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

Beyond the read-only and idempotent annotations, the description explains that the tool breaks down reach by exclusion reason: suppressed, unsubscribed, recently mailed, or over today's limit. That adds useful behavioral context about what the preview reports, though it does not discuss auth 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?

Two sentences, no wasted words. The core purpose is front-loaded, and the prerequisite and workflow instruction follow compactly.

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 preview tool with a full input schema, an output schema, and clear annotations, the description supplies everything an agent needs: what is computed, why exclusions occur, when to use it, and the workspace prerequisite. Return-value details can be left to the output schema.

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 segmentKey and workspaceId are already documented in the schema. The description refers to 'this segment' but does not add parameter syntax, format, or examples beyond what the schema provides, so the baseline of 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?

The description states a specific verb and resource: calculating how many contacts a campaign would reach for a segment and why others would be excluded. It is clearly a preview/reach-count operation, distinguishable from siblings such as sendTestCampaign or scheduleCampaign.

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?

It gives explicit workflow guidance: 'Always do this before proposing a send.' It also states a prerequisite constraint: only for a workspace with subscribed email contacts. It does not name alternative tools, but the when-to-use context is strong.

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