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

Annotations already establish readOnly/idempotent/non-destructive, and the description adds genuinely new behavior: output format is CSV, granularity is day-by-day, data older than 37 months is unavailable, and nothing is persisted server-side. The opening fragment "Unlimited plan." is ambiguous — it reads as an eligibility requirement but is not stated as such.

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?

Three short sentences with no filler, and the retention rationale is front-loaded right after the what-it-does statement. The leading sentence fragment "Unlimited plan." is contextually thin and costs a little clarity for its brevity.

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 must carry the return contract, and it only half does: it says CSV and daily granularity, but not whether the response is inline CSV text, a file URL, or an attachment, nor which metrics/columns are included. For an export tool that an agent must hand data back to a user, that delivery mechanism is a meaningful 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 period, customer_id, and login_customer_id are already fully documented in the schema; baseline 3 applies. The only added semantic is that results are always day-by-day regardless of the period enum, which is modest but real value.

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?

"Returns your campaign metrics day by day as CSV" states a specific resource and output shape, which separates it from report/analysis siblings like campaign_performance or run_gaql_query. It never names a sibling explicitly to differentiate, and the verb is descriptive ("Returns") rather than an action verb like export/download.

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 motivates usage (keep your own copy before Google's 37-month window deletes it) and notes a plan constraint ("Unlimited plan"), which implies when it applies. However, it never says when NOT to use it or which alternative to pick for a one-off query versus a bulk daily export.

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