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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint, covering the safety profile. The description adds useful behavior beyond that: it explains the output format (score and grade) and input formatting rules (US 10 digits, others require + and country code). This enriches the agent's understanding without contradicting 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?

The description is three sentences, each earning its place: the first explains the core function, the second gives a use case, and the third details input formatting. It is front-loaded with the most critical information and contains no filler or repetition.

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 read-only lookup with one parameter and no output schema, the description covers all essentials: purpose, usage context, input format, and output type. Annotations handle safety. Nothing an agent needs to call the tool correctly is missing.

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 baseline is 3. The description adds meaningful parameter semantics by specifying formatting requirements: US numbers can be 10 digits, while other countries need + and the country code. This goes beyond the schema's example and helps the agent construct valid inputs.

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: it tells whether a number is VoIP or a carrier line, with a 0-100 score and letter grade, using Twilio Lookup. This clearly distinguishes it from the sibling tool 'directory', whose name suggests a different purpose. No ambiguity about what the tool does.

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 a concrete use case: 'Use it before relying on a number for SMS or OTP verification.' It does not mention alternatives or exclusions, but the single sibling 'directory' is unrelated, so no competitive guidance is needed. This provides clear context for when to invoke the tool.

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