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

Annotations already cover the safety profile (readOnly=false, destructive=false, idempotent=true, closed world), so the bar is lower. The description adds genuinely new behavioral context: the 100-message cap and the all-or-nothing atomicity ('If any message is not found, nothing changes'), which an agent could not infer from the annotations.

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 short sentences with zero filler; the scope and the atomicity caveat are both front-loaded and each earns its place.

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?

Output schema exists, so return values need not be described, and atomicity is covered. However, for a mutation tool with three parameters the description omits what add_labels/remove_labels do and provides no routing to the single-message or general-message siblings.

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% at the property level, so the description should compensate, but it never mentions add_labels or remove_labels at all. It only echoes the 100-item cap on message_ids, leaving two of three parameters unexplained in both schema and description.

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), resource (sent messages), and batching scope (up to 100). The 'sent' qualifier distinguishes it from the sibling bulk_update_message_labels, and the batch wording distinguishes it from the single-message update_sent_message_labels.

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 gives no when-to-use or when-not-to-use guidance, and never mentions the alternatives (single-message update_sent_message_labels or the non-sent bulk_update_message_labels). The 100-message cap is a hard limit, not a routing rule.

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