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

Annotations cover the safety profile (readOnly, idempotent, non-destructive, closed-world), yet the description adds genuinely new operational context: it is a pure calculation with no network request, nothing is stored, it costs $0.02 USDC via x402 with the payment key location, and invalid input is errored without charge. That payment and billing-failure behavior is not derivable from the annotations.

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?

Four tight sentences lead with the core transformation and then layer the alternative, the no-side-effect guarantee, and the pricing/error model. Every clause carries information an agent needs; nothing is redundant 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?

For a 16-parameter, nested, no-output-schema tool this covers the essentials: what it produces, that it is side-effect free, the cost, and error handling. It only gestures at the return shape ('validated invoice JSON with totals'), which is thin given the absence of an output 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 only 31% across 16 parameters, so the description is expected to compensate, and it partially does by naming hours x rate, rounding, grouping and expenses. However, it adds no meaning for currency, discount, tax_rate, notes, terms, payment_instructions or the buyer/seller sub-objects, leaving the coverage gap only partly closed.

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+resource+transformation: 'Turn time entries ... into a validated invoice JSON with totals.' It also names the sibling it is not ('Use validate_invoice ... instead if you already have an invoice'), so an agent can distinguish it from the other six siblings.

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?

Explicitly routes the agent to validate_invoice with the selecting condition ('if you already have an invoice'), which is clear alternative guidance. It does not address the other calculation/payment siblings (pricing, late_fee_calc), so some ambiguity remains, but the primary branch point is covered.

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