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

Annotations already cover the safety profile (readOnlyHint=false, destructiveHint=false, openWorldHint=false), so the mutation risk is known. The description adds the return behavior ('Returns every label the message carries afterwards'), which matters since there is no output schema, but omits idempotency, behaviour when removing a label that isn't present, and the 20-label cap.

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 sentences, zero filler, with the core action first and the label-syntax constraint immediately after; the return-value note is a brief trailing clause. Every sentence 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?

For a mutation tool with annotations covering safety and no output schema, the description covers the action, the label format, and the return value. Minor gaps remain: the maxItems=20 limit and the default folder are only in the schema, and error behaviour on invalid or duplicate labels is unstated.

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 coverage is 100%, so the baseline is 3, and the description genuinely adds value by specifying the label format (lowercase, letters/digits/dots/dashes/underscores) and giving concrete examples such as 'needs-reply' and 'invoice'. It does not restate the add/remove arrays or the folder default.

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 and resource ('Add or remove labels on a message') and clarifies that a single call can both add and remove. It does not differentiate from near-neighbour siblings like flag_message or move_message, which an agent may reasonably confuse with labelling.

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 phrase 'Useful for keeping track of what has been handled' implies a workflow context, but there is no explicit when-to-use versus flag_message/move_message, and no mention of when not to use it or prerequisites.

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