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

There are no annotations, so the description must fully convey behavioral expectations. It states the core listing behavior but does not disclose whether authentication is required, whether data is real-time, or any side effects (even though listing is inherently read-only, the explicit lack of such disclosure means the agent must assume).

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 a single, front-loaded sentence containing only essential information. Every word contributes value, with no fluff or redundancy.

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 zero-parameter list tool with no output schema, the description sufficiently conveys the output (a list of rails with merchant counts). No additional context is necessary for the agent to invoke it 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?

The tool has zero parameters and an empty schema, so there is nothing to explain. The baseline of 4 applies because the description correctly avoids over-explaining and lets the schema's emptiness speak for itself.

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 tool's function: listing supported payment rails and their current merchant counts. The verb 'List' is specific, and the resource ('supported payment rails') is distinct from sibling tools dealing with merchants, categories, and endpoints.

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?

No guidance is provided on when to use this tool versus alternatives. It does not mention any prerequisites, exclusions, or related tools that might outperform it, leaving the agent without decision support.

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