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

Annotations already declare readOnlyHint, idempotentHint, non-destructive and closed-world, so safety is covered structurally. The description adds genuinely new behavioral context: the gating conditions (Pro subscription, owner's ads switch) and the concrete 403 `scope_ads` error with its remedy. It stops short of describing pagination or result-size behavior, which keeps it at a 4 rather than a 5.

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?

The description is front-loaded with the payload contents before the routing advice and the auth caveat, which is the right order for an agent scanning it. The opening enumeration is a long comma-chained list, so it is slightly dense, but each clause carries distinct information and nothing is filler.

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 zero parameters, a full output schema, and four annotations, the structured data already covers shape and safety. The description fills the only real remaining gap, the authorization prerequisites and their error signal, so an agent has everything needed to call it correctly and interpret a failure.

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?

The tool takes zero parameters, so the baseline is 4 and there is nothing for the description to disambiguate. The description's implicit note that results are scoped to 'the workspace' is a useful but minor addition.

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 enumerates exactly what the tool returns: today's and month-to-date spend against caps, live campaigns, connected ad accounts, every campaign with status and budget, and the count of open Siren suggestions. That is a specific verb+resource statement that an agent can distinguish from siblings like ad_performance or list_ad_campaigns without opening any schema.

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?

'Call first for anything about ads' is an explicit routing instruction that tells the agent this is the entry point before the more granular ad siblings. It also states the prerequisites (Pro plan plus the owner's ads switch for this agent) and names the failure mode (403 `scope_ads`) with where the owner turns it on, so an agent knows both when to use it and what to do if it is blocked.

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