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

Annotations already declare readOnlyHint=true, idempotentHint=true and destructiveHint=false, so safety is covered. The description adds the usage policy that only listed or user-supplied pictures may be used, which is genuinely useful, but nothing about scope limits, freshness of the library, or per-account behavior. The trailing MCP/credits boilerplate is generic and not tool-specific.

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?

The purpose sentence is front-loaded and followed immediately by the practical use cases and the usage constraint; no wasted phrasing in that core. The trailing 'MCP: use the input object... Free: MCP tool calls use no Adrails credits' is generic boilerplate that dilutes focus slightly.

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?

There is no output schema, and the description compensates by naming the fields returned for both pictures and audiences, which is exactly what an agent needs. Combined with the usage constraint and downstream reuse guidance, it is nearly complete for a read-only library lookup; only pagination/size limits are unaddressed.

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 75%, with accountIds, userStatement and idempotencyKey documented in the schema itself. The description says nothing about the required accountId or the nested input wrapper, so it adds no meaning beyond what the schema provides — 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?

States a specific verb and resource ('Reads a Google Ads account's picture library ... and its audiences') and enumerates the returned fields (assetId, address, size; id, name), so an agent knows exactly what data comes back. It is clearly distinguishable from siblings like campaign_media or targeting_options, which do not read account asset libraries.

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 concrete when-to-use guidance: reuse pictures by assetId in google_ads_build and name an audience signal for a Performance Max asset group. It also states a hard constraint ('Only the pictures listed here or given by the person may be used'), but does not name an alternative tool or say when not to call it.

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