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

Annotations already flag not-readonly and open-world, but the description adds substantial side-effect detail beyond them: Substack sends a welcome email, comp grants a complimentary paid plan until comp_until, existing payers get the time appended after their current billing cycle, and the user must be asked to confirm each call by default. This is exactly the kind of operational context annotations cannot convey.

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 the core action and mode selection, then layered constraints. Slightly repetitive ('Adds one email to your list' restates the opening) and the trailing API-key sentence is boilerplate, but nothing is padded or buried.

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 single-parameter write tool with no output schema, the description covers consent, side effects, billing semantics, confirmation and auth. The one gap is what happens on a duplicate or already-subscribed email (idempotency/failure behavior), which an agent invoking this repeatedly would want to know.

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, but the description adds real meaning: it explains the comp/comp_until interaction and how comp applies differently to already-paying subscribers. The email parameter is left to the schema, which is adequate given its triviality.

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 ('Add an email as a free subscriber or give a complimentary paid plan') and disambiguates within the Substack family of list/export tools by framing it as a single-email add. The two modes (free vs comp) are named up front, so an agent knows exactly what the call does.

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?

Provides clear context for use: 'Add only people who agreed to hear from you' is an explicit eligibility precondition, and the comp/billing explanation tells the agent when the comp mode is appropriate. It does not explicitly name an alternative (e.g., substack_list_subscribers or substack_export_subscribers) or state when not to call it, so it stops short of a 5.

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