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

Annotations indicate this is a non-destructive mutation (readOnlyHint: false, destructiveHint: false), which the description doesn't contradict. The description adds useful context about label ID sources (list_labels, system labels) and implies idempotent behavior (adding existing labels likely has no effect), but doesn't cover error cases, rate limits, or authentication needs beyond what annotations provide.

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?

The description is extremely concise (two sentences) and front-loaded with the core purpose. Every sentence earns its place: the first defines the tool, the second provides essential usage guidance. No wasted words or redundant 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?

Given the tool's moderate complexity (mutation with two parameters), 100% schema coverage, annotations covering safety, and an output schema (implied by context signals), the description is reasonably complete. It covers the what and how-to aspects well, though it could better address error handling or idempotency for a mutation 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?

With 100% schema description coverage, the schema fully documents both parameters (emailId, labelIds). The description adds marginal value by mentioning list_labels as a source for labelIds and giving examples of system labels, but doesn't provide additional syntax, format, or constraints beyond the schema's descriptions.

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 clearly states the specific action ('Add one or more labels') and target resource ('to an email message'), distinguishing it from sibling tools like remove_labels (which removes labels) and list_labels (which lists labels). The verb+resource combination is precise and unambiguous.

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 provides clear context for when to use this tool by referencing list_labels to find label IDs and listing common system label IDs. However, it doesn't explicitly state when NOT to use it (e.g., vs. remove_labels) or mention prerequisites like needing a valid emailId from search_email or get_email, though the schema hints at this.

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