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

Annotations already declare readOnlyHint=false, destructiveHint=false and openWorldHint=true, so safety is covered. The description adds real value on behavior that annotations do not: it discloses the return shape ('extension result and available identifier') and the error classes ('validation/platform error'), plus the non-attaching scope. It stops short of mentioning account permissions or rate limits.

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, front-loaded with the action and target, followed by outcome and scope boundary. No filler and nothing repeated from the title.

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 one-parameter create tool with no output schema, the description covers what is created, where it lands, what comes back (result plus identifier), what failure looks like, and that attachment is out of scope — everything an agent needs to call it and interpret the response.

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?

There is only one parameter and schema coverage is 100%, with the schema already documenting the 1-25 character constraint on callout_text. The description adds nothing beyond 'using the supplied fields', so the baseline 3 applies.

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 (Create), a specific resource (callout extension), and the precise target (the account's Google Ads asset library), which immediately separates it from create_microsoft_callout_extension and from the other Google extension types in the sibling list.

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 closing sentence 'Attaching the extension to a campaign is a separate operation' gives a clear boundary and implicitly routes the agent to the attach/add-extensions tooling for that follow-up step. It does not name that sibling tool or state prerequisites explicitly, so it falls short of a full 5.

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