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

With no annotations, the description carries the full burden and does substantial work: it enumerates the returned fields (mid, buy, sell, spread, 1/7/30-day changes, confidence class, citation) and discloses the tiered-access behavior — free data is one day delayed, a Pro key unlocks the current day. It stops short of stating rate limits or the exact auth mechanism.

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 sentences, front-loaded with the core capability and the source, then closing with the access-tier caveat that affects result freshness. No filler, no repetition of the title.

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?

An output schema exists so return values need not be spelled out, yet the description still previews them usefully. Access tiering is covered. The only missing piece for a 3-parameter lookup tool is any mention of error behavior for unsupported currencies or dates outside plan coverage.

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 100%, so date, lang, and currency are already documented in the schema. The description only loosely gestures at them ('the rate on a given day', 'a ready citation'); it adds no format or constraint detail beyond what the schema already provides, making the baseline 3 appropriate.

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 ('latest street exchange rate of an emerging-market currency against USDT') and names the source index, so the agent knows this is a current street-rate lookup rather than history or official rates. It does not explicitly name the distinguishing siblings (get_rate_history, get_official_vs_street), leaving that inference to the agent.

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 the rate on a given day' implies the date-parameter use case, and the free/Pro delay sentence tells the agent when the returned value will be stale. However, there is no explicit statement of when to prefer this over get_rate_history or get_official_vs_street, which are the closest alternatives.

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