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

Annotations already cover idempotency and non-destructiveness, yet the description adds substantially more: it never downgrades an existing opt-out, it requires the Send Message module, it prescribes do-not-retry behavior on 403, and it explains the response partition (Added/AlreadySuppressed/Invalid). This is exactly the behavioral context an agent needs and exceeds what annotations provide.

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?

Front-loaded with purpose and scope, then qualification and error handling, with no filler sentences. Slightly dense and the trailing fetch_messaging_usage guidance is long, but every sentence carries operational value.

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?

With no output schema, the description still communicates the return structure (counts per category) and the key failure mode (403 module-not-enabled) plus the correct recovery path. Nothing an agent needs to call and report on this tool correctly is missing.

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 schema already documents both parameters, including the enum and the 1-500 address range. The description restates the 500 cap and the one-channel-per-call rule but adds little syntax or format detail beyond the schema. 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 and resource ('Add ... to the account do-not-contact list') with an explicit scope constraint ('up to 500 addresses of ONE channel'). An agent can tell this apart from fetch_suppressions and remove_suppression without opening any 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?

Explains the when-to-use posture directly ('the safe direction ... no confirmation is required') and routes the agent to fetch_messaging_usage as the diagnostic alternative when a send is refused. It does not explicitly contrast with remove_suppression, but the usage context is clear.

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