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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, but the description adds genuinely new behavior: deterministic pure calculation with no side effects, and explicit error semantics ('division by zero, non-finite inputs, or mathematically undefined combinations return an explicit error instead of a number'). Those failure modes are not derivable from the annotations and materially change how an agent should handle results.

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?

Clearly front-loaded: purpose and formula first, then WHEN TO USE / WHEN NOT TO USE, BEHAVIOUR, RETURNS, PARAMETERS. Slightly verbose because the PARAMETERS block duplicates schema descriptions, but the sectioning keeps it scannable rather than rambling.

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 three-input deterministic calculator with no output schema, the description supplies everything needed: the formula, the sign conventions, input constraints, error behavior, and the exact return shape ({ equity_value_from_enterprise_value: number, inputs }). Nothing an agent needs to invoke or interpret it is missing.

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 coverage is 100%, so the parameter meanings are already documented; the description largely repeats them. It does add interpretive value by tying each parameter to the formula (what cash is 'added back', why only interest-bearing debt qualifies), making borderline cases like non-interest-bearing liabilities unambiguous.

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 (Calculate) and a precisely named resource (equity value from enterprise value), then defines it as 'the value attributable to common shareholders, derived by subtracting net debt ... from enterprise value — the reverse of the EV bridge.' This explicitly distinguishes it from the sibling calculate_enterprise_value, so an agent can route correctly without opening either schema.

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?

Contains dedicated WHEN TO USE and WHEN NOT TO USE sections: use when holding EV from a DCF or EV multiple and needing implied equity value; do not use when market capitalisation is already known. It also excludes a common misuse ('do not net non-interest-bearing liabilities'), which is exactly the kind of routing guidance that selects among the many calculate_* siblings.

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