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

Annotations already cover the safety profile (readOnlyHint=false, destructiveHint=false, idempotentHint=true, openWorldHint=false), so the bar is lower. The description still adds a genuinely non-annotated behavioral trait: all-or-nothing atomicity ('All targets must be accessible or nothing changes'), which is critical for a bulk mutation. It does not address label auto-creation or permission requirements, keeping it short of a 5.

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 waste; the scope and limit are front-loaded and the atomicity guarantee follows immediately. Nothing could be trimmed without losing information.

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 no explanation, and annotations carry the safety profile. The description supplies scope, limit, and atomicity, which is close to sufficient for this tool; it leaves minor gaps around label existence/permissions and add-vs-remove semantics.

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 parameter level, so the description must compensate, and it only partially does: 'label changes' implies add/remove semantics and 'up to 100' mirrors message_ids maxItems, but it never distinguishes add_labels from remove_labels, nor mentions the 32-label cap or UUID formatting of message_ids.

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 names a specific verb+resource (apply label changes) with clear scope: bulk, same changes, up to 100, received messages. The word 'received' implicitly separates it from bulk_update_sent_message_labels, though the sibling is never named, so differentiation is inferable rather than explicit.

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?

Usage context is implied by 'bulk' and 'received messages' (use when applying identical label changes to many inbox messages), but there is no explicit when-not guidance and no pointer to the single-message update_message_labels alternative for one-off edits.

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