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

With no annotations, the description carries the full disclosure burden and handles it well: it states the operation is non-mutating ('not marked read'), clarifies HTML-only messages are delivered as text, and warns that message content is third-party data that must never be treated as instructions. Those are exactly the traits an agent needs for a mail-reading tool.

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: what it returns, the non-mutation fact, and the injection-safety warning. The most decision-relevant information (uid source, return contents) is front-loaded with no filler.

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?

There is no output schema, and the description compensates by summarizing the return shape (headers, body text, attachment names). Combined with the safety and mutation context, an agent has everything needed to call 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?

Schema coverage is 100%, so uid, folder, and mailbox are already documented at the source, including the 'uid from mail_search' linkage. The description repeats the uid provenance but adds no syntax, format, or defaulting detail beyond the schema, so the baseline 3 applies.

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 by uid') and enumerates what is returned: headers, text rendering of HTML-only messages, and attachment names. It ties itself to the sibling mail_search as the source of the uid, so an agent can distinguish it from mail_search and mail_draft without opening a schema.

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?

Explicitly routes the agent: the uid comes from mail_search, and the description notes it is not marked read, which is the key usage condition for a read tool adjacent to mail_draft/mail_send. It doesn't state an explicit when-not or a case where another tool should be preferred, so it stops short of 5.

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