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

No annotations are provided, so the description carries the full behavioral burden. Saying 'Encrypt and store' usefully implies the card data is not persisted in the clear, but nothing is said about whether it replaces an existing default, whether it charges the card (e.g., zero-dollar authorization), what permissions or billing state are required, idempotency, or how highly sensitive fields (card_ssn, card_cvc, card_password_2digits) are handled.

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 single sentence has zero waste and is front-loaded with the action and resource. It is efficiently written, though its very compactness is part of the under-specification problem rather than a stylistic flaw in itself.

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?

An output schema exists so return values needn't be explained, but for a 9-parameter, PCI-sensitive mutation with no annotations and no parameter documentation, the description is far too thin. It omits replacement semantics, required billing state, and any guidance on the sensitive optional fields.

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 0% across 9 parameters, so the description must compensate, and it supplies no parameter meaning at all. Cryptic, high-risk fields such as payment_type (defaulting to 'LOCAL02'), card_password_2digits, and card_ssn are entirely unexplained, leaving the agent to guess at regional card schemes and security-code conventions.

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 pair ('Encrypt and store') and a concrete resource ('a default payment method') scoped to the caller's billing principal, so the agent knows this is a card-registration write, not a charge. It does not distinguish itself from the closest sibling, purchase_credits_with_card, so the agent must infer the register-vs-charge boundary.

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?

There is no statement of when this tool should be used versus purchase_credits / purchase_credits_with_card, no prerequisites (e.g., whether the caller must already have a billing principal), and no exclusions. The agent is left to infer that this registers a reusable payment method rather than executing a payment.

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