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

With destructiveHint=true and idempotentHint=true already declared, the description usefully says the field type is immutable and that dry_run validates first — real behavioral context beyond annotations. It does not say what happens to the existing description when the parameter is omitted, which is the main remaining gap for a destructive update.

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 short sentences, zero filler, with the scope constraint front-loaded before the immutability note and the preview hint. Nothing repeats the tool name unnecessarily.

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 4-parameter mutation with full schema coverage, rich annotations, and no output schema, the description covers the action, the immutable attribute, and the preview safety valve. Only the behavior of omitting an optional field is unaddressed.

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% and the schema already explains name length limits, description length limits, field_id, and dry_run semantics. The description's 'Set dry_run: true to preview' largely restates the schema's own wording, so baseline 3 is appropriate.

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 and resource: rename a custom contact field or change its description. The clause 'The type cannot be changed' bounds the scope nicely, distinguishing it from a general-purpose update or from create_custom_field/delete_custom_field. It stops short of naming a sibling, but the operation 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?

The only usage guidance is 'Set dry_run: true to preview', which is a conditional hint rather than a when-to-use statement. There is no guidance on when to prefer this over update_contact or delete_custom_field, and no prerequisites (e.g., permissions, whether the field must be unused).

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