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

Annotations already cover safety (readOnly, idempotent, non-destructive, closed-world), yet the description adds substantive traits beyond them: calculations are deterministic, inputs must be explicit and non-identifying, no values are sourced from chats/files/accounts/documents/web, and the output excludes any decision or recommendation. These are exactly the behavioral boundaries an agent needs for routing and for deciding whether a request is in-scope.

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 sentences, front-loaded with the core capability, then the input constraint, then the output boundary. No filler, no restatement of the title, and every clause carries decision-relevant information.

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 an output schema present, return values need no explanation, and the nested scenario schema documents its own fields and enums. The description covers purpose, input sourcing limits, and output boundaries, so nothing an agent needs to decide whether and how to call this tool is missing.

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 67%, so the baseline is around 3, but the description adds meaning the schema does not: scenarios are capped at six (matching maxItems), the product is fixed-rate (constraining how annualRate should be interpreted), and inputs must be explicit numbers. It does not clarify effectiveDate's relationship to the supplied rates or the scenario object's required fields beyond what the schema states.

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 first sentence gives a specific verb+resource+scope: 'Compares up to six Canadian fixed-rate payment and amortization scenarios using deterministic calculations.' An agent immediately knows this is a multi-scenario calculator, not a single-metric one. It does not explicitly name or distinguish itself from siblings like calculate_qualification or estimate_prepayment_penalty, which keeps it below a 5.

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 description sets clear negative scope: 'Returns calculations only: no approval, underwriting, or suitability decisions, no lender or product recommendations,' and restricts inputs to 'explicit, non-identifying numbers in the request.' This tells the agent when this tool is the wrong choice, but it never names an alternative (e.g., refuse_unsupported_mortgage_request) or states the positive trigger conditions, so no explicit alternatives routing.

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