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

With no annotations, the description carries the full burden. It does useful work: it discloses the cost model ('FREE'), the quote base (vs CAD), and the cadence (daily average, not intraday/real-time). It says nothing about failure modes for unsupported currencies or what metadata accompanies the rate, which is a real gap for a zero-annotation tool.

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?

Two short sentences, both front-loaded with the information that matters — cost/freshness first, input contract second. No filler, nothing buried.

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 single-parameter, no-output-schema tool this covers source, base currency, cadence, and input format, which is essentially the whole contract. The remaining omission is the shape of the returned rate (e.g. whether an effective date accompanies the value) and error behavior on invalid codes.

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 coverage is 0% — the schema only says 'string' — so the description must compensate, and it does: it specifies the 3-letter ISO format and gives four concrete examples (USD, EUR, GBP, JPY). It stops short of stating case sensitivity or behavior for a non-supported code.

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 (fetch latest) and a precisely scoped resource: Bank of Canada daily average rate for a currency quoted against CAD. Naming the Bank of Canada source is what separates it from siblings like ecb_reference_rates and global_fx_rate without the agent opening 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?

There is no explicit when-to-use or when-not-to-use guidance, and no sibling is named. 'Latest' implicitly excludes historical use (global_fx_timeseries) and 'CAD' implicitly excludes cross-currency conversion (convert, global_fx_convert), but the agent has to infer all of that. 'FREE' is the only usage-adjacent signal.

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