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

Goes well beyond the annotations: add-ons are removed unless re-listed, only one pending change is kept and replaced by newer ones, an invoice is created and payment attempted, and changes bill or credit the customer. It also discloses the confirmation/API-key requirements, giving the agent real side-effect awareness.

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-loads purpose and timing, then layers side effects and safety notes. Dense and mostly waste-free, though the confirmation/API-key sentences add length that could be tightened slightly.

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 mutating subscription-change tool with no output schema, it covers side effects, timing, proration, and auth/confirm needs. Return-value detail is not required since no output schema exists; completeness is high, missing only explicit sibling routing.

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 coverage is 100%, so the baseline is 3, but the description adds genuine meaning: the add_ons replacement semantics, the proration-vs-timeframe interaction, and the default confirmation behavior. This enriches interpretation beyond the enum/schema text.

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?

Specific verb (change) plus resource (subscription) plus the exact mutable fields (plan or quantity), with timing options named. It clearly distinguishes itself from siblings like recurly_cancel_subscription and recurly_postpone_subscription without needing schema inspection.

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?

Explains when each timeframe applies (now = immediate invoice/payment; bill_date/renewal = later, no proration) and gives operational guidance about confirming before each call. It stops short of explicitly naming alternative sibling tools to use instead, but the context for choosing a timeframe is clear.

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