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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safety profile is covered. The description still adds real value beyond that: it states what the response contains (totals, state, dates, line items) and enumerates the state vocabulary (draft, unpaid, overdue, paid, canceled), which no structured field provides. It omits ordering/pagination behavior, though the schema's next_offset text partially covers that.

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?

Two dense sentences: return payload first, filter surface second – well front-loaded with no filler. The trailing 'invoice.get' is unexplained token-like clutter that does not earn its place.

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?

With no output schema, the description usefully compensates by naming the returned fields and states, and it covers the breadth of the 13 optional filters. Remaining gaps are minor: no explicit escalation path to fastbill_get_invoice for line-item detail and no ordering guarantee.

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 85%, so the baseline is 3. The description adds the grouping of filterable dimensions ('invoice date, due date, paid date, last update'), which clarifies intent for the start_/end_ pairs, but adds no format or syntax beyond the schema's yyyy-mm-dd notes.

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?

Opens with a specific verb+resource ('List outgoing invoices, drafts and credit notes') and enumerates the returned fields and states, so the agent knows this is a read/collection operation. It does not explicitly distinguish itself from fastbill_get_invoice or fastbill_list_recurring_invoices, leaving the list-vs-single and invoice-vs-recurring distinction to inference.

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?

Usage is implied by the filter enumeration (customer, number, type, date ranges), which suggests 'use this to search/filter invoices'. There is no explicit when-to-use or when-not statement, and no pointer to siblings such as fastbill_get_invoice for a single record or fastbill_complete_invoice for a write.

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