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

No annotations are provided, so the description must carry the transparency burden. It discloses the creation action, prerequisite call, connection selection, return value, and token cost. However, it omits potential side effects beyond creation (e.g., whether the number becomes verified immediately, what happens if the number already exists) and any error conditions, leaving noticeable gaps.

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, four short lines, with the primary purpose stated first. Every sentence contributes essential information: function, prerequisite, return, and cost. There is no fluff or repetition of schema details.

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 moderate complexity (6 params, mutation, no annotations), the description covers the core aspects: purpose, prereq, connection handling, return summary, and cost. The schema fully documents parameters and an output schema exists, so the description is largely complete. It could be improved by addressing common failure scenarios, but it is well-rounded for an MCP tool.

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?

All six parameters are fully described in the schema (100% coverage), so the description does not need to explain each one. It adds value by linking clerk_instance_id to clerk.get_connected_accounts and clarifying the default behavior, but otherwise it repeats schema information. 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 opens with 'Create a phone number for a Clerk user,' clearly identifying the specific verb, resource, and target. This distinguishes it from sibling tools like create_email_address or create_user. The additional connection context reinforces the tool's specific scope.

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?

It explicitly instructs to call clerk.get_connected_accounts first and explains how to target a specific connection or use the default. This is clear prerequisite and usage context. However, it does not explicitly mention when not to use this tool versus alternatives, so it stops short of a 5.

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