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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, establishing the safety profile. The description adds value beyond that by disclosing the shape of the response (plan, wallet balance, per-feature usage and remaining allowance) in the absence of an output schema, though it doesn't state units (dollars vs. credits) or the period boundaries for 'this period'.

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?

Two sentences, no filler, with the payload contents front-loaded and the actionable timing advice second. Every clause 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?

For a zero-parameter read-only tool with no output schema, the description covers the essentials by enumerating returned fields. Minor gaps remain around units, period definition, and how it differs from the billing-mutation siblings, but nothing that would cause a misinvocation.

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 no parameters, so the schema carries nothing the description needs to compensate for; baseline applies. The enumerated feature list (searches, enrichment, SMS, postcards, letters, AI) usefully tells the agent what dimensions of data to expect back.

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 enumerates exactly what the tool returns — plan, wallet balance, per-feature usage and remaining allowance for six named channels — so the resource is unambiguous. It does not, however, name or distinguish itself from near-siblings like spur_manage_billing or spur_show_billing_options, which the agent must infer are mutation/option-oriented rather than read-only status.

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?

"Check this before large sends" gives a concrete triggering condition tied to the costly send operations (send_letters, send_postcards, send_sms). There is no explicit when-not or a named alternative for modifying billing, so it stops short of full routing guidance.

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