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

readOnlyHint=true already declares the safe-read profile, so the bar is lower, yet the description still adds genuine context: the operation is account-scoped, it enumerates the returned attributes, and it explains the practical consequence of the API name (usable as a key in customFields). The raw endpoint (GET /v1/customFields) is also disclosed. It stops short of pagination or default-count behavior, but adds real value beyond the annotation.

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?

Very short and front-loaded: the resource and returned attributes come first, the usage hint follows. Every sentence earns a place, though the trailing endpoint string is arguably redundant with the GET semantics and slightly interrupts the flow.

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?

With no output schema, the description compensates well by describing the shape of returned field records (label, API name, type, dropdown choices) and their intended use. The main omissions are pagination behavior and default page size, which matter for a list endpoint with limit/offset parameters.

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 100%, so label, limit and offset are already fully documented in the schema, and the description adds nothing about their semantics or interplay. Baseline 3 is appropriate when the schema does the heavy lifting.

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 names the resource precisely ('the account's contact custom fields') and enumerates what each field record contains (label, API name, type, dropdown choices), which distinguishes it from siblings like fub_list_deals or fub_list_pipelines. It never states the verb but the name 'list' plus the GET endpoint carries that. Clear, though sibling differentiation is implicit rather than explicit.

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?

There is no guidance on when to call this versus alternatives, nor any prerequisite or exclusion. 'Use the name as a key in customFields' is a downstream hint about interpreting results, not a selection guideline for the agent. No when-to-use framing at all.

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