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

Annotations already declare readOnlyHint, openWorldHint and destructiveHint, so safety is covered. The description adds a genuinely non-obvious behavioral guarantee beyond them: that field names are identical across sources, which tells the agent it can reuse one canonical field name regardless of the source it queries. It does not discuss paging or result size, but the presence of an output schema lowers that burden.

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 short sentences, front-loaded with the resource and scope, with no filler. The link to `get_data` appears in the first clause where the agent needs it.

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 tool is simple: one optional parameter, an output schema, and full annotation coverage. The description covers what is listed and the cross-source naming guarantee, which is sufficient. A brief hint that the `source` filter narrows the result would close the remaining 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 100% and the single `source` parameter is documented in the schema itself as "Only fields available in this source." The description never mentions filtering by source, so it adds no meaning beyond the schema; 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 ("List the normalized dimensions and metrics") and ties it to the sibling `get_data` it serves. The phrase "normalized" plus "Names are the same in every source" implicitly separates it from per-source listers like `ads_list_fields` and `ga4_list_dimensions_and_metrics`, but those alternatives are never named directly.

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?

Usage is only implied: an agent can infer this is the discovery step before calling `get_data`. There is no explicit when-to-use statement, no exclusion telling the agent to prefer a source-specific lister in some cases, and no note on whether this should be called once or per session.

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