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

Annotations already declare readOnly, idempotent, non-destructive, non-open-world, so the safety bar is low. The description nonetheless adds real interpretive context beyond them: spendEstimate is engagement-derived, not real spend, spendLowerBound is a true minimum, text fields are null when unavailable, and transcripts are English translations with translatedFrom/transcriptOriginal. Operational details (auth, errors, invalid-key behavior) are absent but not critical here.

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?

Front-loaded with the purpose, then a dense but purposeful enumeration of return fields, null semantics and translation behavior. It is a long run-on paragraph, but with no output schema each clause carries information the agent cannot get elsewhere, so little is wasted.

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?

With no output schema, the description correctly carries the burden of explaining the return shape, and does so thoroughly, including null handling and translation fields. The main gap is what happens when the key doesn't resolve or is malformed, which is minor for a read-only lookup.

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?

One parameter with 100% schema description coverage — the schema already documents the accepted key formats (adKey, bare id, Meta Ad Library id). The description adds nothing about the key, 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?

The first clause states a specific verb and resource: 'Returns the full record of one ad.' The scope (single ad by key, versus the sibling search_ads which lists ads) is discernible from 'one ad' and the reference to tags that 'search_ads shows,' though no sibling is explicitly named as the alternative.

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?

The description never states when to reach for this tool versus search_ads or the other siblings, nor that it should be called after a search to expand a specific result. The search_ads mention is a field-format cross-reference, not usage guidance, so the agent must infer the lookup workflow.

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