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

Annotations already mark it as read-only and open-world, and the description adds valuable behavioral details: it returns null if no primary ENS name is set, and it specifically returns the 'primary' name, implying multiple names may exist but only the primary is returned. This goes beyond the annotation hints.

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 two clear sentences with no unnecessary words. It front-loads the main action and result, making it easy for an agent to quickly understand the tool's purpose and return behavior.

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 tool with one parameter and no output schema, the description adequately explains the input and the return value (primary ENS name or null). It does not mention error cases, but given the simplicity and schema validation, this is acceptable. It's slightly less complete because it doesn't note what happens for an invalid address.

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 input schema has full coverage (100%) with a description for the 'address' parameter and a pattern. The description does not add any parameter-specific meaning beyond what the schema already provides, so the baseline of 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 uses a specific verb ('Reverse-resolve') and clearly states the resource ('Ethereum address') and the target ('primary ENS name'). It distinguishes from the sibling tool eth_resolve_ens, which likely does forward resolution, by explicitly saying 'reverse-resolve'.

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 implies usage by indicating it performs reverse resolution, but it does not explicitly state when to use it versus alternatives like eth_resolve_ens, nor does it mention any exclusions or prerequisites. The distinction is implied but not made explicit.

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