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

Annotations already declare readOnly=false, idempotent=true and destructive=false, so the safety baseline is covered. The description adds genuinely useful context beyond that: batch size limit of 100 and all-or-nothing atomicity ("All targets must be accessible or nothing changes"), which tells the agent how partial failures behave.

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, no filler, with the batch limit and the atomicity guarantee front-loaded. Every clause carries information the agent needs.

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?

An output schema exists, so return values need not be explained, and the annotations cover the safety profile. Between the description's atomicity note and the schema's item-level label constraints, an agent has enough to call it correctly, though the add/remove parameter roles remain thin.

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 should compensate, yet it never mentions add_labels or remove_labels and only vaguely alludes to "label changes." It does convey the message_ids cap of 100, but the add/remove split and label validity rules are left to the schema.

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 (apply label changes) and a precise resource scope (up to 100 saved outgoing messages), which cleanly separates it from the generic bulk_update_message_labels sibling. It never names the sibling explicitly, so it falls short of a 5.

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?

No when-to-use guidance and no routing to alternatives such as update_sent_message_labels for a single message or bulk_update_message_labels for a different resource class. The agent must infer the choice entirely from the tool name and the 100-item cap.

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