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

Annotations already cover the read-only/safe profile, yet the description adds the OAuth scope requirement and the exact HTTP path, neither of which appears in structured fields. Since there is no output schema, it also discloses the response envelope (count, total, next_offset, items) and the paging protocol, which is real added value.

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?

Four short sentences, front-loaded with purpose, then permission, endpoint, and return shape — no padding. The telegraphic fragments ('Needs the "cost_unit" scope. GET /reporting/cost-units.') are efficient, though slightly clipped.

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 6 optional params fully documented in schema and no output schema, the description supplies what's missing: the return envelope and next_offset-as-offset paging loop, plus the scope requirement. It does not explain what a cost unit is or the default ordering, which are minor for a list endpoint.

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 all six parameters are already documented, including the filter syntax and the offset/next_offset paging hint. The description's only parameter-related content restates the pagination convention rather than adding new semantics, so the baseline 3 applies.

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 uses a specific verb ('List') plus resource ('cost units') and confirms the underlying GET endpoint, so the operation is unambiguous. However, it never distinguishes cost units from the nearby sibling smallinvoice_list_units, which an agent could easily confuse.

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 'cost_unit' scope, which is a genuine usage prerequisite, but gives no explicit when-to-use guidance or routing to alternatives (e.g. list_units). Usage is implied by the resource name rather than explained.

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