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

Annotations already declare readOnly=false, destructive=false, idempotent=true and openWorld=false, covering the safety profile. The description only restates the ID-based input, which is already in the schema, and says nothing about duplicate handling, whether non-existent contacts are skipped, or what the call reports back.

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?

One short, front-loaded sentence with no waste. It is arguably under-specified rather than verbose, which is a completeness issue, not a conciseness one.

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?

A mutation tool with full annotation coverage and a fully documented schema, so the description need not carry safety or parameter detail. Still, it omits the outcome semantics (result of a batch add, handling of already-present contacts) that an agent would want before calling a write operation.

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 audienceId, contactIds and apiKey are all documented in the schema; the '(by ID)' note in the description duplicates the contactIds description. Baseline 3 applies since the schema carries the parameter burden.

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 ('add') and resource ('contacts to an audience') with the identity mode clarified ('by ID'), which is clearer than the bare title. It does not, however, distinguish itself from close siblings such as remove_contact_from_audience or import_contacts.

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?

No guidance on when to use this versus import_contacts (which also brings contacts into an audience) or remove_contact_from_audience. The agent must infer the selection criterion from the tool name alone.

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