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

Annotations already declare readOnlyHint=false, destructiveHint=false, and openWorldHint=true, so the mutation surface is known. The description adds the useful behavioral fact that attached extensions are what will actually serve at launch, but says nothing about duplicate handling, whether it appends or replaces existing attachments, or failure modes for invalid extension ids/channel mismatches.

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?

One sentence, front-loaded with verb and resource, with the downstream effect ('so they actually serve') as the payoff. No filler or restated title text.

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 simple 3-parameter attach operation with full annotation and schema coverage and no output schema, the description covers enough to call it correctly. The only material gap is append-vs-replace/duplicate semantics, which matters for repeat invocations.

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% and the schema is unusually explicit — it distinguishes internal library ids from externalId and explains that extensions are a search-channel-only concept. The description adds no parameter-level detail beyond that, so 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?

Names a specific verb (attach) and resource (existing library extensions such as sitelinks/callouts) scoped to native-structure Google Ads / Microsoft Ads campaigns. The qualifier 'existing library extensions' implicitly separates it from the create_google_*_extension / create_microsoft_*_extension siblings, and 'native-structure campaign' separates it from generic campaign element tools.

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 only implied: 'so they actually serve when the campaign launches' hints at pre-launch setup, but there is no explicit when-to-use, prerequisite ordering (e.g., list_extensions / create_*_extension must run first), or pointer to remove_extensions_from_native_campaign as the inverse operation. The extension_ids schema note partially compensates by pointing at list_extensions.

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