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

Annotations already declare readOnly=false, idempotent=true, destructive=false, so the safety profile is carried structurally. The description adds genuinely new behavior: the implicit list creation side effect and the library-membership prerequisite. It doesn't say what the ambiguous-title response looks like, though the schema parameter text does.

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 short sentences, front-loaded with the primary action, followed by the auto-create behavior and the prerequisite. Nothing redundant and no filler.

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 primary action, the side effect, and the precondition — everything needed to invoke it correctly without opening sibling tools.

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% and the parameter descriptions are already rich (exact-title preference, item_id override, case-insensitive list_name). The description's only added hint is 'by title', so it neither regresses nor meaningfully extends the schema — baseline 3.

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+resource ('Add a library item to a list') and immediately adds the two scope facts an agent needs: the list is auto-created and the item must already exist. This distinguishes it cleanly from add_item and create_list in the sibling set.

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 routes to the sibling: 'use add_item first if not' handles the precondition, and 'Creates the list if it does not exist' tells the agent not to call create_list first. Both the when-to-use and the when-to-use-something-else cases are covered.

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