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

Annotations already declare readOnlyHint=true and destructiveHint=false, yet the description adds real semantic context: it cannot start, change or cancel a subscription, and plans alter result depth but never truth, with the response enumerating exactly what does not vary. That guarantee (identical registration verdict, rdap_source and price across tiers) prevents an agent from misattributing tier differences to the data, which no annotation or schema conveys.

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 what the tool returns, then the trigger, then the read-only constraint, then the tier-invariance guarantee. Four sentences each carry distinct information, though the final sentence is dense enough that it slows reading compared with the crisp open.

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 need not be documented, yet the description still summarizes the payload and states the invariance rule an agent would otherwise guess at wrongly. For a zero-parameter, read-only, non-destructive tool, nothing needed for correct invocation 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 there is nothing for the description to disambiguate and the baseline of 4 applies. The description correctly spends no space restating an empty 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?

States a specific verb and resource plus the exact return contents: available plans with inclusions, the caller's current tier, and any allowance used. The resource is unambiguous, though the description never contrasts itself with billing-adjacent siblings like set_budget or get_finance, leaving that separation to inference.

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?

"For when the user asks what a tier costs or has reached a limit" gives an explicit trigger condition, and the read-only sentence rules out subscription mutations. It stops short of naming the sibling an agent should call instead when the intent is actually to change a plan or budget.

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