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

Annotations mark this as a mutating, non-idempotent operation, and the description adds meaningful behavioral detail beyond that: hosted session vs direct settlement, the ability to hand the session to another person, and the explicit rejection of on-chain payment. It does not detail error cases or side effects on the order, but the core behavioral traits are transparent.

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?

The description is front-loaded with the core purpose and then efficiently explains the two payment modes. Each sentence contributes relevant information; the final sentence about on-chain payment is a useful restriction. It is compact without being sparse, though slightly verbose in the middle 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?

With no output schema, the description should convey what the caller receives, and it does say a hosted payment session is returned. However, it does not describe the response shape, completion flow, error behaviors, or what 'order' refers to precisely. The description is adequate for basic invocation but leaves gaps for an agent needing to handle responses.

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 67%, and the description adds important meaning beyond the schema by explaining the behavior associated with tokens: 'Without a token' yields a hosted session, while a 'delegated shared payment token' causes direct settlement. This clarifies the role of accessToken and sharedPaymentToken, though 'order' remains underspecified.

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 opens with a specific verb and resource: 'Start payment for a stage.' It clearly identifies the action and names the result (hosted payment session or direct settlement). It does not explicitly contrast itself with siblings like accept_offer or create_order, but the purpose is concrete and unambiguous.

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 provides clear usage context by explaining two modes: without a token you get a hosted session, and with a delegated shared payment token the payment settles directly. It does not explicitly name alternatives or when-not-to-use conditions, but the behavioral guidance is sufficient for selecting this tool in most cases.

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