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

Only destructiveHint=false is annotated, and the description goes well beyond it: it discloses idempotency (already-listed contacts ignored), a 100-item cap, and the side effect of emitting a contact.list_added event that may trigger automations. That side-effect disclosure is exactly the kind of behavior an agent needs before a write.

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?

Three dense sentences, front-loaded with the action and cap, then routing, then behavior. No filler; every clause carries actionable information.

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 two-parameter mutation with no output schema and minimal annotations, the description covers limits, idempotency, side effects, and alternatives — everything needed to call it safely and correctly.

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 0%, so the description carries the param burden, but it only adds the 100-item limit (which already appears as maxItems: 100 in the schema). The meaning of listId and contactIds is inferable from their UUID formats, but nothing beyond the schema is explained.

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 ('Adds existing contacts to a list') and immediately scopes it with a batch cap, clearly distinct from sibling operations like upsert_contact and remove_contacts_from_list. 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?

Explicitly names the alternative (upsert_contact) and the condition that selects it ('to create them, or to create and link in one go'), and points to list_automations as a pre-write check. This is genuine when-to-use/when-to-use-something-else guidance.

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