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

With no annotations, the description adds useful context: it mentions the prerequisite call, the default account behavior, and that it returns a phone number summary. It doesn't disclose error handling or permission requirements, but for a read operation this is acceptable.

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 with the core purpose. Each sentence serves a distinct function (purpose, prerequisite, output, cost), with no fluff or redundant detail.

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?

Given the tool's simplicity, the presence of an output schema, and complete schema parameter documentation, the description covers the essential context: the prerequisite connection, the optional instance targeting, the return summary, and cost. Nothing critical is missing for confident invocation.

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 parameters. The description adds minimal extra meaning: 'Pass clerk_instance_id to target a specific connection, or omit it to use the default account' essentially restates the schema's existing description, providing no significant new value.

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 clearly states 'Retrieve a Clerk phone number by id', using a specific verb and resource. It distinguishes from sibling tools like delete_phone_number, update_phone_number, and create_phone_number by focusing on retrieval.

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?

Provides explicit prerequisite ('Call clerk.get_connected_accounts first') and explains how to use the optional clerk_instance_id parameter. While it doesn't mention alternatives or exclusions, the context is clear for when and how to use the tool.

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