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

Annotations already declare readOnlyHint=false, openWorldHint=true and destructiveHint=false, so the write/non-destructive profile is known. The description adds real value beyond that: it says the call returns the extension result and identifier, and that failures surface as validation or platform errors. It does not cover auth/account requirements or duplicate-handling.

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 with zero filler; the creation scope leads and the attach-separation caveat follows. Nothing repeated from the title or schema.

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 single-required-parameter create tool with full annotation coverage and no output schema, the description covers purpose, return payload, error behavior, and the follow-up attach operation. An agent has everything needed to call it and interpret the result.

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 single callout_text parameter already carries its 1-25 character constraint in the schema. "Using the supplied fields" adds no meaning beyond that, so the baseline 3 for a fully documented schema 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 and resource ("Create a callout extension in the account's Microsoft Ads asset library"), which separates it from the Google callout sibling and from the sitelink/price/promotion Microsoft extension creators. It never names an alternative tool outright, but the note that attaching is a separate operation implicitly draws the boundary against add_extensions_to_native_campaign.

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?

"Using the supplied fields" plus "Attaching the extension to a campaign is a separate operation" gives the agent clear context for when this tool is the right call and warns that creation alone does not deploy the asset. There is no explicit when-not guidance or named alternative, which keeps it below a 5.

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