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

Annotations only declare the generic write/openWorld/non-idempotent profile; the description adds material behavior the annotations do not: it sends a real message, is cost-bearing ('billed per segment on the user's own Twilio account'), and requires user confirmation. These are exactly the traits an agent needs before invoking a non-idempotent outbound send.

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?

Two compact sentences, front-loaded with the action and its scheduling variant, then the cost/confirmation caveat. Every clause earns its place with no filler.

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 non-idempotent outbound send with annotations covering the safety profile and no output schema, the description covers cost, timing, and the confirmation requirement. It stops short of noting delivery-failure or opt-out handling, which would complete the picture.

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 already 80%, with toPhone, contactId, fromNumber, and scheduledAt all documented in the schema. The description only reinforces the scheduledAt meaning, adding little beyond what structured fields provide, so the baseline 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?

States a specific verb and resource ('Send a text') and adds the scheduling dimension ('now, or later with scheduledAt'). It is clearly distinguishable from sms_list and sms_cancel_scheduled by its resource and action, though it does not explicitly name those siblings.

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?

Gives a concrete usage directive: 'confirm with the user first,' which tells the agent this is a user-approval-gated action. It also clarifies the now-vs-later condition via scheduledAt, but states no when-not-to-use or explicit alternatives.

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