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

Annotations already declare this is a read-only, idempotent, non-destructive, closed-world calculation, so the safety profile is covered. The description adds useful behavioral context beyond annotations by stating the formula, the output unit (months), and investor benchmark ranges.

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 metric and formula are front-loaded, followed by practical input guidance, a benchmark, a related tool, and a guide link. The description is compact and every sentence contributes to correct selection or invocation.

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 read-only calculator with no output schema, the description provides the formula, required and alternative inputs, output unit, benchmark context, and a relevant sibling tool. An agent has enough information to invoke it correctly without needing additional behavioral or return-value details.

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 baseline is 3, but the description adds meaning by showing how parameters relate in the formula and by clarifying the alternative CAC input path using spend and new customers. It does not add much beyond the schema for arpa_monthly_usd and gross_margin_pct, but the relationship between inputs is made explicit.

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 states a specific metric and formula: months of gross profit needed to recover CAC, calculated as CAC / (monthly revenue per account x gross margin). It also distinguishes the tool from the related ltv_cac sibling, so an agent can identify both what it computes and where it fits among alternatives.

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?

It gives clear input alternatives: provide cac_usd directly, or provide sales_marketing_spend_usd and new_customers for the same period. It also points to ltv_cac when lifetime value is also needed, but it does not explicitly state when not to use this tool or contrast it with every relevant sibling.

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