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

With no annotations, the description carries the full behavioral disclosure burden. It adds non-obvious details: cost is $0.025/segment, segment limits are 160 GSM / 70 unicode chars, and providing contactId attaches the message to the contact's thread. This is strong context, though response/error behavior is not covered.

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?

Three sentences, each carrying distinct, non-redundant information: core action, format/cost, and contactId guidance. The main action is front-loaded, 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 straightforward send action with fully described schema parameters, the essential operational context is present: E.164 requirement, segment billing, thread linkage, and default sender behavior (in schema). Nothing material is missing for correct invocation.

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 100%, so the baseline is 3. The description adds extra meaning beyond the schema by quantifying body segmentation/cost and explaining the thread-attachment effect of contactId, lifting it above the baseline.

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 'Send an SMS message to a phone number immediately' – a specific verb, resource, and temporal scope. The word 'immediately' distinguishes it from sms_send_scheduled, making the tool easy to separate from 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?

The word 'immediately' establishes the primary use case (instant send vs. scheduled send), and 'Pass contactId when available' offers practical when-to-do guidance. However, it does not explicitly name sms_send_scheduled or state when not to use this tool, so it stops short of full exclusions.

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