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

Annotations already cover readOnly=false, idempotent=true, destructive=false. The description adds behavior beyond them: the PROOF_PACK state gate, the anti-price-invention constraint, and the CHF 0.18 per accepted call platform fee. It stops short of describing what state the flow ends in or what the call returns, so it is not fully exhaustive.

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 tight sentences, front-loaded with the core action and then the gate and fee. The imperative fragments ('Never: Invent a price or skip the gate') are dense but readable; nothing is wasted.

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 money-mutating tool with no output schema, the description covers the precondition gate, cost, and idempotency contract, which is what an agent needs to invoke safely. The main residual gap is the post-call effect on the flow/invoice state.

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. The description adds relational meaning the schema lacks: that quote_id must come from a prior quote call and that mandate_token and idempotency_key travel together with it on this money call. It doesn't add format detail, but the sequencing context is genuinely useful.

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 (issues) and resource (the invoice), states the inputs it derives from (business tariff + reported work), and pins the gate condition (only after the proof pack). An agent can distinguish this from quote and payment_collect without opening the 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?

It states the required precondition (state PROOF_PACK), forbids specific misuses ('Never: Invent a price or skip the gate'), and gives the call ordering versus the sibling quote tool ('quote first, then send quote_id, mandate_token and idempotency_key'). When-to-use and when-not are both explicit.

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