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

Beyond the readOnly/idempotent/openWorld annotations, the description discloses the return shape ('the count and lists the holidays skipped'), the error contract (400 with JSON {error}), and a non-obvious operational requirement: the call is paid, $0.002 USDC via x402, with the exact retry mechanism (params._meta["x402/payment"]). That payment gate is the single most important behavioral fact here and it is stated plainly.

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 is front-loaded in the first clause, then usage, returns, errors, and pricing in a sensible order with no filler. The closing HTTP endpoint line is marginal for an MCP-only caller but is short and arguably aids debugging, so it does not badly bloat the text.

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, the description takes on the job of explaining the return value and does so ('count and lists the holidays skipped'). Combined with the error format, the holiday-rule authority, and the payment requirement, an agent has everything needed to call this correctly on the first attempt.

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 0%, so the description must carry parameter meaning; it adds one genuinely useful semantic, that the range is inclusive of both endpoints. It does not cover date format expectations, whether from must precede to, or how reversed ranges are handled, leaving gaps the schema does not fill.

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 specific verb and resource ('counts working days between two dates') and pins down the exact rule set ('excluding weekends and observed US federal holidays per 5 U.S.C. 6103 and OPM rules'). This clearly separates it from the sibling us_federal_holidays, which presumably enumerates holidays rather than counting working days.

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?

It gives concrete application contexts ('deadlines, SLAs, shipping and payment due dates'), so an agent knows the class of question this answers. It stops short of explicit routing such as 'use us_federal_holidays instead when you need the holiday list', so no exclusion or alternative is stated.

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