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

Annotations already declare readOnlyHint and openWorldHint, yet the description adds real value: 2-credit cost, results returned in the Library's own ordering rather than relevance, and the caveat that spend/impression ranges only populate for political/issue ads under certain deployment configurations. That deployment caveat is a genuinely non-obvious behavioral trait.

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 compact sentences, front-loaded with the capability before the caveats. No padding, though the 'where the deployment reads the Graph Ad Library API' clause is dense and slows scanning.

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 13-parameter search tool with no output schema, the description covers result ordering, the hasMore/cursor pagination contract, field-availability gaps, and cost — everything an agent needs before invoking. Remaining parameter details are adequately handled by the 85% schema coverage.

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 85%, so the schema documents nearly every parameter with enums and defaults. The description's only parameter-adjacent content (political ads unlocking spend data) restates the ad_type schema text, and no light is shed on the undocumented cache_max_age.

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+resource ('Search the Meta Ad Library') and enumerates the surfaces covered (Facebook, Instagram, Messenger, Threads ads). It does not explicitly differentiate from near siblings like facebook_ad, facebook_adLibraryAdvertisers, or facebook_pageAds, so it stops 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?

No guidance on when to pick this over facebook_adLibraryAdvertisers, facebook_pageAds, or tiktok_adLibrarySearch — the agent must infer from names alone. The only operational note is the credit cost, which addresses economics rather than tool selection.

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