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

With no annotations, the description carries the burden and does disclose a key behavioral outcome: the field becomes immediately available as a merge tag in email/SMS composers. It also mentions auto-generation of field_key. It doesn't cover uniqueness constraints or error behavior, but for a create tool this is reasonably transparent.

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 sentences, front-loaded with purpose and usage, then a note on field_key and a concrete behavioral result. Every sentence earns its place with no filler.

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 create tool with no output schema, the description covers what it does, when to use it, and a key side effect (merge tag availability). It doesn't mention the response format, but that's a minor gap given the tool's simplicity and the absence of an 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 coverage is 100%, so baseline is 3. The description adds minimal param-specific value: it repeats the auto-generation rule already in the schema and gives name examples, but doesn't clarify the semantics of options or entityType beyond what the schema already states.

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 verb 'Define a new custom field' with the resources (contacts, companies, deals) and provides concrete examples of when to use it. It distinguishes from siblings by focusing on creation, and the examples (e.g., tracking loan amount) make the purpose unmistakable.

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?

Explicitly instructs to use this tool when the user mentions tracking data without a built-in column, with examples. It also notes the auto-generation of field_key, guiding on when to provide it vs not. This gives clear when-to-use and implicitly contrasts with other custom field operations.

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