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

Annotations already declare readOnly/idempotent/non-destructive, but the description adds genuinely non-obvious behavior beyond them: the precise definition of what 'counts' as a reply (Sent message in same conversation or same subject, or the answered flag) and the explicit 'Reading marks nothing as read' caveat. It stops short of describing pagination or rate limits, but the side-effect disclosure is the important piece.

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?

Dense but well-front-loaded: the core purpose leads, then exclusions, the reply definition, and the return shape. Most sentences earn their place, though the reply-definition clause is long enough to slow scanning slightly.

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 correctly covers the return contract (oldest-first ordering, preview, days waited, unread flag, toYou vs Cc). Combined with the exclusion rules and reply definition, an agent has enough to call and interpret results, though edge cases like what happens at the 30-day cap are unaddressed.

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 every parameter (days, limit, mailboxes, directOnly, includeRead, excludeAddresses) is already documented in the schema, including defaults. The description adds behavioral framing but no parameter-specific syntax or constraints beyond what the schema provides, so the baseline of 3 applies.

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 ('Real people waiting on a reply... every person whose latest message the owner has not answered') and a clear scope. It implicitly distinguishes itself from siblings like waiting_on (the mirror direction) and catch_up (whose filtering logic it reuses), so an agent can tell them apart.

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?

Explains the filtering contract clearly (newsletters/marketing/auto mail excluded, 'judged as catch_up judges them') and sets expectations by noting 'Whether a message actually needs an answer is left to the reader.' However, it never names a sibling to call instead (e.g. waiting_on for the reverse direction), so usage is contextual rather than fully directive.

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