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

Annotations already declare readOnlyHint=false, idempotentHint=true, destructiveHint=false, and openWorldHint=false, so the safety and repeatability profile is covered. The description adds only that the operation is additive and subtractive in one call, and says nothing about behavior for unknown labels, partial failures, or limits, so it is adequate but thin.

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?

One short sentence with no filler and the key operation front-loaded. It is efficient, though its brevity comes at the cost of omitted detail rather than being a model of well-packed 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?

With an output schema present, return values need not be described, and annotations cover the mutation safety profile. Still missing is any routing versus the closely named sibling tools and any statement of failure semantics, which leaves the definition minimally viable for a mutating 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% at the property level, so the description carries the burden for three parameters. 'Add or remove labels' loosely maps to add_labels/remove_labels but conveys nothing about the required message_id, the 32-item cap, or the case-sensitive/128-byte label constraints.

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 pair ('Add or remove') and resource ('labels') scoped to a 'received message', which distinguishes it from update_sent_message_labels and update_thread_labels. It does not, however, distinguish itself from bulk_update_message_labels or explain the single-vs-bulk boundary.

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?

There is no guidance on when to use this tool versus update_sent_message_labels, update_thread_labels, or bulk_update_message_labels, nor any stated prerequisites (e.g. label must already exist). The agent must infer usage entirely from the name.

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