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

Annotations only declare readOnlyHint=false, openWorldHint=true, idempotentHint=false, destructiveHint=false. The description goes well beyond: requires auth, stores the message as received mail, fires message.received webhooks, carries no DMARC/DKIM results, and only creates an owner task when require_sender_auth=false — precisely the side-effect caveats an agent 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-loaded with purpose and the test-only constraint, then behavior, inputs, and return value. It is one dense paragraph but nearly every clause carries information; only the input enumeration lightly duplicates the schema.

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 7-parameter mutation tool with no output schema, the description covers auth, side effects, webhook firing, the sender-auth caveat, the parameter list, and even the return value ('Returns the stored message'). Nothing needed to call it 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 all 7 parameters including client_id as an idempotency key and attachments as base64. The description mostly restates these ('from (required; sender address)', 'client_id (optional idempotency key)'), adding little beyond the schema, so the 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: simulate inbound email arriving in a Cooper inbox without sending real mail. It distinguishes itself from the normal path by noting 'real inbound mail arrives automatically,' so an agent can tell it apart from send/reply siblings.

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?

Explicit when-to-use ('only for testing... to try a webhook or task flow end to end') and when-not ('never use this to fake mail for a user'), with the real-inbound alternative named. Nothing is left to inference.

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