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

With readOnlyHint=true already covering safety, the description still adds useful behavior beyond annotations: the purchased-number prerequisite and the empty-inbox semantics. It doesn't discuss pagination limits or how large result sets behave, which is a minor gap for a 9-parameter list tool.

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?

Four short sentences, tightly front-loaded with the core purpose before prerequisites and format details. Every sentence carries information the agent needs and none restates the schema.

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 read-only, nine-parameter list tool with no output schema, it covers purpose, prerequisites, empty results, and date parsing. Pagination and sorting defaults are left entirely to the schema, so it is complete enough but not exhaustive.

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, but the description goes further by disclosing accepted date formats ('UNIX timestamp, ISO 8601 or plain English') for start/end, which the schema only constrains by length and does not explain. That is genuine added meaning over the structured fields.

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 ('List SMS received on the account's Virtual Mobile Numbers') and clarifies the scope as replies and keyword messages, which cleanly separates it from sibling list tools like list_delivery_reports and list_scheduled_messages. The endpoint mapping reinforces the intent.

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?

Provides a clear precondition ('Needs a purchased number') and a result expectation ('an empty inbox returns an empty result'), which tells the agent when this tool is meaningful. It stops short of naming when to prefer an alternative sibling, so it is clear context rather than full routing guidance.

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