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

Annotations already declare a non-destructive, open-world write, so safety is covered. The description adds outcome behavior beyond that: it returns the created extension result and available identifier, or surfaces a validation/platform error, which tells the agent what to expect on failure.

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?

Two sentences, front-loaded with the action and scope, then the critical boundary condition. The only mildly redundant phrase is 'using the supplied fields', but it does no harm.

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 two-parameter creation tool with no output schema, the description covers what is created, where it lives, what comes back, and the fact that attachment is out of scope — nothing an agent needs to call it correctly is missing.

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% (header 1-25 chars, 3-10 values of 1-25 chars each), so the schema fully documents both required params. 'Using the supplied fields' adds nothing beyond it; baseline 3.

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 ('Create a structured snippet extension') plus scope ('in the account's Google Ads asset library'), which separates it from the campaign-attachment sibling add_extensions_to_native_campaign and from other extension types.

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?

Explicitly routes the agent away from attaching the extension: 'Attaching the extension to a campaign is a separate operation.' That is a genuine when-not clause naming the follow-on step, though it doesn't name the specific sibling tool to use for attaching.

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