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

Annotations already carry the read-only/idempotent safety profile, yet the description still adds real value: the masked return fields, the 'required for any spend' constraint, the needsReentry=true failure mode (charges rejected until re-entered in console), and the billing:read permission scope. That is more context than the annotations provide.

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-loaded with the resource and what it returns, then the usage constraint, then the edge case. Dense but every clause earns its place; the trailing bracketed account/key note and double parentheticals add minor visual clutter.

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 zero-param, no-output-schema read tool, the description covers the return shape (masked card), the precondition for calling it, the degraded-state behavior, and the required permission. Nothing an agent needs to invoke or interpret it is missing.

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 tool takes zero parameters, so no per-parameter semantics are required — schema coverage is 100% and there is nothing to document. Baseline 4 applies; there is no parameter detail the description could add.

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+resource and even scopes the return ('The saved card (masked: brand, last 4 digits, expiry)'). It is trivially distinguishable from the sibling gpucloud_add_payment_method because this reads the stored card rather than creating one.

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?

'Required for any spend by an agent' tells the agent exactly when this call is a prerequisite, and the needsReentry note explains a state where re-entry must be routed to the console. It does not explicitly name an alternative tool, so it falls just short of a 5.

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