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

The annotations already carry the safety profile (readOnlyHint=false, idempotentHint=true, destructiveHint=false), and the description adds value beyond them by disclosing upsert semantics, the finance.edit permission requirement, and the allowed values for three settings. These details are consistent with the annotations — an upsert is idempotent and non-destructive — so there is no contradiction.

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 description is compact, with the purpose front-loaded in the first clause and the enum lists compressed into terse pipe-separated forms. Each sentence earns its place; the only minor redundancy is restating schema enum values, but that keeps the description self-sufficient for quick scanning.

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 14-parameter tool with no output schema, the description is under-specified: it does not describe the return value, does not explain how frequency-related parameters interact (e.g., specific_date requiring invoice_specific_day_of_month), and leaves several parameters undefined. It covers core purpose, permission, and key enums, but an agent would still struggle to invoke it correctly in edge cases.

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 only about 50%, so the description should compensate for the undocumented parameters, but it merely repeats the enum values already present in the schema (billing_model, invoice_frequency, approval_type). It adds nothing about payment_terms_days, invoice_interval_unit, invoice_interval_value, fx_rate_to_company_currency, or the relationship between invoice_billing_day_of_month and invoice_specific_day_of_month, leaving half the parameters unexplained in both places.

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 opens with a specific verb and resource — 'Create or update a project's invoicing settings' — and the '(upsert on project_id)' parenthetical clarifies the exact scope and merge behavior. The name and title are also unambiguous. It does not explicitly name a sibling it is not (e.g., projects_update or projects_get_invoicing), so it stops short of a 5.

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?

There is no guidance on when to use this tool versus the many relevant siblings such as projects_update, projects_get_invoicing, or projects_update_billing_milestone. The only constraint offered, 'Requires finance.edit,' is a permission prerequisite rather than alternative-selection guidance, so an agent gets little help deciding which tool to pick.

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