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

Annotations already declare readOnlyHint, destructiveHint, and idempotentHint, so the description's 'Read-only' is redundant. However, it adds valuable context beyond annotations: the 402 error behavior with upgrade_url and the 'signed-in account' scope, which informs the agent about authentication context and error handling.

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 two sentences with no wasted words. It front-loads the primary purpose and appends the relevant error detail. Every sentence earns its place.

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 no parameters, no output schema, and annotations covering safety, the description provides all necessary context: what data it returns (plan, usage, inbox count, limits), that it is read-only, and the upgrade_url detail. The 'signed-in account' implies authentication. Nothing essential 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 has zero parameters, so the description does not need to explain parameter semantics. The baseline for 0 params is 4, and the description adds no parameter info because none exists, which 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 clearly states the action (read) and the resource (the signed-in account's Cooper plan, monthly send usage, inbox count, and limits). It is specific and distinguishes itself from siblings by being the only billing-status read tool; no other sibling covers this domain.

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 implies usage for checking the current account's billing status and notes that a 402 from send or inbox create includes upgrade_url, which hints at when this tool is relevant (e.g., when handling such errors). It does not explicitly name alternatives or exclusions, but the context is clear given the tool's unique purpose among siblings.

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