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

Annotations only declare the mutation/safety profile; the description goes well beyond them by disclosing the validation rules (accessible-accounts check plus isManager=true), the rejection behavior for sub-accounts, the global side effect on every subsequent API call, and the downstream effect on sibling name resolution in list_ad_accounts.

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?

Three tight paragraphs with the core action and its header side effect front-loaded, followed by validation rules and downstream impact. Every sentence carries information; only the parenthetical 'My Client Center' gloss is mildly expendable.

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 one-parameter configuration tool with no output schema, the description covers the precondition, the failure mode, and the observable consequences for other tools, leaving nothing an agent needs in order to call it correctly.

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 100% and the schema already documents the customerId format, so the baseline is 3. The description adds genuine semantic value by constraining the parameter to a manager account ID and explaining the failure mode if a sub-account ID is supplied.

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 ('Set which Google Ads account is the user's MCC') and immediately names the concrete effect (the login-customer-id header). It is clearly distinguishable from siblings like set_brand_ad_account and list_ad_accounts.

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?

Explicitly states when it is needed ('Required for agency users with multiple client accounts under one MCC') and when it is not ('single-account users normally don't need to call this'). It also gives a hard precondition: the ID must be a manager account, so passing a sub-account is rejected.

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