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 structurally. The description still adds real value beyond that by naming the authentication requirement ('needs account token') and preempting the obvious billing-related fear with 'Does not charge', though it says nothing about pagination or result limits.

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 clauses, front-loaded with the prerequisite and the core action, then the return contents, then the non-effect reassurance. Zero filler.

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?

Even with no output schema, the description enumerates what comes back (date, amount, printable link), and the auth requirement is stated. For a zero-argument read-only listing tool this covers everything an agent needs.

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?

The tool takes zero parameters and the schema is empty, so there is nothing for the description to disambiguate. Per the zero-parameter baseline, a 4 is appropriate; no misleading parameter hints are given.

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 ('Lists the account's invoices') and even summarizes the returned fields (date, amount, printable link). It does not need to differentiate against siblings, since none of the listed tools overlap with invoice retrieval, but it also does not explicitly position itself relative to any related tool.

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 prerequisite '(needs account token)' gives a usable trigger condition, and the fact that no sibling deals with invoices means there is no real alternative to disambiguate. There is, however, no explicit when-to-use or when-not-to-use statement, only implied context.

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