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

Annotations already declare readOnlyHint=true, openWorldHint=true and destructiveHint=false, so the safety profile is covered. The description earns credit beyond that by disclosing the analytical output shape (CPM/CTR/conversion-rate decomposition and campaign-level drivers) in the absence of an output schema, though it says nothing about latency, account-scope requirements or period defaults.

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 sentence with the purpose front-loaded and no filler; the clause after the colon exists to specify the analytical method, so every word earns its place.

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 seven-parameter, zero-required analytical tool with no output schema, the description tells an agent what question it answers and what the result contains. It stops short of clarifying how the comparison window is chosen (the schema defaults date_range to LAST_30_DAYS) or what happens when the account is ambiguous, which would fully close the 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 coverage is 100% with all seven parameters documented, including enum explanations for detail, language and date_range, so the schema carries the parameter burden. The description adds no parameter-level guidance of its own (no mention of detail=full vs summary, folder scoping, or date_range versus start_date/end_date), which is the expected baseline when the schema is complete.

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 verb and resource ('Explain why results or cost changed between this period and the previous one') and then details the mechanism: decomposition of cost per result into CPM, CTR and conversion rate, plus attribution to campaigns. This is clearly distinguishable from sibling reporting tools like meta_performance or meta_insights_query, which return metrics but do not explain change.

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 opening clause establishes the triggering context — the user asks why results or cost moved versus the prior period — which is a clear use condition. It does not, however, name alternatives or state when to use meta_performance/meta_insights_query instead for raw numbers, so routing among the meta reporting siblings is left partly to inference.

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