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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and closed-world, so the safety profile is fully covered. The description adds the comparison behavior (current CPA is checked against the computed ceiling), which hints at the result shape, but says nothing about output format or precision. Reasonable but thin given the low bar set by annotations.

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?

Front-loaded single operational sentence that states the computation and the check; efficient. The trailing promotional link to 'the full version' is the only sentence that doesn't serve invocation, a minor deduction.

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 four-parameter, fully documented, read-only calculator with no output schema, the description conveys what is produced (break-even CPA, max CPA, and a ceiling comparison). Return values are only implied rather than enumerated, which is the main remaining gap.

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% and all four parameters (aov, grossMarginPct, targetProfitPct, currentCpa) are self-documented with units noted. The description adds no additional meaning, syntax, or unit clarification beyond the schema, so the 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?

Names a specific verb (calculate) and concrete outputs (break-even CPA, maximum CPA) plus a validation step (check current CPA against the ceiling). This is clearly distinct from generic CPA/CAC siblings like cpa_to_cac_scalability_matrix or roas_calculator, though it never explicitly names an alternative. Clear but without sibling differentiation in text.

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 implies the scenario (protecting a target margin while evaluating paid acquisition costs), but states no explicit when-to-use condition, prerequisites, or which sibling calculator to prefer. Usage is inferable from the outputs described rather than stated.

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