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

Annotations already mark this as read-only and open-world, so no safety contradiction exists. The description adds genuinely useful behavioral context: it transforms opaque card URIs into structured data and clarifies that images are media_keys, not URLs. This is valuable disclosure beyond what the annotations provide.

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 purposeful and front-loaded. The core transformation is stated in the first sentence, and the second sentence adds a critical clarification about media_keys without any filler.

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 read-only expansion tool with no output schema, the description provides enough to invoke it correctly: input URI format, output categories, and a pointer to the correct sibling for media retrieval. The account_id is covered by the schema, and annotations handle the read-only/open-world expectations.

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 only 50%, but the description compensates for the undocumented card_uris parameter by explaining the card:// format and the expansion semantics. The other parameter, account_id, is already described in the schema with a source and example. The description adds meaningful meaning to the main parameter.

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 description states a specific verb, 'Expand', and resource, 'card_uri values', and clearly defines the output shape: type, components, buttons, media_keys. This makes the tool's unique role obvious and distinguishes it from sibling tools like x_ads_get_media or x_ads_create_card.

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 gives clear context for when to use the tool: whenever you have card:// URIs and need their expanded structure. It also provides an explicit routing instruction, sending media_keys to x_ads_get_media rather than treating them as URLs. It does not explicitly enumerate when not to use this tool, but the guidance is specific enough for an agent.

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