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

Annotations only declare readOnlyHint/destructiveHint/openWorldHint, but the description adds real behavioral depth: the meaning of access='write' vs 'read' (write-changing tools refused), the folder restriction semantics for file tools, the equivalent calendar restriction, AgentMail having neither, and the empty-state guidance. This is exactly the context annotations cannot carry.

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 the core purpose, then progressively details the returned fields and edge cases in tight sentences. It is long but nearly every sentence conveys distinct semantics; a minor tightening of the folder/calendar clauses is possible.

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?

With no output schema, the description still explains what is returned (provider, status, access level, folder/calendar restrictions) and how to interpret each field, plus the no-connection fallback. An agent has everything needed to call it and act on the result.

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 baseline is 4; the schema is empty and there is nothing further the description could add about inputs. Instead it documents the semantics of returned fields (access, folders, calendars), which is a bonus rather than a parameter gap.

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?

Names a specific verb and resource (list every mailbox) and enumerates the providers covered (Gmail/Workspace, Microsoft/Outlook, AgentMail). It clearly separates itself from siblings like list_workspaces, list_mail_folders and list_messages by scoping to mailbox-level authorization.

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?

Gives concrete usage context: only mailboxes with status 'active' can be used by other tools, mailboxes with no access are omitted, and it tells the agent what to do when nothing is connected. It stops short of explicitly stating it should be called first to discover available mailboxes before invoking provider-specific tools.

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