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

Annotations only supply destructiveHint=true; the description adds the concrete destructive mechanic — omitted custom fields are cleared — plus the WRITE marker and the underlying API endpoint. It stops short of auth/permission requirements, but it substantially exceeds what the annotations convey and is consistent with them.

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 'WRITE:' and the core action, then the data-loss rule, then the endpoint. Two tight sentences plus a routing detail; nothing is padded, though the endpoint string is of marginal value to an agent.

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 an 8-parameter destructive mutation with a nested custom_fields object and no output schema, the description covers the highest-risk behavior (field clearing) that an agent must know before calling. Missing auth/permission context and explicit sibling routing keeps it from a 5.

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 the schema already documents all 8 parameters, including the custom-field clearing warning. The description's field list and 'include every custom field' note largely restate that structured content, so the baseline of 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 ('update one subscriber (by ID) in a mailing list') and enumerates the updatable surface (email, name, custom fields, tags, preferences). The 'by ID' scoping and the WRITE marker clearly separate it from moosend_add_subscriber and the get_* 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?

The description implies this operates on an existing subscriber and gives an operational rule ('include every custom field you want to keep'), but it never names alternatives or states when to prefer this over moosend_add_subscriber or moosend_unsubscribe_subscriber. Usage is inferable rather than explicit.

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