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

Annotations only declare readOnlyHint and openWorldHint, but the description adds substantial behavioral context: the lookup is free ('nothing is paid'), the results are data-only with no phone number, and the follow-on purchase flow (x402 request, USDC on Base or Solana, no account, eSIM provisioned before payment settles). This goes well beyond the annotations.

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 when-to-use clause is well front-loaded, but the final two sentences are entirely about the separate buy/payment flow, including an endpoint URL and wallet mechanics, which are tangential to a read-only search tool. That space is dense and arguably belongs to a purchasing tool's description.

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 output schema, the description usefully characterizes the return (plan list, prices in USD, ordered by cheapest per GB) and pairs it with readOnly/openWorld annotations. It doesn't mention result limits or pagination, but for a simple filtered lookup it is nearly 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?

Schema coverage is 100%, so the schema already documents all four parameters, making 3 the baseline. The description restates the country/region enumeration and the cheapest-per-GB ordering, but adds little syntax or format detail the schema doesn't already carry.

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 verb and resource ('find'/'plans' for data-only travel eSIMs) with scope qualifiers: no phone number, country name or 2-letter code, or named region. Sorting ('cheapest per GB first') and price currency are included, so an agent knows exactly what this returns and how it differs from any unrelated sibling.

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?

Front-loads a clear trigger ('Use this when someone needs mobile data abroad') and adds that the search is free while purchasing requires a separate POST to the buy endpoint. No sibling overlaps exist in the list, so it doesn't need explicit exclusions, but it also doesn't spell out when NOT to use it.

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