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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint, and non-destructive behavior, so the safety profile is covered. The description adds that these are configured reply addresses and their downstream use, but it doesn't disclose ordering, pagination defaults, or other behavioral traits beyond 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?

Two sentences, front-loaded with purpose, then a clear usage note. No filler, and every sentence earns its place.

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 simple read-only list tool with no output schema and rich annotations, the description covers purpose and downstream integration. It doesn't specify returned fields beyond 'IDs' and 'email addresses', but that is a minor gap for such a straightforward 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 coverage is 100%, so the schema fully documents 'page' and 'limit' with their descriptions. The tool description itself adds no parameter-specific meaning (the 'reply_id' mentioned belongs to create_campaign), so the 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?

The description states a specific verb ('List') and a unique resource ('reply-to email addresses configured in your MailerCloud account'), which distinguishes it from sibling list tools like list_senders or list_domains. An agent can immediately tell what this tool returns.

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 second sentence gives clear downstream context: 'Use the returned IDs when creating campaigns with the reply_id parameter.' This tells the agent when the result is needed. It doesn't explicitly exclude alternatives, but no sibling overlaps, so the guidance is sufficient.

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