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

Annotations already declare readOnlyHint=true and destructiveHint=false, so safety is covered; the description reinforces and extends this with billing behavior ('execute bills the tool'), the free-read model, and the 20-account fan-out cap. It does not explain the non-obvious idempotentHint=false, which is the one behavioral trait left unexplained.

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?

Purpose and scope are front-loaded in the first two sentences, followed by invocation syntax, account semantics, and the sibling exclusion. The 38-tool category listing is long but earns its place as a routing index that saves a list_tools round trip; a slightly tighter format would push this higher.

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 router with a nested arguments object and no output schema, the definition is nearly complete: it tells the agent to resolve tool names via list_tools/get_tool_schema and describes the account fan-out. It stops short of describing what a read returns or how results are shaped across multiple accounts.

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%, so baseline is 3, but the description adds real meaning beyond the schema: 'never guess a tool_name' operationalizes tool_name, accounts="all_active" semantics and the 20-account read fan-out are contextualized, and it clarifies that arguments is the target tool's own 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?

States a specific verb and resource ('Reads and tool lookup for Google Ads') and immediately delimits scope: 'nothing called here changes anything.' It explicitly contrasts itself with google_ads_write, so the agent can separate the read router from the write router without opening either schema.

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?

Explicit invocation pattern is given (action="execute" with tool_name/arguments, list_tools and get_tool_schema free), plus the constraint 'never guess a tool_name.' It names the write alternative (google_ads_write) and even clarifies that google_get_campaign_performance is called by name outside this router.

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