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

Beyond the annotations (readOnlyHint=false, openWorldHint=true, idempotentHint=false), the description discloses the two facts an agent most needs before firing: this sends a real message and it is billed per segment on the user's own Twilio account, plus a confirm-first requirement. That is meaningful side-effect and cost transparency the annotations cannot encode.

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 tight sentences with no filler; the primary action is front-loaded and the safety/billing caveat follows immediately. Every clause 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?

For a 5-parameter, no-output-schema mutation tool, the description covers the essential risk profile (real send, billed, confirm first). The only gap is what a successful call returns (e.g., message identifier or scheduling confirmation), which matters when scheduledAt is used.

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 80%, and the parameter descriptions already cover E.164 formats, contactId threading, fromNumber defaulting, and scheduledAt semantics. The description only echoes the scheduledAt behavior ('omit to send now') rather than adding syntax, constraints, or edge cases, so 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?

The description opens with a specific verb+resource ('Send a text') and immediately qualifies the timing variant (now vs. scheduledAt). This is clearly distinguishable from sms_list and sms_manage, the adjacent SMS siblings, without needing to open any schema.

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 states an explicit precondition ('confirm with the user first') and clarifies the scheduling fork ('or later with scheduledAt'). It does not name alternative tools (e.g., email_send) or exclusion criteria, so it falls short of a 5, but the operational context is clear.

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