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

Annotations already cover the safety profile (readOnlyHint=true, openWorldHint=false), so the description's added value is the sentinel result semantics and the retry pattern, which are genuinely useful behavioral details beyond structured fields. It leaves out whether the OTP can be absent from an existing message or any rate/expiry behavior on the mailbox.

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?

Two sentences, zero filler, with the primary outcome front-loaded and the failure/retry case immediately after. Every sentence carries information the agent needs.

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 low-complexity, read-only lookup with two fully documented parameters, the description covers the success concept, the not-found sentinel, and the retry loop. With no output schema, a fuller accounting of the returned message fields would help, but nothing essential to calling it correctly is missing.

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 both `to` and `since` are already fully documented with examples, and the description adds no new meaning about them. The mention of 'an address' and 'newest' loosely maps to the schema but is baseline-equivalent.

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 names a specific verb (return), a specific resource (the newest message plus the OTP extracted from it), and the address it is keyed to, which clearly separates it from list_messages and get_message. It stops short of explicitly naming those siblings, so an agent must infer the distinction from 'newest' and 'extracted'.

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?

It gives concrete operational guidance for the polling case: found=false means nothing has arrived yet, wait a few seconds and retry. It does not state when to prefer get_message or list_messages instead, so there is no explicit exclusion, but the polling context is clearly established.

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