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

Annotations already provide readOnly, idempotent, and non-destructive hints. The description adds meaningful behavioral context beyond those annotations: it requires a caller-provided ABR web-services GUID, explains the BYOK model, and gives the registration path. It also names the returned fields, so the agent knows what to expect from the response.

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 compact and front-loaded: it opens with the critical BYOK constraint, then states the purpose and return fields, then the requirement and an example. Every sentence earns its place, and the example makes the expected invocation unambiguous.

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 two-parameter read-only lookup with strong annotations, the description is complete: it covers purpose, expected return fields, authentication prerequisite, registration path, and an example call. There is no output schema, but the listed return fields are sufficient for an agent to understand what will come back.

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 the schema already documents both acn and _apiKey well. The description adds a concrete usage example and reinforces the meaning of _apiKey, but it does not provide substantial semantic value beyond what the input schema already says. Baseline 3 is appropriate.

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 states a specific verb and resource: 'Look up an Australian Company Number (ACN)'. It also clarifies what data is returned and explicitly distinguishes this from abn_lookup by saying it returns the same detail for the company's associated ABN. An agent can immediately tell what the tool does and how it relates to its sibling.

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 gives clear context: use an ACN as input and get the same details as abn_lookup, with an explicit BYOK prerequisite and registration link. It does not explicitly state 'use abn_lookup when you have an ABN', but the phrasing strongly implies the ACN-vs-ABN routing and names the sibling tool that provides the parallel behavior.

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