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

Goes well past readOnlyHint/openWorldHint by disclosing the unread-state side effect (it does not mark the message read), the HTML-to-text conversion behavior, the attachment list in the response, and a prompt-injection warning that content is untrusted. That untrusted-content caveat is safety-critical context the annotations cannot express.

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, front-loaded with what is returned, then the state side effect, then the security caveat. No filler and nothing buried.

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, yet the description enumerates the return shape (headers, body, attachment list), states the state side effect, and warns about untrusted content. All inputs are covered by a fully-described schema, so an agent has everything needed to call and interpret it.

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 uid, folder, account, maxChars and format are already fully documented, including the note that uid comes from search_messages. The description's 'text part, or HTML converted to text' slightly reinforces the format enum but adds nothing the schema does not already carry.

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 (read) and resource (one message) and enumerates what comes back: headers, body, attachment list. The singleness of 'one message' implicitly contrasts with search_messages, but the description never names that sibling 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?

'Does not mark it as read' is a genuine routing hint that tells the agent a separate call to `mark` is needed, effectively distinguishing this tool's scope from the mutation sibling. No explicit prerequisites or 'when not to use' statements beyond that.

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