Retrieve Upcoming Invoice

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

The readOnlyHint annotation already declares this as a safe read operation, so the description does not need to repeat that. The term 'Preview' aligns with the annotation, but the description adds no additional behavioral context beyond what the annotation provides (e.g., what happens if no upcoming invoice exists, or whether it requires specific permissions). It is adequate but not enriching.

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 consists of two concise sentences with no unnecessary words. The first sentence immediately states the tool's core purpose, and the second adds a practical use case. Every sentence earns its place, making it highly efficient.

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?

Given the tool's simplicity (two parameters, read-only, no output schema), the description covers the essential purpose and a typical use case. It does not explicitly mention that the subscription parameter is optional or what the return object contains, but the core functionality is clear. The lack of output schema is compensated by the inferred return type (invoice preview). Mild gaps exist but are not critical.

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?

The input schema provides descriptions for both parameters (customer ID and subscription ID) with regex patterns, achieving 100% coverage. The description does not add any extra meaning or clarification beyond what the schema already states. With complete schema coverage, the baseline score of 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?

The description uses a specific verb 'Preview' and identifies the resource 'next upcoming invoice for a customer,' clearly distinguishing it from sibling tools like retrieve_invoice (which retrieves a specific invoice) and list_invoices (which lists all invoices). The additional sentence 'Useful for showing what will be charged' reinforces the intended use case.

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 states it is 'useful for showing what will be charged,' which provides some context for when to use it. However, it lacks explicit guidance on when not to use it or what alternatives exist (e.g., retrieve_invoice for existing invoices, or create_invoice to generate). The guidance is implied but not fully explicit.

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