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

With no annotations, the description carries the full burden. It does disclose a meaningful behavioral trait — the $0.005 USDC x402 payment requirement — which is valuable and not derivable from structured fields. However, it says nothing about return format, error behavior, or whether the extra capabilities (next/previous day, first/last of year) are returned by the same call.

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 question, followed by the extended capabilities and then the price. Three tight sentences with no padding, though the middle enumeration is a slightly loose list.

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 no output schema and no annotations, the description partially compensates by enumerating the answers it can produce (open/closed, next/previous trading day, first/last of year). But how those extra answers are triggered by a single date-only parameter is left ambiguous, and no output shape is described.

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%: the single date parameter already documents the YYYY-MM-DD format and the 1955-01-01..2027-12-31 supported range. The description adds no parameter-level meaning beyond the schema, so the baseline of 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?

The description names a concrete resource and question ('Is the Tokyo Stock Exchange (JPX) open on a date?') plus additional derived answers, so the agent understands the domain immediately. It does not, however, distinguish itself from siblings like is_business_day or is_business_day_free, leaving overlap in scope unaddressed.

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?

There is no explicit when/when-not guidance and no mention of the obvious alternatives (is_business_day, is_business_day_free, jpx_record_date, jpx_sq_date). Usage must be inferred from the JPX-specific framing alone.

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