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

Adds substantial context beyond the annotations: the exact return shape, the cost model (1 credit = 1 CAD cent per successful call), and the rejection-before-billing guarantee for malformed input. That billing and failure semantics are things an agent genuinely cannot infer from readOnlyHint/openWorldHint.

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?

Three short sentences, front-loaded with the operation, then the return shape, then the cost/error contract. Nothing is repeated from the schema 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 no output schema, the description compensates by spelling out the returned fields, and it covers billing and invalid-input handling. The only gap for a money tool is how rounding is handled on a tax-included split, which is left unstated.

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 amountCents, rate and included are already fully documented, making 3 the baseline. The description echoes the two modes and adds the CRA rate-confirmation nudge, but no syntax or rounding detail beyond the schema.

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 and resource (adds NS HST / splits a tax-included total) and covers both operating modes in one clause. No sibling in the list does tax arithmetic, so the agent can route to it unambiguously.

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?

"or splits a tax-included total" plus the schema-linked mode selection gives clear context for which behavior applies, and "Confirm the rate with CRA" flags the one real caveat. It stops short of explicit when-not-to-use guidance (e.g. non-NS provinces), so it is a 4 rather than a 5.

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