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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so safety is covered. The description adds the useful detail that the estimate is derived by chaining click cost through two conversion rates, i.e. a pure projection with no side effects, but says nothing about rounding, currency handling, or what the returned figure includes.

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 front-loaded sentence with no filler; the purpose and the input drivers arrive immediately. It is efficient, though the trailing enumeration of inputs partially duplicates what the schema already says.

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 five-parameter calculator with full schema coverage and no output schema, the description covers the purpose but not the return shape – an agent does not know whether it gets a single budget number, a breakdown by funnel stage, or implied clicks and leads. That 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%, so every parameter including the optional currency default is already documented in the schema. The description restates the same three inputs in prose without adding notation, units, or edge-case guidance (e.g. what happens with very low conversion percentages), so it sits at the baseline.

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 and resource ("Estimates the monthly ad budget") plus the exact target (new customers) and the three drivers of the calculation. An agent can distinguish it from calculators like campaign_roi or max_affordable_cac, though it never names a sibling to draw the line explicitly.

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?

Usage is only implied by the described inputs (CPC, conversion rates, target customers) – an agent can infer 'use when planning paid acquisition budget against a customer goal'. There is no explicit when-not-to-use or pointer to alternatives such as ppc_forecast or max_affordable_cac, which occupy adjacent territory.

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