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

Annotations already declare readOnlyHint, openWorldHint, and idempotentHint. The description's 'read-only' reinforces this, and 'No input' and 'Free' add a little context, but it does not disclose any additional behavioral traits (e.g., staleness of rates, rate limits). Since annotations cover the safety profile, a score of 3 is appropriate—it adds some value but not rich behavioral detail.

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?

The description is extremely concise, using three short sentences that each add value: what the tool does, that it requires no input, and its free/read-only nature. No wasted words, and it front-loads the core purpose immediately.

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 simple no-parameter read-only tool, the description fully covers what the agent needs: it explains what the tool returns (current market rates), provides concrete examples (USDC, EURC, major crypto rates and FX), and clarifies the resource (AsterPay). No output schema is present, but the description adequately conveys the expected return at a high level. The sibling tools are unrelated, so no further context is needed.

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 has zero parameters, and the schema coverage is effectively 100% (empty schema). The description explicitly states 'No input,' which is redundant but harmless. There is no parameter meaning to enhance, and the baseline for 0-parameter tools is 4. The description meets this baseline without needing to compensate.

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 description clearly states a specific action ('Get current reference market rates') and identifies the resource ('used by AsterPay') with examples of the rates included. This unambiguously distinguishes it from the sibling tools (check_agent_trust, merchant_resolve, settlement_estimate), which serve entirely different purposes.

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 provides clear context that this tool is for retrieving market rates with no input, and it is free and read-only. While it does not explicitly name alternatives or exclusions, the uniqueness of this tool among its siblings makes the intended usage obvious. It lacks an explicit 'when not to use' but still gives sufficient guidance.

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