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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false. The description adds meaningful context by stating the tool counts matching customers and is a preflight step, reinforcing the non-mutating nature. It does not contradict annotations and adds the workflow-level behavior of previewing before commit.

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 description is a single, front-loaded sentence that clearly states what the tool does and when to use it. Every word contributes to understanding, with no filler or redundancy.

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?

Given the tool has one optional parameter, a rich input schema, a known output schema, and clear annotations, the description is sufficient to convey purpose and usage context. The 'before creating or updating a segment' clause completes the workflow picture without needing more detail.

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?

The single parameter 'criteria' has comprehensive schema-level documentation (100% coverage), including the meaning of omitting it. The description only loosely refers to 'dynamic segment criteria' and does not add parameter-specific details beyond the schema; hence baseline 3 is appropriate.

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 uses a specific verb ('Preview') and a clear resource ('how many customers would match dynamic segment criteria'), and explicitly ties it to the workflow of 'creating or updating a segment'. This distinguishes it from sibling tools like create_segment and update_segment.

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 description explicitly says the tool is for use 'before creating or updating a segment', which provides clear contextual guidance for when to invoke it. It does not name alternative tools explicitly, but the workflow timing is sufficiently clear.

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