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

The readOnlyHint annotation already establishes the read-only safety profile. The description adds that this maps to 'GET /plans' and supports an optional state filter, which is useful context. However, it does not disclose return format, pagination behavior, or any operational caveats beyond the annotation and schema.

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 a single sentence that front-loads the primary purpose and then provides the API endpoint mapping. There is no filler, redundancy, or unnecessary explanation.

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 read-only list operation with six parameters all described in the schema, the description plus schema is sufficient for an agent to select and invoke the tool correctly. The lack of an output schema means return-value details are not specified, but the operation's purpose is clear enough that this is a minor gap rather than a blocking omission.

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 description coverage is 100%, so all six parameters are already fully documented in the input schema. The description's mention of 'optionally filtered by state' simply echoes the existing 'state' parameter, adding no new semantic meaning beyond the structured 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?

The description uses a specific verb ('List'), a concrete resource ('the site's subscription plans'), and an additional dimension ('optionally filtered by state'). It is immediately distinct from sibling tools like recurly_list_accounts, recurly_list_invoices, and recurly_list_subscriptions because it names 'plans' explicitly.

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 context signals show this is the only tool for listing plans among the siblings, so the intended use case is clear: call this when you need subscription plans rather than accounts, invoices, or subscriptions. It does not explicitly state when not to use the tool or name alternatives, but the resource naming provides sufficient contextual guidance.

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