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

Annotations already declare readOnlyHint=true, destructiveHint=false, openWorldHint=true, so the safety profile is covered. The description adds one genuine behavioral trait not in the annotations — "Free and unlimited" (no cost, no quota) — but says nothing about determinism, rounding, or what happens on invalid input combinations.

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 sentences, zero filler, with the core input/output relationship front-loaded before the ancillary "Free and unlimited" note. Nothing repeats the schema or annotations verbatim.

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 stateless three-number calculator with no output schema, the description covers the input contract and names the returned value, which is essentially all an agent needs. It leaves minor gaps: the response field name/format and behavior when zero or three arguments are supplied. Low tool complexity keeps these gaps minor.

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% and each property is documented with units, so the baseline would be 3. The description earns a point by stating the cross-parameter constraint — any two must be provided — which the schema cannot express, since it marks all three properties optional with required: [] (0 required parameters).

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 operation on a specific resource: given any two of budget/impressions/CPM, return the third. The title and description together make the calculation semantics unambiguous, and the siblings (breakdown_ad, generate_hooks, rewrite_ad_for_product, write_ugc_script) are all creative tasks with no overlap, so no differentiation is needed.

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?

"Give any two of ... and get the third" is an explicit invocation rule that tells the agent exactly how many and which inputs to supply. It stops short of naming when-not-to-use cases or error behavior (e.g. all three supplied, or only one), but no alternative tool competes for this task.

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