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

Goes well beyond the annotations: explains HTTP 201 vs 200 (confirming the idempotentHint), the recipient_blocked 403 behavior with no send/store/count, inbound block non-storage and the message.blocked event, the 1000-entry cap, and specific error codes. This is exactly the side-effect disclosure a destructive mutation needs.

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-loads purpose and auth requirement before the input recap and behavioral consequences, which is good structure. It is dense and a few clauses (e.g. the progress-email exemption, the task allowlist note) sit at the tail, but each carries real behavioral information rather than filler.

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 mutation tool with no output schema, the description covers return shape ('Returns the list entry'), status codes, precedence of per-inbox vs account-wide lists, downstream effects, and failure modes. An agent has everything needed to call it correctly and anticipate consequences.

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 all five parameters, including the enum semantics for direction and the account-wide-list meaning of inbox_id. The description restates those meanings ('send, receive, or reply', 'omit for the account-wide list') without adding new syntax or format detail, so baseline 3 is appropriate.

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 ('Add an allow or block entry') and immediately frames intent ('when an agent should only mail approved recipients, or should ignore unwanted senders'). This distinguishes it from siblings like cooper_add_domain and cooper_delete_list_entry without needing to open 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?

The opening sentence gives a clear when-to-use scenario (restrict to approved recipients vs. ignore unwanted senders). It does not explicitly name the alternative tools (e.g. cooper_delete_list_entry to remove, cooper_add_domain for domain management), so it stops short of full routing guidance.

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