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

Beyond the annotations (readOnlyHint=false, destructiveHint=false, non-idempotent), the description adds real behavioral context: the operation is free, creates the assets and links them in one atomic mutate, and mirrors add_sitelinks. Atomicity is a useful trait not captured by the annotations. It does not mention idempotency/re-run behavior, so it falls short of 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?

Three sentences, front-loaded with the definition before the value proposition and mechanics. Efficient overall, though the sitelinks analogy is stated twice ('the same way sitelinks do' and 'same as add_sitelinks'), a slight redundancy that could be tightened.

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 campaign-level mutation with no output schema, the description covers what it does, its atomic nature, and its cost/benefit. Combined with the annotation safety profile and the richly documented schema (validateOnly, extraFields, customerId), an agent has what it needs. Return values and permissions are not addressed, keeping it short of 5.

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 80%, so the schema already carries most parameter meaning (including the 25-char/20-item limits and the detailed customerId and extraFields docs). The description adds no parameter-specific detail, which is acceptable at this coverage but earns no bonus.

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 (add) and resource (callouts) and defines what callouts are with concrete examples ('Read-only access', 'No card required'). It also anchors the concept against a known sibling ('same way sitelinks do', 'same as add_sitelinks'), so an agent can place it precisely without opening a 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?

Usage is implied: this is the tool to add callout extensions at the campaign level, and it is framed as parallel to add_sitelinks. However, there is no explicit when-to-use/when-not statement or a directive telling the agent to prefer this over a sibling for a given goal, so routing is left to inference.

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