list_mailboxes

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

With no annotations, the description carries full burden for behavioral disclosure. It reveals that the parentId filter is applied client-side after the Mailbox/get call, which is a crucial behavioral note. However, it does not explicitly state that the operation is read-only or safe, though that's implied by listing. Still, it provides enough transparency to avoid misuse.

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 concise, consisting of two sentences with no unnecessary words. It front-loads the primary purpose and immediately addresses a known limitation (result window), then provides actionable advice. 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?

For a listing tool with no output schema, the description is fairly complete: it defines purpose, limitations, optimization tips, and a behavioral detail. It could be enhanced by mentioning that the result is a list of mailbox objects, but that is standard for list operations. The given context is sufficient for correct usage.

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% with descriptions for both parameters. The description adds value beyond the schema by explaining the optimization benefit of 'properties' (roughly halves payload size) and clarifying that 'parentId' filtering is client-side. This contextual insight helps the agent use parameters more effectively.

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?

Description states explicitly that the tool lists mailboxes in the Fastmail account, using the verb 'list' and specifying the resource 'mailboxes'. It clearly distinguishes from sibling tools like get_mailbox_by_name or create_mailbox by focusing on listing multiple mailboxes.

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 provides clear usage guidance: it notes the default returns all mailboxes with full metadata, warns about result window limits on large accounts, and offers specific parameters ('properties' and 'parentId') to slim down the result or filter by parent. This helps the agent decide when and how to use the tool effectively.

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