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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so the safety profile is covered. The description adds behavior the annotations cannot express: an x402 payment of 0.001 USDC is required and there is no free path, which materially changes how an agent should invoke it. It stops short of saying what happens on payment failure or whether the snapshot is live or cached.

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 clauses, both high-value: contents first, then cost and the paid-only constraint. No filler, no repetition of the tool name.

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 no inputs and no output schema, the description correctly spends its words on what the report contains and what it costs. Only minor gaps remain, such as freshness of the pricing snapshot and error behavior on a failed payment.

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 and the schema is empty with full coverage, so the baseline of 4 applies. There is nothing for the description to clarify about inputs.

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 names concrete contents (catalog probe, supported Workers AI models, pricing snapshot), so an agent knows exactly what comes back. It falls short of a 5 because it does not distinguish itself from the sibling 'fidelity_models', which appears to cover the same Workers AI model listing.

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?

'Strictly paid — no free tier' implies the agent should reach for a free sibling (e.g. fidelity_quick_check) when cost matters, but no alternative is named and no condition for choosing this over fidelity_models or fidelity_audit is stated. Usage is implied rather than prescribed.

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