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

Annotations only supply readOnlyHint=true, so the description carries the semantic load, and it does: it defines classify scope, explains what the count excludes ('No match' people already counted in `classified_people`), and rules out double-counting with list_segment_groups. It adds no notes on cost, credit usage, or edge cases like an empty scope, keeping it short of a 5.

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 scope-defining sentence is front-loaded and the pairing with `classified_people` and the 'No match' distinction each add real disambiguation value. The XML-style <summary>/<returns> wrapper tags are structural overhead that slightly dilutes an otherwise tight description.

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?

There is no output schema, but the description compensates by spelling out the return shape ({group_id, unclassified_people}). Combined with the scope definition, exclusions, and the acting-account parameter documented in the schema, an agent has everything needed to call this correctly.

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% and both parameters (group_id, on_behalf_of) are documented in the schema, including the on_behalf_of acting-as semantics. The description only indirectly enriches `group_id` by explaining the group's classify scope; it adds nothing about on_behalf_of or the integer source, so the baseline 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+resource: counting people in a group's classify scope with no classification row yet. It goes further to define what 'classify scope' means (tracked campaigns and teammates, `criteria`) and explicitly separates this population from 'No match' people, so an agent can distinguish it from get_segment_rates or query_segment_people without opening a schema.

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 concrete usage context — it is the population a classify_segment_group run with force off would process — which implies the natural workflow of previewing scope before classifying, and points to list_segment_groups as the source of `classified_people` to pair with. It stops short of explicit when-not-to-use guidance or naming an alternative count tool such as get_message_tag_unclassified_count.

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