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, which tells the agent this is a safe, repeatable local mutation. The description adds only the fact that both addition and removal are supported in one call. It says nothing about what happens when a label is unknown, whether add and remove may overlap, or any auth requirement — acceptable given annotations, 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?

A single front-loaded sentence that states the operation with zero filler. Nothing is repeated from the title beyond what is needed for clarity.

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 described, and annotations cover the safety profile. What is missing is the routing information an agent actually needs in a 40-tool namespace: differentiation from update_message_labels and the bulk variant, and edge-case behavior for unknown or duplicate labels.

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?

Top-level schema description coverage is 0%, but property names (add_labels / remove_labels / message_id) are self-explanatory and the array item schemas carry the label constraints (128 bytes, no control chars, max 32). The description confirms the add-or-remove semantics that map to the two array parameters, but adds no extra syntax or format detail. Baseline 3.

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?

Specific verb pair (add/remove), resource (labels) and scope (sent message) are all present, so an agent knows exactly what mutation is performed. It does not, however, explicitly disambiguate itself from the close sibling update_message_labels or from bulk_update_sent_message_labels; only the name and the word 'sent' carry that distinction.

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 states the operation but gives no when-to-use guidance: no indication of when to prefer update_sent_message_labels over update_message_labels, over bulk_update_sent_message_labels, or of any prerequisite (sent message must exist, labels must already exist). Usage is only inferable from the name.

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