Twilio SMS MCP Server

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

Annotations already provide readOnlyHint=true, destructiveHint=false, idempotentHint=true, and openWorldHint=true, covering safety and idempotency. The description adds value by specifying the validation aspect and output formats (E.164, national format, country code), but doesn't disclose behavioral traits like error handling for invalid numbers, rate limits, or authentication requirements beyond what annotations imply.

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 a single, efficient sentence that front-loads the core action ('Validate a phone number') and immediately lists the three return values. Every word contributes essential information without redundancy or unnecessary elaboration.

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 low complexity (1 required parameter), rich annotations covering safety and idempotency, and the presence of an output schema (which likely details the return structure), the description is mostly complete. It could improve by mentioning use cases or prerequisites, but it adequately conveys the tool's function and outputs for an agent to invoke it correctly.

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 0%, but the description compensates by explaining the purpose of the parameters: 'phone_number' is for validation and reformatting to E.164, and 'country_code' is for disambiguating national numbers. However, it doesn't detail the exact input formats or constraints beyond what the schema's min/max length and type indicate.

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 the specific action ('Validate a phone number') and the outputs ('return its E.164 form, national format, and country code'), distinguishing it from sibling tools like sms_lookup_number or sms_list_numbers which have different purposes. It uses precise terminology that indicates transformation and validation rather than lookup or listing.

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 provides no guidance on when to use this tool versus alternatives like sms_lookup_number (which might provide additional carrier info) or sms_list_numbers (which lists owned numbers). It mentions validation and reformatting but doesn't specify use cases such as preparing numbers for sending SMS or standardizing user input.

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