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

Goes well beyond readOnlyHint=true by naming exactly what does NOT happen ('nothing is marked read, moved or deleted'), disclosing folder/spam scope, flagging untrusted sender content as data not instructions, and clarifying the receive-only nature of the toolset. These are non-obvious behavioral traits an agent cannot infer from annotations or 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?

Three tight sentences, front-loaded with the core action, followed by the safety constraint and the trust caveat. No redundancy or filler.

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 compensates by naming the returned fields (sender, subject, short preview) and the read-only guarantee. Minor gap: it does not discuss pagination or what happens when limit is reached, though the limit parameter is documented in the schema.

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 already 100%, but the description adds meaning the schema lacks: 'newest' implies result ordering, and 'all folders, spam included' defines the addressable scope of the address parameter. It does not elaborate on limit or since beyond what the schema states.

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?

States a specific verb and resource ('List the newest messages delivered to one of this account's mailboxes') and scopes it to all folders including spam. It does not explicitly distinguish itself from siblings like get_message or wait_for_message, though the 'short preview' phrasing hints that full bodies come from elsewhere.

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?

Usage is only implied: 'newest messages' suggests a listing/browsing use case, but the description never says when to use this versus get_message (full content) or wait_for_message (waiting for arrival). No explicit exclusions or alternative routing are given.

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