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

Beyond the readOnly/idempotent annotations, it discloses cost ('free, not counted against tasks'), timezone semantics, currency semantics, and the important null-vs-zero behavior for CPA/ROAS with an instruction on how to report blanks. It also advises asking the user rather than guessing ids or dates. No contradiction with 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 with a safety/cost banner, then use/don't-use routing, then parameter notes. Slightly long, but nearly every sentence carries operational value; only the emoji-banner line is arguably decorative overhead.

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?

With no output schema, the description compensates by naming the returned metrics, the period-over-period comparison, and null-handling conventions. Combined with complete schema documentation, an agent has everything needed to call and interpret this tool correctly.

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 value the schema does not: the date_range vs start_date+end_date mutual exclusivity is emphasized as a hard rule, and periods are defined as calendar days in the account's timezone with money as decimal in account currency. It does not explain limit, raw_data, or campaign_ids beyond what the schema says.

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 (reports campaign performance metrics) and enumerates the exact measures returned: spend, impressions, clicks, CTR, conversions, conversion value, CPA, ROAS, plus period-over-period change. It also explicitly distinguishes itself from generate_report_now, google_list_campaigns, and google_analyze_search_terms.

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?

Provides explicit 'Use when' triggers (performance questions, improvement checks, cost per conversion), explicit 'Do not use when' cases with named alternative tools, and a routing note to generate_report_now for report deliverables. No inference required.

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