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

Annotations already declare readOnly/idempotent/closed-world, and the description adds non-obvious behavior the annotations cannot express: no provider round-trip occurs, a `found` result does not imply a complete thread body, and a `not_cached` result is not a definitive negative. It also discloses that RFC IDs are scoped to the selected mailbox, which materially affects interpretation of misses.

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?

Four tight sentences, front-loaded with the operation and its defining constraint before the caveats. Every clause carries unique information — result states, follow-up tools, negative-result caveat, scoping — with no restatement of the name or annotations.

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 must characterize return states, and it does: `found` yields cached opaque message/thread IDs while `not_cached` is a non-definitive miss. Combined with the follow-up path for completing thread bodies, an agent has enough to call this correctly and interpret both outcomes.

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?

Coverage is 50% (only rfcMessageId is schema-documented), so the description needs to carry weight, and it does by clarifying the scope semantics that accountId controls ('RFC IDs are scoped to the selected mailbox', implied in the schema description too). It does not explain format constraints or what happens if accountId does not own the cached thread, leaving one parameter thin.

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 ('Resolve an exact RFC Message-ID') and pins the scope hard with 'using only durable mailbox cache' / 'never searches the provider', which functionally separates it from find_message_by_rfc_message_id. It does not explicitly name that provider-search sibling, so the agent must infer the name from the contrast rather than being routed to it.

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 concrete next-step guidance for the success case ('queue request_cached_thread_refresh and then poll get_cached_thread') and sets expectations for the miss case ('not_cached is not proof that the provider mailbox has no matching message'). It stops short of saying which alternative tool to call when the caller needs an authoritative provider lookup, leaving that inference to the reader.

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