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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is covered. The description adds meaningful behavioral context: it writes a file to a path, returns the path, includes specific content blocks, and notes that Free vs Pro affects branding and logo. It also warns that the bytes written are always PDF, which is a useful edge-case disclosure. It doesn't mention potential errors or side effects, but the annotations cover the main safety concerns.

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?

The description is two sentences, front-loaded with the main action and output, and every clause adds useful information. The first sentence states the purpose and the second explains the Free/Pro difference. No filler or repetition of the schema.

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 tool with 2 parameters, 100% schema coverage, and no output schema, the description is quite complete. It explains what the PDF contains, the default output path, and the branding difference. It doesn't describe the return value format (e.g., whether it returns just a path or a JSON object), but since there is no output schema, a brief note on the return shape would have been helpful. Still, the description is strong for the tool's complexity.

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 the schema already documents both parameters. The description adds value by explaining the output path default and the content of the PDF, which helps the agent understand what 'out_path' means and what the result will look like. It doesn't add much about the 'number' parameter beyond what the schema says, but the schema already says it's the invoice number returned by invoice_create. Overall, the description complements the schema well.

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 specific verb ('write'), a specific resource ('one stored invoice'), and the output format ('A4 PDF'), and it distinguishes itself from sibling tools by focusing on rendering an existing invoice to PDF rather than creating or listing invoices. It also clarifies the difference between Free and Pro branding, which is unique to this tool.

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 description explicitly says 'Call this tool to write one stored invoice as an A4 PDF', which gives clear context for when to use it. It does not explicitly name alternatives or exclusions, but the sibling list and the focus on rendering an existing invoice make the usage context clear enough. A small gap is that it doesn't explicitly say 'use invoice_create first' or 'use invoice_get to view instead', but the context is strong.

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