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

Beyond the annotations (non-read-only, non-destructive), the description discloses important side effects: validation of an output schema, settlement of seller proceeds and existing fees, and issuance of a network-signed economic receipt. It also reveals the authorization requirement of a seller-signed Ed25519 artifact and the exactly-once guarantee. The phrase 'output schema is validated' is slightly ambiguous, preventing a 5.

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?

Two sentences, about 40 words, lead with the core action and constraints, then list the consequential outcomes. There is no filler or redundant restatement of schema fields.

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 side-effectful tool with no output schema, the description covers the essential contract: what is fulfilled, under what conditions, and what happens afterward (settlement, receipt). Parameter names are self-explanatory, and the only real gap is an explicit statement of the return value, though the receipt issuance partially covers that.

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?

With schema coverage at only 33%, the description compensates by linking fulfillment to the seller-signed artifact, purchaseId to the paid purchase, and idempotencyKey to the 'exactly once' behavior. It does not describe each parameter explicitly, but it adds meaningful context beyond the bare schema types.

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?

The description clearly states the action ('Fulfill'), the resource ('paid dynamic purchase'), and the key conditions ('exactly once', 'seller-signed Ed25519 artifact'), making it distinct from sibling purchase/execute/recover tools. It conveys a precise post-payment fulfillment step.

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?

The description implies this tool is used for paid purchases and exactly-once fulfillment, but it does not explicitly state when to use it versus alternatives such as execute_authorized_purchase or recover_commerce_purchase. No direct routing guidance is provided, so the agent must infer usage from context.

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