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

With no annotations, the description carries the behavioral burden. It clarifies that field_key is the identifier used in merge tags and custom_fields_set_value, which tells an agent how the output connects to other tools. The verb 'List' and the absence of mutation language signal a read-only operation.

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 three sentences, front-loaded with the purpose and followed by a dense but useful enumeration of return fields and a clear usage cue. No filler is present, though the return-field list makes it slightly heavier than strictly necessary.

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 list tool with no output schema, the description compensates well by enumerating the returned fields and their meanings, and by giving a concrete trigger condition. Pagination and empty-response behavior aren't mentioned, but they are minor gaps given the tool's low complexity.

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?

The single optional entityType parameter is fully documented in the schema with an enum and a clear description ('Optional — filter to fields on one entity type. Omit for all.'). Since schema description coverage is 100%, the description doesn't need to add parameter-level meaning; baseline 3 applies.

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 opens with a specific verb-object pair, 'List the user's defined custom fields,' and then enumerates the exact fields returned (field_key, name, type, options, entity_type). This clearly distinguishes it from mutation siblings like custom_fields_create/update/delete/set_value and from more general merge_tag tools.

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 final sentence gives an explicit trigger: 'Call this before authoring email/SMS templates or workflows that reference custom field merge tags.' It doesn't name an alternative or state when not to use it, but the context is clear enough for an agent to select it.

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