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

Annotations only declare readOnlyHint=true; the description corroborates that ('Safe and read-only') and adds genuinely new behavior: nothing is sent, and it returns three computed values. It discloses the underlying REST route and payload shape, which helps an agent reason about semantics, though it does not cover failure modes or rate limits.

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?

The lead clause is front-loaded with the dry-run nature and the three computed outputs, which is the highest-value information. The trailing REST endpoint reference is marginally useful for debugging but is the one sentence that does not directly aid tool selection.

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?

With no output schema, the description must convey return semantics, and it does so explicitly (segments, SMS/MMS classification, estimated cost). For a 2-parameter read-only tool, nothing needed to call it correctly is missing.

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 both parameters (text required, mode optional with example values AUTO|SMS|MMS) are already documented. The description restates the payload as {text, mode?} but adds no syntax or constraint detail beyond the schema, 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 action (DRY-RUN evaluate) on a specific resource (message body) and enumerates exactly what is computed: segment count, SMS vs MMS, and estimated cost. 'WITHOUT sending anything' immediately separates it from simpletexting_send_message and the get_* 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 DRY-RUN framing plus 'WITHOUT sending anything' makes the intended context clear: use this before sending to preview cost/segments. It never names simpletexting_send_message as the alternative or states exclusions, but the contrast is unambiguous from the phrasing.

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