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

Annotations carry destructiveHint=true, and the description reinforces this by flagging that it MODIFIES live data and adds a subscriber. It also discloses the return ('Returns the created contact'), which matters because there is no output schema. It stops short of stating idempotency/duplicate handling or permission requirements.

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 tightly packed sentences with the mutation warning front-loaded before the parameter summary and the endpoint. Every clause carries information; nothing is padding.

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 5-parameter mutation tool with full schema coverage and no output schema, the description covers the mutation semantics plus the return value. The remaining gap is duplicate/already-subscribed behavior and any auth or rate-limit constraints.

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 baseline is 3. The description restates that email_address is required and names the optional fields (fields, tags, status) with the key-by-field-tag convention, but adds little syntax or format detail beyond what the schema already documents.

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 ('CREATES a new contact in a list') and immediately scopes it as a live-data mutation that adds a subscriber. The word 'new' plus the resource 'contact in a list' lets an agent separate it from update_contact and delete_contact without opening any 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?

Usage is only implied by the verb: use it when you need to add a new contact to a list. There is no explicit when-to-use/when-not guidance, no mention of what happens if the email already exists, and no routing to sibling tools such as update_contact.

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