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

Annotations already indicate read-only, idempotent, and non-destructive behavior. The description adds meaningful behavioral context: null conditions for smart purchases being off, no named card, or nothing to say; the guarantee that a card number is never returned; and the plain-words reason in the output. No contradiction with annotations.

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 compact and front-loaded with the core purpose, then gives invocation hints, output shape, null semantics, and the safety note. Every sentence carries useful information without fluff.

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?

With an output schema present, the description does not need to restate return types. It covers selection context, required inputs, null behavior, fallback behavior, and a critical privacy guarantee. This is complete for an agent to decide when to call it and what to expect.

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%, so the schema already documents all four parameters. The description adds slight practical guidance ('when you know it' for the website, 'amount' for the total), but does not materially expand parameter meaning beyond the schema.

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 and resource: asking the vault which of the user's own stored cards earns the most for a purchase. It clearly scopes the tool to smart-purchase recommendation and distinguishes it from related tools like list_cards and name_card. The return value (card id, reason, runners-up) is also explicitly stated.

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 tells the agent what to pass (merchant name, website when known, amount) and how to interpret null results, including paying as usual. It points to name_card when no stored card has been named. It does not broadly compare against other sibling tools, but the usage context is clear enough for selection.

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