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

Annotations already cover the safety profile (readOnlyHint=true, destructiveHint=false), so the bar is lower; the description nonetheless adds genuinely non-obvious behavior: results are annual by default, with monthly or daily detail only if A-Cube has activated it. It does not explain what happens for years outside the 2000-2100 range or how empty/unauthorized cases behave.

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?

A single compact sentence that front-loads the resource and the key scoping caveat, followed by the endpoint path. Nothing is wasted, though the raw 'GET /invoices/stats/{year}' is marginal value for an MCP caller.

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 only two parameters and no output schema, the description should hint at the shape of the returned counters (which counts, by status or period). It says 'counters of invoices and notifications' but leaves the response structure largely unspecified, so an agent knows the intent but not what it will receive.

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 both parameters are already documented and the baseline is 3. The description adds only a modest clarification that omitting fiscal_id aggregates the whole account rather than a single customer, which is useful but thin.

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 ('counters of invoices and notifications') scoped to a year and optionally a fiscal id, which is clearly distinct from the list_* siblings that return individual records. It does not name any sibling explicitly, but the aggregate-vs-list distinction is inferable from the wording.

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 description implies the use case (getting aggregate counts for a year) but never states when to prefer this over acube_list_invoices or acube_list_invoice_notifications, nor any exclusions. An agent must infer that this is the summary path rather than the detail path.

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