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

With annotations covering safety (readOnlyHint=true, openWorldHint=false), the description adds meaningful behavioral detail: HTML is converted to text, links preserve their addresses, attachments are listed but not returned, and message content is untrusted sender data. This goes well beyond what the annotations provide and helps the agent interpret the result correctly.

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 tightly written sentences with no waste. The core purpose and return shape are front-loaded, followed by important behavioral and safety notes.

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 simple two-parameter read tool with no output schema, the description explains what is returned, how HTML and links are transformed, and the attachment behavior. Combined with annotations and 100% schema coverage, it gives the agent everything needed to call and interpret the tool 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?

Schema description coverage is 100%, so both parameters (id and address) are already fully documented in the schema. The description reinforces that this reads one message from a mailbox but adds no syntax or format details beyond the schema, so the baseline of 3 is appropriate.

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: 'Read one message of a mailbox'. It also enumerates the returned fields (sender, recipients, subject, text), which clearly differentiates it from list-level siblings like list_messages and wait_for_message.

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 implies usage by saying it reads one message, but it does not explicitly say when to use this instead of list_messages or wait_for_message, nor does it name alternatives. The id parameter schema mentions those siblings, but that guidance is not in the description itself.

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