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

Annotations already declare the mutation profile (readOnly=false, destructive=false, idempotent=false), so the bar is lower. The description still adds real value by disclosing the side effect that it 'raises the order total' and the order-state restrictions, plus the underlying endpoint. It does not say whether calling twice creates duplicate entries, which matters given idempotentHint=false.

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 clauses: what it does and the total impact first, then the state restrictions, then the endpoint. No filler, and the most decision-relevant facts are front-loaded.

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 5-parameter mutation with no output schema, the description covers purpose, side effect, and eligibility constraints. It leaves open the return value and duplicate-entry behavior, but the schema fully documents every parameter, so nothing blocking remains.

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 100%, so the baseline is 3. The description illustrates the name/amount pair via the delivery-fee example, but says nothing about tax_free or comments, so it adds little beyond the schema's own per-field documentation.

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 ('Add a custom line ... to an order') and immediately clarifies what a custom line is with a concrete example (a delivery fee). This distinguishes it cleanly from sibling mutators such as add_assets_to_order, which attach inventory rather than arbitrary named amounts.

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?

Explicitly excludes orders in sold, void, completed or cancelled states, which is the key precondition an agent must check before calling. It stops short of naming an alternative tool (e.g. add_assets_to_order for real inventory) or telling the agent how to verify order status first.

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