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

Annotations already declare readOnly/idempotent/non-destructive, and the description adds materially new behavior: malformed input is rejected, and each successful call costs $0.001 USDC via x402 on Solana with an HTTPS fallback endpoint. That payment/error context is exactly the kind of detail annotations cannot carry.

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?

Front-loaded with the core definition and example, then constraints, then pricing/access. Every sentence carries information; the pricing and endpoint sentences are long but earn their place for callers using this as a paid API.

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 one-parameter arithmetic tool with no output schema, the description covers input format, edge cases, cost, and transport alternatives. There is nothing an agent needs in order to call it correctly that 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 0%, so the description must compensate, and it does: it clarifies that the input is an integer whose sign is ignored, that no leading zeros are allowed, and that malformed values are rejected. It never names the parameter 'n', but the semantics of the single input are covered well.

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?

States a precise operation (sum of decimal digits) with a worked example (199 -> 19) and explicit scope rules (sign ignored, no leading zeros). It distinguishes itself implicitly from siblings like digital_root/digit_power_sum, though it never names an alternative.

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 explains what inputs are valid but gives no when-to-use guidance and never points to related siblings (digital_root, digit_rev, byte_sum) that an agent might confuse it with. Usage must be inferred from the operation name.

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