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

Annotations already declare readOnlyHint=true and idempotentHint=true, so safety is covered. The description adds useful behavioral context: segments evaluate live, so contact_count reflects current contacts. This is beyond what annotations provide, though it doesn't discuss pagination or error cases; but with zero parameters, the description is sufficient.

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 concise, around three sentences, with the primary purpose and key behavioral detail front-loaded. Every sentence adds value: what the tool does, when to use it, and the live nature of contact_count.

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 there are no parameters and no output schema, the description fully covers what an agent needs: it explains the return structure, the semantics of contact_count, and how to use the result. No critical information 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?

The tool has zero parameters, so there is no schema to describe. The description still explains the output format (array of { id, name, filter, contact_count }), which adds value beyond the empty schema. Since no parameters exist, a baseline of 4 is appropriate because the description clarifies the output and usage.

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 its purpose: listing all saved segments with their live contact_count. It explicitly distinguishes itself from creation by advising to use it to find an existing segment's id rather than recreating one.

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 explicit guidance on when to use this tool: to find an existing segment's id for preview_broadcast or send_broadcast, and implies it should be used instead of creating a duplicate segment. This clearly routes the agent to the appropriate action.

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