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

Annotations already declare readOnly, idempotent, non-destructive, closed-world. The description adds genuinely new context: an access prerequisite tied to brand tracking that an agent could not infer from any structured field. It does not describe rate limits or failure behavior when access has lapsed.

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, both front-loaded with the constraint that matters most. The saved-lists/favorites clause is slightly tangential but still operational information, not padding.

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-parameter read tool whose safety profile is fully covered by annotations, the description supplies the missing dimension: the tracking prerequisite. Without an output schema, a hint at what fields a creative returns would have made it complete.

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 coverage is 100%, so the baseline is 3; the description earns an extra point by hammering the key failure mode (internal AdSpendSpy ID, never a Meta ad ID) and cross-referencing the 'internal_ad_id' field name in listings.

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 ('Retrieve one ad') plus the scoping constraint that distinguishes it from list-style siblings such as get_advertiser_ads or list_brand_ads. It does not name an alternative tool explicitly, keeping it just short of a 5.

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?

Gives a real precondition: the advertiser must be actively tracked, and access is lost when a tracked brand is removed on the website, including via saved lists and favorites. It stops short of pointing to a different tool for listing or searching when the ID is unknown.

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