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

Adds substantial context beyond the destructiveHint=false/readOnlyHint=false annotations: recipients and senders are server-resolved, opt-in/DNC/quiet-hours gates run server-side and cannot be overridden by parameters, and a refusal is a normal answer rather than an outage. It also discloses that an opt-out line is appended automatically, which prevents a real misuse.

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?

Dense but front-loaded: what it does, then the hard constraints, then the classification rule, then the opt-out reminder. A couple of sentences duplicate the schema's wording on the opt-out line, which is the only wasted space.

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 destructive, side-effecting send with no output schema, the description covers the things that actually cause failure: resolved numbers, compliance gates, refusal handling, and message classification. It never says what comes back on success (message id, delivery status), which would have made it fully self-sufficient.

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 baseline is 3, but the description adds real meaning the schema does not: it explains why lead_id/recipient resolution is fixed, and supplies a tie-breaking rule for message_class classification that the enum description alone lacks. The body-length and opt-out constraints are restatements of schema text.

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?

Specific verb and resource: 'Send one SMS to a lead... and record it on the lead,' and it immediately clarifies that neither phone number is agent-chosen. It is clearly distinguishable from send_lead_email and the list_lead_messages siblings by the medium and the one-message scope.

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 an explicit decision rule for the only genuinely ambiguous choice ('marketing for anything promotional, a follow-up, or anything with a call to action... If you are unsure, it is marketing') and states the preconditions under which sending occurs. It stops short of naming send_lead_email as the alternative channel, which is the one routing choice an agent might still have to guess.

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