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

There are no annotations indicating read-only or destructive behavior, so the description carries that burden. It clearly states that the operation changes read status only and explicitly excludes content-reading and triage behaviors. It does not mention response shape or side effects like unread-count updates, but for this simple mutation the stated boundary is sufficient.

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?

A single, well-structured sentence covers the action, the target, the two modes, and a clarifying exclusion. No filler, no redundancy, and key information appears upfront.

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 gives enough context for an agent to invoke the tool correctly for a simple mail mutation. It omits return-value details, but the tool has only two well-documented parameters and no output schema, so the missing information is unlikely to cause incorrect usage.

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 input schema already fully describes both parameters (ids and all) and their intended use. The description adds minor value by framing them as 'specified' versus 'explicitly all current inbox messages', but it does not meaningfully expand beyond the schema.

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 names a precise action ('Mark... as read'), a precise target ('specified inbox message IDs, or explicitly all current inbox messages'), and distinguishes the selective case from the bulk case. It is immediately obvious what the tool does.

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 explains the two invocation modes (specific IDs vs. all current messages) and adds an explicit exclusion clause: it does not read, summarize, or triage content. It does not explicitly name sibling tools like mail_read or mail_search, but the boundary is clear enough for an agent to choose correctly.

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