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

Annotations already declare readOnly/openWorld/idempotent/non-destructive, so the bar is lower. The description adds meaningful behavior beyond that: it is free, costs no wallet or payment, and requires no arguments – exactly the traits that distinguish a preview from the paid call.

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?

A single sentence with the [FREE] marker front-loaded, immediately conveying the cost benefit. Zero wasted words.

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 zero-arg preview tool with no output schema, the description explains what it returns (a sample signal_latest response) and the cost model, which is enough to call it correctly. A hint at what the sample contains would make it fully complete.

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, which sets the baseline at 4, and the description reinforces this with 'no arguments needed.' There is nothing further to document.

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 purpose: a free sample response for signal_latest to inspect the output shape before paying. The verb+resource (preview of the signal_latest payload) is clear, and it explicitly distinguishes itself from the paid sibling signal_latest.

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?

Gives clear when-to-use context ('see the shape before paying 0.03 USDC') and names the paid alternative by implication. It does not spell out when-not to use it (e.g. when real data matters), so it stops short of a fully explicit routing statement.

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