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

Annotations include readOnlyHint:false, destructiveHint:true, idempotentHint:false. The description adds that it 'never permanently deletes mail', which clarifies the destructiveHint is limited to non-permanent destructive actions (moving to trash, modifying flags). This adds useful context beyond annotations. It also implies multiple operations are possible but doesn't detail side effects like folder required existence (exact folder name is in schema). Overall, good supplemental behavioral context.

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?

Two sentences, front-loaded with the core purpose and operations, followed by a safety guarantee. No fluff, every word earns its place. The structure is concise and informative.

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?

The tool has nested operations, 100% schema coverage, and an output schema (though not shown, its presence reduces need for description). The description covers the essence: multiple mutations, never permanent deletion. It does not mention idempotency or key requirements, but these are well-documented in the schema (idempotencyKey description). For an agent selecting and invoking correctly, the description is sufficient. A small gap: it doesn't state that folder must already exist for move/copy, but that's in the schema. Overall, nearly complete.

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 the baseline is 3. However, the description provides top-level guidance: 'Set message flags, move, copy or trash a message' which maps to the operation object. It also highlights 'Choose one operation' summarizing the oneOf constraint. The nested object descriptions are thorough, but the description adds the high-level intent that helps an agent grasp the operation's purpose. This exceeds the baseline.

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 states a specific set of operations (set flags, move, copy, trash) with a clear resource (message) and explicitly notes it never permanently deletes mail. It distinguishes itself from siblings like send_email or forward_email by focusing on message mutations. Although it doesn't name a sibling, the verb+resource+operation list is specific and unambiguous.

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 clarifies that it modifies messages (flags, move, copy, trash) and explicitly excludes permanent deletion. It implies it's for post-delivery message management, but does not explicitly say when to use this vs. alternatives like get_message or save_draft. The lack of explicit exclusion of non-mutation operations is a minor gap; still, the intended use case is clear.

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