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

Annotations already declare readOnlyHint=true and destructiveHint=false. The description goes beyond them by disclosing that each response signs a new invoiceUrl, that the link expires 1 hour after the response, and that it is safe to share with the buyer, plus livemode test-record behavior. It does not cover error modes or rate limits, but the added behavioral context is substantial.

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 the core action, then layers usage routing and behavioral detail in short, purposeful sentences. Slightly dense with the URL-expiry tangent, but no sentence is wasted.

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 two-parameter read tool with a full output schema, the description covers routing, mode selection, and the key return field's behavior. An agent has everything needed to call it correctly; output-schema details need not be restated.

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 id and livemode. The description's mention of livemode: false for test-mode records largely duplicates the schema, so it sits at the baseline of 3.

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 ('Retrieve a single invoice by id') and clearly scopes it as a single-record fetch, distinguishing it from list_invoices and regenerate_invoice by name. An agent can route to it without opening any 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?

Explicitly tells the agent to use list_invoices first to discover an id (by customer, invoice number, PO number, or chargeId), and names regenerate_invoice as the alternative for rebuilding the PDF. Both the when-to-use and the when-to-use-something-else conditions are provided.

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