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

readOnlyHint=true already establishes this is a safe, non-mutating read, so the description needn't restate safety. It adds useful payload context (totals, status, dates, contact, contact person) and cites the underlying endpoint, but says nothing about errors on missing ids, auth requirements, or whether the embed expansion changes cost/latency.

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?

Two tight clauses with the resource and lookup key front-loaded, followed by the payload summary and the API mapping. No filler, no redundancy.

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?

For a single-record read with full schema coverage and readOnlyHint annotations, the essentials are present, and the field listing partially compensates for the absent output schema. It falls short of 5 only because return-shape details (e.g., nested contact object structure) are left entirely unaddressed.

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 (invoice_id, embed) are already documented in the schema and the baseline of 3 applies. The description only echoes the id-based lookup and never mentions the embed parameter's effect.

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 ('Fetch a single invoice by id') and enumerates the payload's key fields, which cleanly separates it from sevdesk_list_invoices by scope. It does not explicitly name the list sibling as an alternative, so it stops short of a 5.

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 implied by 'by id' — an agent can infer this is the lookup path when a specific invoice id is known. There is no explicit when-to-use vs when-not-to-use statement and no routing to sevdesk_list_invoices for discovery, which is what a 4-5 would require.

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