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

Annotations already declare the safety profile (not read-only, not destructive, not idempotent, closed-world). The description adds important behavioral context beyond annotations: labels apply to every message currently in the conversation, both received and sent, and future messages are excluded. It does not cover permissions or return behavior, but the annotations and output schema reduce the burden.

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 tightly written sentences with no waste. The core behavior is front-loaded, and the temporal caveat follows immediately without unnecessary detail.

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?

Given the annotations and the presence of an output schema, the description covers the key non-obvious behavior: scope to current messages and exclusion of future messages. It could say more about permissions or repeated-call behavior, but for an agent selecting and invoking the tool, the essential context is present.

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% for the top-level parameters, so the description must compensate. It only implies that add_labels and remove_labels are used to add or remove labels and that thread_id identifies a conversation; it adds no label format, array limits, case-sensitivity, or add/remove edge-case semantics beyond what the parameter names already suggest.

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 states a specific action: adding or removing labels on every message currently in a conversation, received and sent. This scoping clearly distinguishes it from message-level and sent-message-level label tools, which update individual messages rather than an entire conversation's existing messages.

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 implies the tool is for applying labels across all current messages in a thread and warns that later messages will not receive them. However, it does not explicitly name alternatives such as update_message_labels or update_sent_message_labels, nor does it state when-not to use this tool.

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