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

Annotations cover the safety profile (readOnlyHint, openWorldHint), and the description adds genuinely non-obvious behavior: this is a pay-per-call endpoint billed in x402 USDC on Base, and a missing payment yields a payment challenge in the response rather than an error. That discloses a multi-step protocol an agent could not infer from the schema.

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 compact sentences with zero filler. The core capability is front-loaded and the payment caveat follows, which is the right ordering for an agent scanning for fit.

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 an output schema present, return values need no explanation, and the unusual payment model is called out. The only remaining gap is the mechanics of satisfying the payment challenge (whether it is automatic or requires a separate call), which the agent must infer.

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 description coverage is 100% and the single 'q' parameter is already documented with examples ('London or Madrid'). The description adds no syntax, format, or constraint beyond what the schema supplies, so the baseline of 3 applies.

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 a specific resource and scope: weather for any city, with current conditions plus a 3-day forecast. This is immediately distinguishable from the sibling tools (fx, gas, yields, etc.), none of which cover weather. No ambiguity about what the tool returns.

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?

Usage is implied by the description ('weather for any city'), and there is no competing sibling to route against, so no explicit when/when-not is needed. However, it never states prerequisites such as needing a funded wallet before the first call, leaving the caller to discover the payment flow by failure.

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