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

Beyond the annotations, it discloses the downstream effect (the cash flow forecast counts the invoice on that day, including overdue ones it would otherwise omit) and that null reverts to default. Combined with idempotentHint=true and destructiveHint=false, the mutation's behavior is well covered; only reversion details for existing values are left implicit.

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?

A single dense sentence front-loads the purpose and effect, with no filler. The trailing 'Free.' fragment is terse to the point of being cryptic and adds minor noise.

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 two-parameter idempotent mutation with no output schema, the description covers prerequisites, reset semantics, and the reporting consequence. Sufficient to call correctly; only edge cases such as behavior on voided or unpaid-deleted invoices are undocumented.

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%, so the baseline is 3, but the description adds genuine meaning beyond the schema: it explains what the date represents and that null returns to default, which matters for overdue invoices. The invoice_id parameter is left to the schema, hence not a 5.

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 names a specific verb and resource (set the expected payment day on an invoice) and scopes it precisely: invoices that are sent and not yet paid. The Xero 'expected date' analogy and the mention of books_report cash_forecast make the tool's identity unmistakable against siblings like invoices_update or purchasing_set_planned_date.

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?

It states the precondition clearly (invoice must be sent and unpaid) and the reset behavior ('null clears it'). It does not name an alternative sibling to use for other cases, which keeps it short of a 5, but the usage context is unambiguous.

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