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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is fully covered by structured data. The description's 'without sending' usefully reinforces that no message is dispatched, but it adds nothing about result size, sampling, or pagination limits on the preview.

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?

A single front-loaded sentence with zero filler; the key constraint ('without sending') arrives immediately after the core action.

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 with no output schema, the description should at least hint at what comes back (a count, a sample list, a capped number of contacts). It states the action and the no-send guarantee but leaves the return shape entirely unspecified.

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% for both parameters (apiKey with env-var override, segmentId), so the schema carries the load. The description adds no extra meaning such as the segment ID format or where to obtain it, 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?

States a specific verb (preview) and resource (contacts matching a segment), plus a scoping qualifier ('without sending') that tells the agent no email is dispatched. It is distinguishable from send_* siblings, but it does not differentiate itself from the adjacent preview_campaign_recipients or explain how a segment preview differs.

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?

Usage is only implied: the phrase 'without sending' signals a safe inspection step before a send, but there is no explicit when-to-use statement, no prerequisites (e.g. segment must exist), and no pointer to preview_campaign_recipients as the alternative for campaign-level previews.

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