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

Annotations already declare readOnlyHint, destructiveHint=false and openWorldHint=false, so the explicit 'Read-only' line is largely redundant. The description does add genuinely new behavior: that on a plan without invoices the tool returns status 'blocked' with the plans on sale, which an agent could not know from structured fields.

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, front-loaded sentences: what it returns, what questions it answers, and the edge-case behavior. The only waste is restating 'Read-only', which the annotations already assert.

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, listing the returned fields is exactly the right compensation, and the plan-blocked fallback is a useful edge case. The remaining gap is the undocumented limit parameter (default, ordering, truncation behavior), which is small but real for a list tool.

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?

There is one parameter (limit, min 1, max 50) with 0% schema description coverage, and the description never mentions it — no default value, no explanation of what happens when omitted, no ordering implied. With low coverage the description was the only place to compensate, and it does not.

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 states a concrete resource and scope — 'This company's invoices still waiting for money' — and enumerates the fields returned (number, client, amount due, due date, status). That distinguishes it from a generic invoice_list, though it never names the sibling that lists all invoices, so differentiation is inferred rather than stated.

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 conveyed indirectly through example user utterances («покажи неоплачені рахунки», «хто мені винен»), which make the intent clear. There is no explicit when-not guidance or routing to invoice_list/invoice_get for other invoice needs, so the agent must infer the boundary.

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