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

Annotations already declare readOnlyHint=false, idempotentHint=false and destructiveHint=false, and the description adds genuinely new context: the original invoice stays unchanged and this is a legal requirement. It also discloses the integration dependency. It does not mention the effect of repeat calls, though idempotentHint=false already flags that no de-duplication happens.

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?

Three short sentences led by the 'Use when' trigger, so the routing information is front-loaded. "As the law requires" is slight color but earns its place by signaling the behavior is mandatory rather than a setting.

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 mutation tool with no output schema, the description never indicates what comes back (credit note identifier, status, document link), so an agent cannot plan follow-up calls. Preconditions and behavioral facts are otherwise adequately covered.

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?

With 0% schema description coverage the description must carry both parameters, and it does: invoice_id is the invoice to be credited in full, and reason is optional and is printed on the credit note ("shown on it"), which is real semantic value beyond the bare string type. Length/format constraints (maxLength 300, uuid) are left to the schema.

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+resource ("issues a credit note") and pins the scope: "for the whole invoice," which implicitly separates it from any partial-credit or invoice-editing path such as create_invoice. The only ambiguity is that the trigger phrase says "cancelled or reduced" while the operation covers the whole invoice only, leaving partial reduction undefined.

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?

"Use when an invoice must be cancelled or reduced" gives an explicit trigger condition, and the availability precondition (Billify connected to the user's account) is stated up front. It does not name a sibling or alternative for the partial-reduction case it alludes to, leaving that route to inference.

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