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

readOnlyHint=true is consistent with the 'without sending mail' claim, so there is no contradiction. The description usefully discloses the three validation categories, but adds nothing about what a validation actually produces (errors thrown vs. a report), whether it consumes quota, or account/permission requirements.

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 front-loaded sentence with no filler; every clause adds information. Appropriate size for the message.

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?

With 11 parameters, no output schema, and only readOnly/openWorld annotations, a one-line description is thin. An agent still cannot tell what the tool returns on success or failure, which is the central question for a validation tool.

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 0%, so the description carries the full burden. It names only recipient, sender, and attachment concerns, leaving subject, bodyHtml, bodyText, replyAll, accountId, and replyToMessageId completely undocumented in both schema and prose.

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 gives a specific verb ('Validate') plus the exact resources checked (recipients, sender, attachment limits) and explicitly negates sending mail, which cleanly separates it from the sibling send_email. It stops short of naming send_email as the alternative it pairs with, so differentiation is implied rather than stated.

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?

'Without sending mail' implies the pre-flight/dry-run context, so an agent can infer this runs before send_email. However, it never states when to call it versus just calling send_email or create_draft, and names no alternatives explicitly.

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