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

Annotations already declare readOnlyHint=false, idempotentHint=true, destructiveHint=false, and openWorldHint=false, so the safety profile is covered. The description usefully supplies the mechanism behind idempotency ('keyed on email'), but it omits the critical behavioral question for an upsert: whether unmatched existing fields are preserved or overwritten on an update.

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?

One short sentence with the identity key front-loaded and no filler. It is efficient, though the 'a invoice_client' typo and the terseness leave the upsert merge semantics unaddressed.

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?

A 9-parameter, partly-enum'd mutation tool with 0% schema coverage and no output schema needs the description to carry merge semantics, enum meanings, and return behavior. The single sentence leaves all of these unstated, so an agent cannot safely predict the result of an update path.

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% across 9 parameters, and the description only identifies 'email' as the key plus 'name' implicitly via the create-or-update verb. The remaining seven fields (notes, phone, address, company, contact_id, message_language, reminder_cadence) get no mention, and the non-obvious enum values such as 'from_due' are left unexplained.

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 pair ('Create-or-update') and resource ('invoice_client'), plus the identity key ('email') that defines the operation. That keying detail distinguishes it functionally from sibling invoice_client_create and invoice_client_update, though neither is named explicitly.

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?

'Create-or-update ... keyed on email' implies you use this when the record may or may not already exist, but the description never states when to prefer the dedicated create/update siblings or what happens if the email matches an existing client. Usage is inferable rather than stated.

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