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

Annotations only declare readOnlyHint and openWorldHint; the description adds the payment model ($0.03 USDC on Base, eip155:8453), that billing occurs only on HTTP 200, that 4xx is free, and that the tool returns a pointer rather than the dossier itself. It also discloses the full route URL and manifest locations, which is exactly the operational context an agent needs before invoking a paid endpoint.

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 pricing and free-vs-paid disclaimer are correctly front-loaded, but the 'does not fetch or include that data' warning is repeated three times across the text, and the service attribution is appended at the end. It is accurate but noticeably longer than necessary for a single-parameter tool.

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 pointer-returning tool with no output schema, the description covers everything an agent needs: what is validated, what is returned (a payment pointer, not data), price and network, billing conditions, rejection behavior, and where to find manifests. Nothing material about invoking it correctly is missing.

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%, and the rfc property description already states the 12-character persona moral format with the 13-character rejection. The description restates the same constraint and adds only the HTTP 400 consequence, which is behavioral rather than semantic, so this sits at the baseline where the schema does the heavy lifting.

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 uses a specific verb-and-resource pairing: it validates a persona moral RFC locally and returns an x402 payment pointer, explicitly stating it does not fetch the dossier. It immediately separates itself from the free siblings by opening with 'PAID, not part of the free quota and not a free tool,' so an agent can tell it apart from mx_rfc_validate or mx_sat_69b_lookup without opening a 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?

Usage context is clear and concrete: only 12-character persona moral RFCs, 13-character persona física RFCs are rejected with HTTP 400, and no account or API key is needed. It stops short of naming a specific alternative (e.g., 'use mx_rfc_validate for free local validation'), so the agent must infer which sibling to prefer when payment is undesirable.

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