The description clearly states that no input is needed, which is the primary usage requirement. It implies this is the tool to call when a parsec length in meters is needed, though it does not explicitly name alternative tools such as x402-astronomical-unit or say when not to use them.

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