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

Annotations already declare readOnlyHint=false, destructiveHint=false, and idempotentHint=false, so the write-ness is covered. The description adds genuinely new behavioral facts: field_key is immutable after creation and the account is capped at 15 active fields. It stops short of describing what happens on a duplicate key or what the call returns.

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, front-loaded with the action, then the two constraints an agent most needs before committing. No filler or restated schema detail.

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 simple three-parameter creation tool with annotations covering the safety profile, the core facts are present. Missing pieces are failure behavior on duplicate field_key, whether creation is scoped to the current account, and any return value, which matter because there is no output 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 0%, so the description carries the burden; it meaningfully explains field_key (permanent, so choose carefully) but says nothing about label or how the type enum values affect behavior. With one of three parameters enriched beyond the schema, the compensation is partial.

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?

"Create a custom field" states a specific verb and resource, and the body clarifies what a custom field's key semantics are. It does not, however, position itself against siblings like update_custom_field or list_custom_fields, so an agent must infer the create-vs-update distinction from the name alone.

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 'Up to 15 active fields' clause gives an implied precondition that an agent can check before calling, but there is no explicit when-to-use vs. when-not guidance and no mention of the update_custom_field alternative for existing fields. Usage context is inferable rather than stated.

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