Count profile segment contacts

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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safety profile is covered. The description adds useful behavioral context about not returning a contact list and about cost efficiency compared to paging, but does not disclose anything further such as response shape or rate limits. This is adequate given the annotation coverage.

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 with no wasted words. The primary action is stated first, followed by the key usage rationale. Every sentence earns its place.

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 simple count operation with two required parameters and a clear title, the description is largely complete: it states what is counted, that contacts are not listed, and why this tool is preferable. It stops just short of explicitly stating the return value, though 'Count' and 'size' strongly imply a numeric result.

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 the parameters are already documented, even if only generically ('Profile uuid parameter', 'Segment uuid parameter'). The description does not add meaningful semantic detail beyond implying these identify the profile and segment whose contacts are being counted. 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?

States a specific verb ('Count') and resource ('contacts currently matching a segment'), and explicitly contrasts with listing ('without listing them'). This distinguishes it clearly from sibling tools like reach_listProfileSegmentContactsV1.

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?

Gives clear when-to-use guidance: when only the size is needed, because it is 'cheaper than paging through the segment contacts endpoint.' It names the alternative endpoint, though not the exact sibling tool name, and the 'when only the size is needed' clause effectively implies when not to use it.

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