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

Annotations already indicate this is a non-read-only mutation with destructiveHint=false and idempotentHint=false. The description adds useful approval context: MCP approvals are configurable by the organization administrator, and business workflow approvals always apply. It does not explain overwrite behavior, validation rules, or what happens if the field is not defined.

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 front-loaded and efficient: two sentences with no filler. The second sentence about approvals is relevant context, though its phrasing is somewhat generic and organizational.

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?

This is a mutation tool with a nested arbitrary-value parameter, 0% schema description coverage, and no parameter guidance in the description. The approval note is helpful, and an output schema exists to cover return values, but the description is insufficient for an agent to understand how to structure or validate the required inputs.

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%, so the description must carry parameter meaning. It only loosely implies siteId, fieldId, and data.value through the phrases "on a site" and "custom field value," without naming parameters, describing UUID formats, or explaining the nested arbitrary value structure.

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 and resource: "Set a defined custom field value on a site." This is clear enough to distinguish it from definition-management siblings like update_custom_field or retrieval siblings like get_site_custom_fields, though it does not explicitly name those alternatives.

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 when the tool is used—setting an existing custom field value on a site—and notes that approval requirements may apply. However, it does not explicitly state when to use this tool versus siblings such as update_custom_field or get_site_custom_fields, nor does it describe prerequisites beyond the approval note.

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