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

Annotations declare readOnlyHint=false (a write) and destructiveHint=false, so the safety profile is covered. The description adds valuable behavioral context beyond annotations: the default visibility scope, how to scope to a property, and the restriction that platform-wide annotations are reserved for system events.

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?

Three tightly written sentences with zero waste. The purpose is front-loaded, followed by scope details and a restriction, all clearly structured.

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 and annotations cover the safety profile, so return values and write safety are handled. The description fills in scoping behavior and the platform-wide restriction, but omits any mention of required parameters or the idempotency key, though those are covered by the schema.

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 low (43%), so the description should compensate for undocumented parameters. It only mentions siteUrl, which is already described in the schema, and provides no meaning for color, label, category, or description. For a 7-parameter tool, this leaves significant gaps.

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 verb ('Adds') and resource ('a chart annotation'), making the tool's purpose immediately clear. It does not explicitly differentiate from sibling tools like update_annotation or delete_annotation, but the create action is 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?

It explains the default scope ('account', visible only to the user) and instructs when to pass siteUrl versus omit it. It also states a when-not condition: platform-wide annotations cannot be created via this tool, which is a clear exclusion.

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