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

Annotations already cover readOnly, idempotent, non-destructive, and closed-world, and the description adds real behavioral context beyond them: a caching recommendation because the data is stable, plus the precise degradation path for an unknown resource name (a note plus valid_resources in the response). The invalid-input behavior is exactly the kind of detail an agent cannot infer from annotations.

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 lead sentence states the outcome first, followed by usage, caching, and edge-case behavior, so it is well front-loaded. The trailing Args block largely restates the parameter already covered above, which is a small redundancy but not damaging.

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?

The description covers purpose, mandatory-use guidance, caching, and the invalid-name response path. With an output schema present, return values need not be detailed, and its reference to 'the search tool' is understandable in context, though naming the sibling explicitly would close the last gap.

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 0%, so the description carries the burden for the single parameter, and it does: it names resource_name, supplies example values ('campaign', 'ad_group'), and explains what happens when the value is not a real resource. It does not state format/casing rules, so it is not fully compensating, but it adds substantial meaning beyond the bare schema.

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 names a specific verb and resource: retrieves the selectable, filterable, and sortable fields for a Google Ads resource. It implicitly distinguishes itself from the sibling `search` tool by noting that the returned valid_resources are 'the resource names the search tool accepts', so an agent knows this tool discovers schema rather than executing queries.

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?

It gives clear context ('use this tool to find out which fields you can select, filter by, or sort by when querying a specific resource') and a strong directive against guessing field names. It stops short of an explicit when-not statement or a fully named alternative for running the actual query, but the routing intent is unambiguous.

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