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

readOnlyHint=true already declares the safe-read profile, and the description goes further by disclosing the exactly-one selector constraint, the 'last invoice wins' tie-break for order numbers, and the account-scoping limitation. No permissions or error behavior described, but the added context is substantive.

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?

Front-loads purpose, then constraints, then the secondary use case, with no filler. The long parenthetical is dense but every clause carries information; marginally less scannable than it could be.

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 usefully enumerates the returned field groups, and it covers identifier selection, scope, and a secondary purpose. It stops short of describing not-found/failure behavior, 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 coverage is 100% with per-parameter descriptions, so the baseline is 3; the description adds that only one of the three identifiers may be supplied, a mutual-exclusion rule the schema itself does not express (all params optional, none required).

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 (read) and resource (one issued invoice), enumerates what is returned (seller, buyer, lines, totals, VAT breakdown, payments), and names the three lookup keys. It is clearly distinguishable from siblings like szamlazz_get_invoice_pdf or szamlazz_create_invoice.

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 the selection rule ('give exactly one'), the multi-match behavior for order numbers (last one returned), the scope restriction (only invoices in this account), and an additional use case (existence check before re-issuing). Lacks an explicit named alternative tool for that re-issue check, but the guidance is otherwise concrete.

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