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

Annotations already declare idempotent=true, destructive=false, readOnly=false, so the safety profile is covered. The description adds genuinely new behavior: labels replaces the existing set, and an inbound message from outside will reopen a thread, plus cost-per-call with no charge on failure — useful for a paid mutation.

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?

Purpose is front-loaded, but the pricing is stated twice ('$0.001.' followed by 'Costs $0.001 per call; nothing if it fails'), which is pure redundancy. The single long colon-chained sentence also buries the reopen rule at the end.

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 3-param mutation with no output schema, the description covers state semantics, label replacement, re-open behavior, and billing — enough to call it confidently. It never clarifies that state is optional and labels can be updated alone, which is a minor omission.

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 67% and already documents message_id and the labels replacement behavior. The description adds meaning the enum lacks by glossing states — open as needs-a-reply, waiting as on-the-other-side — which is real value over a bare enum list.

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 ('mark') plus the resource ('email thread') and enumerates the accepted states and label setting. It also names the sibling consumer, email_list_messages, so the agent can distinguish this mutation from the read-side filter tool.

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 motivating context ('so you never reread or answer twice what you already dealt with') and points at email_list_messages as the filtering counterpart. It does not, however, give an explicit when-not or contrast with email_update_inbox, so alternatives are only implied.

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