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

No annotations are supplied, so the description carries the full behavioral burden. 'Lists' correctly signals a non-mutating read, but it says nothing about permissions, pagination, ordering, or what happens when a unit has no custom fields defined — relevant gaps for a tool whose siblings include set_unit_custom_field and remove operations.

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?

Two short sentences, purpose stated first with the use case immediately after. Nothing is redundant and no sentence could be removed without losing information.

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 single-parameter read with no output schema, the description covers what it does and a representative use case. It does not indicate the shape of the returned custom fields (names/values) or that it pairs with set_unit_custom_field, leaving an agent to discover the write counterpart and return structure on its own.

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% for the single required parameter, so the phrase 'by unit id' is the only explanation that unitId identifies the inventory unit whose fields are returned. It covers the parameter's meaning adequately but doesn't clarify whether unitId is the internal record id versus an identifier set via set_inventory_unit_identifier.

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?

States a specific verb ('Lists') and resource ('custom field values on a stock line (inventory unit)'), and clarifies the scope is per-unit rather than per-item, which implicitly separates it from the sibling get_inventory_item_custom_fields. It stops short of naming that sibling, so the differentiation is inferred rather than stated.

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?

'Use for lot number, expiry, and other per-line attributes' gives a concrete use case, but there is no explicit when-not guidance and no mention of the likely alternative find_units_by_custom_field (which searches units by field value rather than returning a unit's fields). Usage is implied rather than routed.

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