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

Annotations already establish read-only, idempotent, non-destructive behavior, so the bar is lower. The description still adds real value beyond them: the upstream source (Bank of Canada Valet API), that figures are read live, that each carries its own observation date, and the important caveat that mortgage numbers are system AVERAGES rather than a lender offer.

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 trigger and the headline list of rates, then layered with source, date semantics, and the averages caveat. It is on the longer side for a single paragraph, but each clause carries distinct information an agent would otherwise have to guess.

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 carries the burden of explaining what comes back, and it does so thoroughly (the specific rate types plus per-figure observation dates). Source provenance and the mortgage-average caveat round out everything needed to call and interpret it.

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?

The tool takes zero parameters, so the baseline of 4 applies. The description instead spends its budget describing the returned figures, which is the useful information for a no-input tool.

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?

Names a specific resource (current Canadian reference interest rates) and enumerates exactly which figures it returns: BoC policy rate, prime rate, system-average mortgage rates, and GoC benchmark bond yields. An agent can distinguish this from 'stats_marche' or other siblings without reading any schema.

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?

Opens with an explicit trigger ('Call this for the current Canadian reference interest rates'), giving clear context for when it applies. It does not, however, name an alternative sibling or state when-not to use it, so routing against 'stats_marche' is left to inference.

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