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

Annotations already declare the mutation profile (readOnlyHint=false, idempotentHint=true, destructiveHint=false), and the description adds real context beyond them: an authentication requirement, a permission scope on the target list, and partial-failure semantics ('Unknown or unavailable products are reported and not added'). The duplicate-handling sentence reinforces the idempotent hint with concrete specifics, though it does not describe the response payload or ordering.

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?

Five short sentences, front-loaded with the core action before the constraints. Every sentence carries information (identity, inputs, duplicate behavior, permissions, error handling), though the trailing 'Requires sign-in' would read better next to the permission sentence.

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 write tool with no output schema, the description covers auth, permissions, duplicate behavior, and partial-failure reporting, which is most of what an agent needs. It stops short of saying what the response contains (e.g., which items were added vs. rejected) or whether the market parameter affects the write itself.

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 documents list_id, items, product_id, slug, note, and market thoroughly. The description only restates the id/url duality and the 140-character note limit, and never mentions the market enum or the 1-50 item cap, so it adds little beyond the schema. Baseline 3 applies.

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 products to a list on the signed-in shopper's account') plus the identifier inputs (list_id, product id or url). The 'signed-in shopper' framing implicitly distinguishes it from the sibling add_to_wishlist, but the description never names that sibling, so the agent must infer the boundary.

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?

Adds useful preconditions ('Requires sign-in', 'Works on lists the shopper owns or may edit'), which tells the agent when the call is valid. It does not, however, explicitly contrast with add_to_wishlist, get_my_wishlist, or update_my_wishlist, so the choice among siblings is only implied.

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