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

Annotations already indicate this is a mutating, non-idempotent operation (readOnlyHint=false, idempotentHint=false). The description adds the prerequisite that the agent must have a provisioned phone number, which is useful beyond the annotations. It doesn't disclose failure modes or costs, but given the annotations cover the basic behavioral profile, a 3 is appropriate.

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 concise sentences with no fluff. The action is front-loaded, followed by a key requirement and usage context. Every sentence earns its place.

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 complexity (4 params, output schema present, annotations available), the description covers purpose, usage, and a prerequisite. It doesn't explain return values, but the output schema handles that. It could mention rate limits or costs, but those are not essential for a correct call. The description is adequate and slightly above minimum.

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 coverage is 100%, so all parameters are already well-documented in the schema. The description mentions mediaUrls for MMS, but this is already in the schema. The description adds no new parameter-specific information, so the baseline of 3 applies.

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 ('Send') and resource ('SMS' or 'MMS'), and distinguishes from sibling tools like sms_get, email_send, and phone_call_create by explicitly mentioning SMS/MMS and mediaUrls. It clearly states the action and the optional MMS variant.

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 explicit usage context: 'Use this for transactional texts or conversational messaging.' It doesn't explicitly mention when not to use it or name alternatives, but the context is clear enough to guide selection among the sibling tools.

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