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

No annotations are supplied, so the description carries the full burden, and it does deliver two genuinely useful behavioral facts: duplicate members are silently ignored (idempotent, no error) and the operation is free at 0 credits. It still omits partial-failure behavior on large batches, auth requirements, and what happens when segment_id is unknown.

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 short sentences, front-loaded with the action and scope, then the two non-obvious facts (dedupe, cost). No filler, no restatement of the title.

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 two-parameter mutation with fully documented schema, the description covers the two traits an agent most needs: idempotency and zero cost. The gap is return behavior — there is no output schema, so the description should say what comes back (counts added vs. skipped) rather than leaving it unknown.

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% — person_ids documents the 1-1000 limit and segment_id points at list_segments — so the schema already does the work. The description only echoes 'by person_id' and adds no batch-size, format, or failure semantics beyond it.

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 and resource ("Add one or more people ... to an audience segment") with batch scope made explicit. It does not name the sibling it replaces (add_to_collection) or its inverse (remove_from_segment), so differentiation is inferred from the term 'segment' rather than stated.

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 implied by the verb and resource — you use it when you want people in a segment — but there is no explicit when/when-not, no mention of prerequisites (segment must exist, ids from list_segments or search results), and no routing to add_to_collection for the analogous collection case.

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