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

Beyond the annotations (idempotentHint=true, destructiveHint=false), the description discloses cost ($0.01/call), the billing guarantee that errors and unverified rules are never billed, and that output is deterministic and cites primary sources. Those are meaningful behavioral traits an agent cannot get from the annotation block; only the x402 payment mechanics are left to the schema.

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?

Purpose, pricing, guarantees, and mode routing are front-loaded and the whole thing is compact. The only waste is the long 'Required by mode...' clause, which duplicates the schema's mode description almost word-for-word.

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 nine parameters, nested objects, an output schema, and annotations, the description supplies the missing commercial context (price, billing rules, deterministic sourcing) plus mode routing. It is nearly complete; a hint about as_of selecting the rules version and the uncharged x_payment discovery flow would close the gap, though the latter is covered in the schema.

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 the schema already documents every parameter, including the mode->parameter mapping. The description reproduces that mapping verbatim rather than adding syntax, formats, or defaults that the schema lacks, so this is the baseline 3 case.

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?

It names a specific domain (Japanese consumption tax) and enumerates concrete capabilities: per-rate invoice rounding, qualified-invoice validation, simplified taxation, and the 20% special case. That scope cleanly separates it from neighbors like jp_crypto_tax, jp_retirement_tax, and verify_invoice, so an agent can pick it without opening the 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?

The mode->parameter mapping implicitly tells the agent which mode fits which input shape (invoice lines vs. document vs. taxable sales), which is real routing guidance. However, it never states when to prefer this tool over alternatives such as verify_invoice or quote_parse, nor any prerequisites, so usage guidance is only implied.

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