Create a contact field

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

The description adds valuable behavioral context beyond the minimal annotations: it reveals that the slug is derived from the label and cannot be changed later, that the field type is likewise immutable, and that the response includes a uuid to be used for setting contact values. This meaningfully informs the agent about side effects and response content.

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 compact sentences, with the core purpose stated first, followed by an important constraint and a usage pointer. Every sentence earns its place; there is no filler or repetition of schema content.

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 create operation with four parameters and no output schema, the description covers the essential behavioral facts: it defines a field, notes immutable attributes, and tells the agent how to use the returned uuid. The remaining gaps (such as the meaning of profileUuid or option requirements) are already covered by the schema, so this is sufficiently complete for correct invocation.

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%, so the baseline is 3. The description adds semantic value by explaining that the slug derives from the label and that the field type is immutable, which enriches the otherwise thin schema descriptions ('label parameter'). The options and profileUuid parameters remain adequately covered by the schema, though not further explained.

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 operation: 'Define a new custom contact field in a profile.' This is a specific verb + resource, and the title matches. While it doesn't explicitly name a sibling like reach_updateAContactFieldV1, the create intent is unambiguous and distinct from the surrounding list/update/delete siblings.

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 no explicit guidance on when to use this tool versus alternatives such as update or list. The instruction 'Use the returned uuid to set values on contacts' is post-usage, not tool-selection guidance. No exclusions or alternatives are mentioned, leaving the agent to infer usage from the name alone.

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