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

Annotations already indicate readOnly, idempotent, non-destructive behavior; the description goes well beyond this by disclosing refresh cadence ('Rebuilt every 5 minutes'), authentication options (Bearer key, x402 payment), and free sample mode behavior with truncation and marked results. This is rich behavioral context.

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 front-loaded with purpose and examples, then follows with refresh, auth, and fallback details. It is economical despite length; the only minor issue is some repetition of api_key guidance already present in the schema.

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 one optional parameter, no output schema, and readOnly annotations, the description covers all essential aspects: purpose, usage, refresh behavior, authentication, and unauthenticated fallback. It doesn't describe return shape, but that's not critical for a straightforward snapshot tool.

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 already documents api_key thoroughly with header preference and sample mode. The description adds complementary practical meaning: rate limits, x402 alternative, and the no-credential fallback, making the parameter more actionable.

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?

Description states 'Current headline values for all 25 zones in one call', giving a specific resource and scope. The examples ('which European zone has the lowest price right now', 'rank US ISOs by demand') make its comparative purpose unmistakable and distinguish it from per-zone or history siblings.

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 gives clear usage context with concrete examples of when to use this tool ('cheapest way to compare zones'). It does not explicitly name alternatives or state when not to use it, so it stops short of full exclusions.

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