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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description goes further by enumerating the returned content (printed value, machine-readable value, uncertainty, unit, note, page, source) and, critically, disclosing that a null standard uncertainty means an exact constant — a genuinely useful interpretive detail annotations cannot supply.

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 dense sentence with the resource and accepted key forms front-loaded, followed by the return payload. No filler and nothing redundant given the tool's simplicity.

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 one-parameter read with no output schema, the description covers both the input forms and the shape of the result, including the null-uncertainty convention. There is no obvious gap an agent would need filled before calling it.

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?

Schema description coverage is 100%, so the single query parameter is already documented with the same example forms. The description restates that id, symbol or name are accepted but adds no format or disambiguation rules beyond the schema; baseline 3 applies.

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+resource (one CODATA 2022 constant) with an explicit input space (id, symbol or name) and concrete examples. The singular "one" implicitly distinguishes it from the sibling list_constants, which returns multiple.

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 context is clear: this is the single-record lookup path, and the examples ('c', 'speed of light', 'planck-constant') show the accepted identifier forms. It never explicitly names list_constants as the browse alternative or states when not to use it, so it stops short of full routing guidance.

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