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

The description adds value beyond annotations by stating that the tool is read-only in its effects ('Does not convert anything') and groups by quantity. While annotations already indicate readOnlyHint=true, the description provides functional context that the tool is a listing operation. It does not contradict annotations, and it adds the nuance of the UNKNOWN_UNIT use case, which is helpful.

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?

The description is three sentences with no wasted words. It front-loads the core purpose, then the usage trigger, and finally a clear exclusion. Everything is essential and adds value.

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 listing tool with one optional parameter, full schema coverage, and readOnly annotations, the description is complete. It covers the purpose, usage trigger, and non-conversion behavior. The output schema is absent, but as a listing tool, the output is self-explanatory, so no additional description is needed.

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 schema has 100% coverage for the optional 'quantity' parameter, which is described in the schema. The description does not reiterate the parameter but adds context that it can be used to filter by quantity (e.g., 'length'), which goes slightly beyond the schema's generic description. Since the schema covers the parameter, a baseline of 3 is expected; the description's mention of filtering adds a small extra value, so 4 is justified.

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 clearly states the tool's function: it lists every unit symbol supported by convert_units, grouped by quantity, with names. It also specifies a specific trigger condition (when a unit symbol is rejected with UNKNOWN_UNIT) and explicitly states what it does not do (does not convert anything). This distinguishes it from the sibling convert_units tool and other calculators.

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 explicitly states when to use this tool: when a unit symbol is rejected with UNKNOWN_UNIT. It also implies that it is not for conversion, which differentiates it from convert_units. This gives clear, actionable guidance for an agent deciding between tools.

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