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

With no annotations, the description carries the full burden. It discloses that the tool performs parallel live probes, names the specific endpoints, and even states the cost per call ($0.003/call). It doesn't describe return format or side effects, but for a read-only health check, the behavior is sufficiently 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 two sentences long, front-loaded with the primary purpose as a question, and then provides the key specifics (facilitators, probes, cost). Every word adds value, with no redundancy or fluff.

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 health check tool, the description is complete: it states what it does, what it checks, and the cost. It also differentiates from siblings by naming both production rails. The lack of output schema is acceptable since the purpose is straightforward and the agent can infer a status response.

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 the schema coverage is 100% (empty object). Since there are no param details to document, the baseline of 4 applies. The description adds contextual meaning by explaining what the tool actually checks, which is more valuable than parameter documentation.

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 purpose: checking if the x402 payment rail is up. It specifies the exact resources probed (Dexter facilitator and Coinbase CDP facilitator) and the attributes checked (supported kinds, schemes, networks, reachability, HTTP status, latency). This distinguishes it from sibling tools like x402_endpoint_verify, which likely targets a single endpoint.

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 opening question 'Is the x402 payment rail up right now?' establishes a clear use case for real-time health monitoring. It does not explicitly discuss alternatives or exclusions, but the specific mention of the two production rails implies when this tool is appropriate compared to more granular checks. This is clear context without explicit alternative guidance.

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