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

Annotations already declare readOnlyHint=true, openWorldHint=true, and destructiveHint=false, so the safety profile is covered structurally. The description adds only that the result is fee lines scoped to one invoice; it says nothing about empty results, missing-invoice errors, or pagination, which would be the value-add for a remote read.

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?

One compact sentence that front-loads the resource and scope. The trailing 'GetInvoiceFees.' merely restates the operation name and earns nothing, a minor redundancy in an otherwise tight definition.

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 one-parameter read-only lookup this is close to sufficient, and annotations cover the safety profile. But with no output schema, the description leaves the shape of a 'fee line' entirely unexplained, and it omits any note on behavior for an invalid or nonexistent invoice id.

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 a single parameter at 100% schema coverage the baseline is 3, but the description goes further by telling the agent where the invoice_id originates (ecourier_list_invoices), which is genuine semantic guidance not present in the schema's bare 'Invoice id.'

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 resource and scope: the fee lines of a single invoice, retrieved by invoice id. This distinguishes it from the nearby ecourier_get_order_fees (order vs. invoice) implicitly through the resource noun, though it never names that sibling explicitly.

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?

The parenthetical '(the id from ecourier_list_invoices)' gives a useful workflow pointer for obtaining the required id, which is real guidance. However, there is no statement of when to use this versus ecourier_get_order_fees or ecourier_list_payments, and no exclusions or prerequisites beyond the id source.

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