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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so the safety profile is covered. The description adds real context beyond that: it names which fields come back and explains that hosted_invoice_url is 'the stable link to send the customer', which tells the agent the link is durable and customer-facing rather than ephemeral.

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 front-loaded sentence with no filler; the verb and resource come first and the field list plus the one piece of genuinely non-obvious semantics follow immediately.

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?

With no output schema, the description carries the return-value burden and does so by naming the four key fields. It is complete enough to call correctly, though it omits error behavior (e.g. what happens for an unknown or deleted 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?

Schema coverage is 100% and the single parameter is documented as 'Invoice id (inv_…)', so the description has no burden to compensate for. It adds no further meaning about the id beyond what the schema states, which is the baseline-3 case for a fully documented single-param tool.

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 ('Read a subscription invoice') and enumerates the fields returned (status, amount, due date, hosted_invoice_url), so the agent knows exactly what it gets. It does not explicitly distinguish itself from siblings like xaalis_list_invoices or xaalis_get_payment, relying on the resource name alone for that separation.

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?

Usage is only implied: the presence of a required single `id` makes it clear this is a single-record lookup, and the mention of hosted_invoice_url hints at the send-to-customer use case. There is no explicit 'use this when… / use list_invoices instead when…' guidance or statement of prerequisites.

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