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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the description need not restate safety. It adds valuable behavioral detail by specifying the response fields (ids, addresses, subjects, previews, times) and explicitly excluding full bodies, plus the default-inbox behavior. This goes beyond what the annotations convey.

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 sentences, each earning its place: the first gives the core action and ordering, the second defines the return scope and exclusion, and the third covers the optional parameter and the sibling alternative. No redundancy beyond the harmless 'Read-only' reinforcement.

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?

The description makes key context explicit: default inbox behavior, return field set, and lack of full bodies. Since there is no output schema, this is important. Slight gap: the meaning of 'limit' and pagination are not addressed, but for a list operation with only two simple parameters, the description is mostly complete.

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?

The schema documents inbox_id well, and the description reinforces it ('defaults to the newest inbox'). However, the 'limit' parameter has no description in the schema and receives no explanation in the description. With schema description coverage at only 50%, the description partially compensates but leaves limit semantics unclear.

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 states a specific verb ('List'), a resource ('messages in a Cooper inbox'), and an ordering guarantee ('newest first'). It also distinguishes itself from cooper_get_message by clarifying it returns only previews, not full bodies. An agent can clearly understand what this tool does and how it differs from the closest sibling.

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?

The description clearly indicates this is for listing messages and explicitly points to cooper_get_message for reading a single message, giving useful routing guidance. It does not, however, mention any exclusions such as when to use cooper_search or other sibling tools, so it falls slightly short of fully explicit when/when-not guidance.

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