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

Annotations already declare readOnly/idempotent/non-destructive, and the description reinforces 'read-only; changes nothing' while adding behavior the annotations cannot: what messages_left and unseen mean relative to wait_for_email, that active=false means mail is refused until renewal, and the x402 renewal endpoint. This is substantive context beyond structured fields.

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 the return shape, then usage, then field semantics, then the expiry/renewal caveat, then sibling routing and the no-parameter note. Every sentence carries distinct information; nothing is redundant padding.

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 takes on the burden of enumerating return fields and their meaning, plus expiry consequences, and it is rich enough that an agent can call and interpret the tool correctly without further documentation.

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?

Zero parameters, so baseline is 4, but the description goes further by explaining why none exist: the inbox is selected by the connection's auth token, so there is no address or id to pass. That resolves an ambiguity an empty schema would otherwise leave.

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?

Opens with a specific verb+resource (show the inbox's email address and state) and enumerates the exact returned fields. It also names sibling tools ('to receive mail use wait_for_email; to browse it use list_emails'), making it unambiguous against the other five tools.

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 explicit when-to-use conditions ('use it first to get the address to give a website, or to check the inbox is still active before waiting for mail') and routes the agent to the correct alternatives for receiving vs browsing mail. Both the trigger and the alternatives are stated, not inferred.

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