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

Annotations already establish readOnly and non-destructive behavior; the description adds the important behavioral trait that unknown ids resolve to found:false rather than an error. This soft-validation behavior is meaningful operational detail beyond the structured annotations.

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?

A single sentence delivers the action, target, key, and edge-case behavior with no filler. The most decision-relevant information is front-loaded.

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 one-parameter read-only lookup backed by an output schema, the description covers the essential call semantics and the not-found contract. No critical missing piece is apparent for an agent to invoke it correctly.

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 schema only declares 'id' as a minLength-1 string, with no description. The description compensates by mapping that id to a variable_id and indicating that it selects one canonical construct, which is the core semantic an agent needs.

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 uses a specific verb ('Lookup'), names the resource ('canonical construct'), and specifies the key ('variable_id'). It contrasts with list/search siblings by emphasizing a single canonical item and explicitly describing the not-found case, making the tool's role 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?

The description conveys a clear use context: retrieve exactly one construct when a variable_id is available, and safely probe existence via found:false. It does not explicitly name alternatives or say when not to use it, but the singular lookup semantics are enough to route an agent away from list/search tools.

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