download_invoice

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

Goes well beyond the readOnlyHint/openWorldHint annotations by disclosing a non-obvious runtime trait: the upstream API returns binary PDF but MCP wraps it as JSON with base64 'data' that must be decoded. With no output schema, this is exactly the behavioral detail an agent needs. Annotations already cover the safety profile, and the description adds the payload-handling quirk.

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 tightly packed sentences, front-loaded with what the tool does and followed by the essential handling instruction. Nothing is redundant and every clause earns its place.

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-parameter read tool with no output schema, the description supplies the missing return-value information (JSON with encoding=base64 and data, decode to .pdf), which is the main gap the structured fields leave. Nothing needed to invoke it correctly is absent.

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 a single documented parameter ('invoice_id: Номер инвойса'), so the schema already carries the semantics. The description adds no further parameter detail (no format, range, or where to obtain the ID). Baseline 3 is appropriate.

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 ('Скачать инвойс в PDF') plus the output format, which implicitly separates it from the metadata-oriented siblings get_invoice/get_invoices. The parenthetical WHMCS method name ('whmcs/download_invoice') anchors it to a concrete operation. An agent can tell what it does without opening the 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?

The PDF output format implies when to use it (when the actual file is needed rather than invoice data), but no alternative such as get_invoice is named and no when-not condition is given. Usage is only implied, which is the definition of a 3.

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