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

Goes well beyond the annotations by disclosing the credit cost model (1 credit per new domain), the mandatory user-confirmation flow, and the exact failure behavior when credits are short. The additive "merge, nothing is removed" note usefully scopes the destructiveHint=true flag rather than contradicting it.

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-loads the cost warning in capitals before any procedural detail, which is the right ordering for a spend-incurring tool. The confirmation instruction is stated twice ("Before you call it..." and "Never call it without"), a mild redundancy that reinforces rather than obscures.

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?

Covers the cost, the required confirmation gate, the insufficient-credit outcome, and the plan restriction — everything an agent needs to call this safely. No output schema exists, and the description supplies the one return behavior that matters (cost plus balance on failure).

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, but the description adds real meaning: domains are stores, and only domains new to the list consume credits, which explains the merge/dedup behavior behind the domains array. listId is left entirely to the schema, which is acceptable.

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 ("Add domains to a watchlist") and immediately clarifies the merge semantics ("nothing is removed"), which separates it from removeFromWatchlist and deleteWatchlist. 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?

Gives explicit preconditions (confirm cost with the user first, paid plans only) and a failure path (insufficient credits = nothing added). It does not explicitly route between this and sibling list-management tools like createWatchlist, but the add-vs-create distinction is implied clearly enough.

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