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

With no annotations supplied, the description carries the full burden and does substantial work: it enumerates the returned line fields, fixes the status enumeration (InStock, Ordered, Outbound, Damaged), notes optional custom fields, and explains that item totals are a per-status sum of line quantities. Gaps remain around permission requirements, pagination/limits, and behavior for an unknown or deleted item id.

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 tight sentences, front-loaded with the operation, then the return shape, then the aggregation semantics. Every sentence conveys information an agent would otherwise lack, with no repetition of the name or schema.

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?

There is no output schema, so the description must describe results, and it does so concretely including field list, status values, and totals. For a single-parameter read tool this is close to complete; only error/empty-result behavior and any result-size limits are unaddressed.

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 0% for the single parameter, which is only typed as an integer with no description. The prose says 'by its id,' which confirms the id is the inventory item identifier rather than a unit id or barcode, but adds no format, source, or lookup guidance beyond that. Partial compensation for a one-parameter tool.

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 and resource: 'Lists the stock lines of an inventory item by its id.' This clearly separates it from mutation siblings (add_inventory_unit, remove_inventory_unit) and from item-level reads (get_inventory). It does not explicitly distinguish it from the closest alternative, find_units_by_custom_field, but the id-based retrieval scope is unambiguous.

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?

Usage is only implied: the phrase 'by its id' signals this is the lookup path when you already have an item id, as opposed to find_units_by_custom_field or get_inventory. There is no explicit when-to-use/when-not-to-use statement or named alternative, so the agent must infer routing from sibling names.

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