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

Annotations only declare readOnlyHint=true; the description adds real behavioral context beyond that: the Merit 3-month hard limit on the period, that results are header-level (no line detail), and the underlying endpoint. It omits paging/auth behavior, but the material constraint is disclosed.

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 sentences, zero filler, front-loading what is returned, then the period constraint, then the sibling for line detail. Nothing is padded or redundant.

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 compensates by enumerating the returned header fields, and it covers the key constraint and the sibling hand-off. Minor gaps remain around result paging/ordering, but an agent has enough to call it correctly.

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 date_type, max_results, unpaid_only and both period bounds are already documented in the schema, including the same 3-month cap. The description restates the period limit but adds no syntax or format detail beyond the schema, so 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?

States a specific verb and resource plus the exact header fields returned (number, customer, dates, totals, paid flag), and explicitly distinguishes itself from merit_get_sales_invoice, which returns lines. An agent can select it against siblings without opening a schema.

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?

Gives a clear condition and remedy for long spans ('at most 3 months... for longer spans make several calls') and routes line-level needs to merit_get_sales_invoice. It does not mention merit_search_sales_invoices, which is a plausible alternative for filtered lookups, so the guidance stops short of full alternative coverage.

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