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

Annotations already declare readOnlyHint=true, openWorldHint=false and destructiveHint=false, so the safety profile is covered. The description adds one genuine behavioral fact beyond that — zero volume is rejected (a validation error) — plus the unit of the result. It says nothing about precision, rounding, or numeric limits (negative mass, non-finite inputs).

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 clauses, formula front-loaded, unit and the one hard constraint immediately after. No filler or repetition.

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?

An output schema exists, so return-value documentation is not required, and both params are fully described. For a two-input arithmetic tool the description is nearly sufficient; only edge-case handling (negative mass, precision of the quotient) is unaddressed.

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%: both parameters carry descriptions with units (kg, m³), and the non-zero constraint on volume is already encoded in the schema text. The description's 'Rejects zero volume' duplicates rather than extends that, so it sits at the baseline for a fully documented schema.

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 an exact verb-and-resource pair (Density ρ = mass / volume) with output units (kg/m³), which pins the tool down more precisely than a name restatement. It does not explicitly contrast with near-neighbours like calculator_concentration or calculator_volume, but the formula itself is disambiguating.

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?

There is no statement of when to prefer this over sibling calculators, nor any prerequisites or preconditions. The only contextual cue is the 'Rejects zero volume' rule, which is a constraint rather than usage guidance.

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