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

The description adds the behavioral detail that the message is marked 'by current user', which is not in the annotations. Since annotations already indicate non-read-only and non-destructive, this extra context is a slight bonus, though it does not cover all potential side effects like return values or persistence.

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?

The description is two concise sentences with no redundant information. It front-loads the core action and directly follows with the usage alternative, making it easy to parse and act on.

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?

Given the simple tool with a single parameter and no output schema, the description provides sufficient context: the action, the scope (current user), and a clear alternative for multiple messages. No additional context is necessary for an agent to invoke it correctly.

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 fully describes the single required parameter 'id' with a clear description. The tool description does not add any additional parameter-specific meaning beyond the schema, so it meets the baseline for full schema coverage.

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 clearly states the verb 'Mark' and the resource 'message as read', and it explicitly distinguishes this tool from the bulk variant by naming mark_messages_read_bulk. An agent can immediately understand the tool's purpose and scope.

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 gives explicit usage guidance: for several messages, prefer mark_messages_read_bulk with one call instead of many. This directly tells the agent when to use this tool versus the alternative, leaving no ambiguity.

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