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

Annotations already declare readOnly/idempotent/non-openWorld, but the description adds substantive behavior: the four possible outcomes (parsed, ambiguous, malformed, unsupported), how ambiguity is resolved (options differ in recipient, network or amount and none is chosen), the limited token reference list, and the no-fetch guarantee. These prevent an agent from treating a parse as verification or expecting a network resolution.

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?

Five dense sentences, front-loaded with the cost signal and the core action, then outputs, then limits. Every clause carries information (formats, statuses, guardrails) and nothing is padding.

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 an output schema present, the description need not enumerate return fields, and it still supplies the status vocabulary and the ambiguity rule. Combined with complete parameter coverage and full annotations, an agent has everything needed to call and interpret this tool 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?

Schema description coverage is 100%, so both parameters already carry full definitions including the string/object union and the transport enum. The description largely restates those input forms and adds only the note about the limited token reference list, so the baseline 3 applies.

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 specific verb (decode) and resource (one x402 v2 payment request), enumerates the accepted input encodings, and says exactly what the output is: every accepts option with atomic and decimal amounts. The scope line ('Does not verify the merchant, authorize a transfer...') draws the boundary against neighbors like get_receipt and x402_known_assets without ambiguity.

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 'Free.' prefix and the explicit negative list (does not verify merchant, authorize transfer, assess fraud, guarantee payment; URLs never fetched) tell the agent when this tool is the wrong choice. However, it never names an alternative sibling or states a positive trigger condition (e.g. 'use before signing a 402 response'), so routing is only implicit.

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