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

Goes well beyond the annotations: it discloses the saved_total price-snapshot semantics per item type (product keeps a total for `deliver_to_country`, a plan keeps none), that re-adding an existing item only mutates the note, the 200-item list cap, and the not_found outcome for foreign lists. These are non-obvious side effects and limits that annotations (readOnlyHint=false, idempotentHint=true, destructiveHint=false) 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 action and the exclusive-or rule, then layered with behavior and limits; nearly every clause carries real information. It is dense and runs long as a single sentence, which costs it a point on scannability, but there is 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 mutation tool with no output schema, the description covers the important behavior: what gets stored, what happens on repeat calls, capacity limits, and the failure mode. What it omits is the success response shape (whether the added item or the updated list is returned), leaving the agent to infer the return value.

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 baseline is 3 and the schema already documents every parameter including note length and locale format. The description does add genuine meaning — the mutual exclusivity of `product_slug`/`digital_id`, that digital_id comes from compare_digital / compare_mobile rows, and that `deliver_to_country` drives the captured total — but it does not explain note visibility or the list_id source, which stay with the schema.

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 (adds) and resource (a product or digital plan) into a named list, and immediately disambiguates the two item types by their identifier parameters (`product_slug` vs `digital_id`, the latter tied to compare_digital / compare_mobile). An agent can distinguish this from remove_from_list, create_list, or update_list 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?

Gives a hard usage rule — "send exactly one of the two" — plus the operational condition that a list is capped at 200 items and that another person's list returns not_found. It does not explicitly route to siblings (e.g. create_list when no list exists, or update_list for renaming), but the invocation conditions are otherwise unambiguous.

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