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

Annotations already establish readOnlyHint=false, destructiveHint=false, and openWorldHint=false, so safety framing is covered. The description adds substantive context beyond that: only the specified IDs are affected, and the per-conversation cursor behavior means acknowledging one conversation does not hide another. It still does not disclose idempotency or whether unread state is restored on failure.

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 short sentences with the action and scope front-loaded and no filler. The middle sentence about independent conversation cursors is slightly abstract and reads as rationale rather than directly actionable instruction, which keeps it from a 5.

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 single-parameter mutation with no output schema, the description covers the action, the scope limit, and the per-conversation isolation behavior. What it omits is the outcome surface, such as whether the call reports how many messages were changed or what happens to IDs that no longer exist.

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?

With one parameter at 0% schema description coverage, the description must carry the load, and it does partially by tying the IDs to "returned" messages, implying they come from a prior inbox fetch rather than arbitrary UUIDs. It does not mention the 100-item cap, uniqueness expectations, or behavior for IDs outside the caller's inbox, so the compensation is incomplete.

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?

"Mark these returned message IDs as read" states a specific verb and resource, and the title/name clarify this is a state mutation rather than a fetch. It distinguishes itself from read-side siblings like get_inbox or list_messages, though it never names an alternative explicitly.

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?

"after reviewing them" implies the intended sequence (fetch, then acknowledge), which gives an agent moderate context. There is no explicit when-to-use vs when-not guidance, no mention of prerequisites such as needing IDs from a prior get_inbox call, and no mention of what happens if an ID is stale or already handled.

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