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

Annotations only supply destructiveHint=true, so the description adds real value: it discloses that the invoice is retained as cancelled rather than deleted, that fiscal status changes, that user confirmation is required, and that the action is reversible via restore. It stops short of stating permission/authentication requirements or any side effects on downstream documents (e.g. payments already recorded).

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 compact sentences plus an endpoint fragment; the persistence behavior and the sibling pointer are front-loaded where they matter. The trailing 'PUT /invoice/cancel' is slightly redundant for an agent that never issues HTTP directly, but it is short.

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, no-output-schema mutation tool whose parameters are fully documented in the schema, the description covers what an agent needs: the state change, its reversibility, and the confirmation requirement. Only the cancel-vs-reverse distinction is left unaddressed.

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 two parameter descriptions already explain the leading-zero convention for number and point to smartbill_list_series for series_name. The description adds no parameter-level meaning beyond that, so the baseline of 3 applies.

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 ('Cancel an existing invoice') and immediately scopes it with 'it stays in the books as cancelled', which distinguishes it from hard-delete semantics. It also names the sibling that reverses the effect (smartbill_restore_invoice), so the agent can place it in the lifecycle without opening a schema.

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?

Gives clear operational guidance — confirm with the user, use smartbill_restore_invoice to undo — which is unusually strong for a mutation tool. It does not, however, distinguish this from smartbill_reverse_invoice, a sibling with an overlapping but materially different effect, leaving that choice to inference.

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