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

Annotations already flag mutation, idempotency, and non-destructiveness; the description adds real value on top by stating the merge semantics (existing members keep their status) and that source deletion is optional. The gated deletion point is useful context that explains why destructiveHint=false despite a delete capability. It stops short of saying what happens to non-subscribed/removed contacts in deleted sources or whether the merge is synchronous.

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?

Three short sentences, front-loaded with the core action, then the two behavioral facts that matter. No filler or restatement of the tool name.

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 3-parameter mutation with full schema coverage and annotations, the description covers what the tool does and the key side effects. Only minor gaps remain: no statement of return shape (no output schema) or what an agent should expect for the 50-source limit and large merges.

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 coverage is 100%, so the baseline is 3; the description only loosely restates delete_sources as "optionally delete" without giving a default. The 50-source cap and the array-of-ids shape are left entirely to the schema.

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 ("Copy the subscribers of other audiences into this one") and clarifies the membership rule for existing subscribers. It does not explicitly distinguish itself from near-neighbours like bulk-add-contacts-to-audience, but the 'audience into audience' framing is distinctive enough to be unambiguous.

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 agent can infer this is the many-audiences-at-once path, but nothing states when to pick it over bulk-add-contacts-to-audience or update-audience-subscriber. No preconditions, permissions, or exclusions are mentioned.

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