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

Annotations already declare idempotentHint, destructiveHint and openWorldHint, but the description adds genuine specifics beyond them: quantity is a new total rather than an increment (explaining why repeats are safe), and 0 is the destructive trigger that removes the item. It also notes the return value. Minor gap: no mention of auth requirements or what happens to a subscription item when quantity is set to 0.

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 tight sentences, front-loaded with the core action, then the non-obvious semantics, then the conditional flag. Every sentence carries information an agent needs; nothing is padding.

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?

With an output schema present and full annotation coverage, the description supplies exactly the missing pieces: set-vs-increment semantics, the destructive zero case, and the disambiguation flag. An agent has everything required to invoke this 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 100%, so the schema already documents all three parameters, including 'new total', '0 removes it', and the both-types condition for subscription. The description largely restates this at the prose level, adding the repeat-safety framing but no new syntax or format detail. Baseline 3 is appropriate.

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 with an explicit scope constraint: 'Set the quantity of a product that is already in the signed-in user's cart.' The 'already in the cart' qualifier immediately separates it from suppco_add_to_cart and suppco_remove_from_cart without needing to open either 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?

Clearly establishes the trigger condition (item already in cart) and gives an explicit conditional rule for the optional subscription flag when a product exists as both a one-time purchase and a subscription, plus the 0-means-removal rule. It stops short of naming sibling tools (e.g. remove_from_cart or add_to_cart) as alternatives, so routing is implied rather than explicit.

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