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

Annotations include destructiveHint: true, and the description adds that the tool is 'Additive' and mentions the API endpoint. It explains the upsert behavior and what it does not do (send email), providing context beyond annotations.

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 with no redundant information. Every part serves a purpose: operation description, additive nature, no email, API reference.

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?

Given the tool's simplicity (no output schema, one required param), the description covers the operation, key side effects, and API details. It is sufficient for an agent to understand the tool's purpose and behavior.

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 describes parameters. The description adds a helpful example for 'fields' ('e.g. { name, last_name, company }') but no additional meaning for other parameters. 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 uses a specific verb 'upsert' and resource 'subscriber', clearly stating the operation (create or update) and the key 'by email'. It distinguishes itself from sibling tools like 'mailerlite_list_subscribers' and 'mailerlite_assign_subscriber_to_group'.

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 explicitly says 'Additive — does NOT send any email', clarifying when not to expect email actions. It implies usage for creating/updating subscriber data, but could be more explicit about when to use this tool over siblings.

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