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

Annotations already declare this as a destructive write, but the description adds crucial details beyond them: the default custom_fields_mode is replace_all and fields not given are cleared, and subcategories_mode replace makes the provided list the full list. These specifics tell the agent exactly what can be destroyed.

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?

The description is short, front-loaded with 'WRITE', and every sentence explains mode behavior or the endpoint. There is no wasted text, and the mode semantics are grouped efficiently.

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 five-parameter mutation tool with no output schema, the description covers the destructive mode behavior and defaults that an agent most needs. Required email validation and parameter types are documented in the schema, and annotations cover safety hints.

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 baseline is 3. The description nonetheless adds meaningful semantics beyond the schema, especially explaining 'remove' and 'replace' for subcategories and spelling out the destructive effect of the default replace_all mode for custom fields.

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 and resource — changing an existing subscriber's subcategories and/or custom fields — and the word 'existing' distinguishes it from enginemailer_add_subscriber and get_subscriber. An agent can identify the operation without opening the 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 context is clear: this is the update path for an existing subscriber, not creation or retrieval. However, it does not explicitly mention when to prefer this over siblings such as add_subscriber or unsubscribe_subscriber, so it falls short of full alternative routing guidance.

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