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

With no annotations, the description must carry behavioral disclosure, and it does. The warning that the key is fetched from the receipt's own `iss`, not from whoever handed it over, is a critical non-obvious behavior. The note about external receipts also clarifies scope, though it does not explicitly state side effects or error behavior.

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 compact and front-loaded: one purpose sentence, one security warning, one usage sentence. The warning is emphatic but earns its place because it prevents a critical security misuse. There is no filler.

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?

Given a single required string parameter and an output schema, the description covers the purpose, the key security caveat, and the supported input scope. It does not fully compare against sibling tools or document the receipt's wire format, but those are minor given the tool's simple surface.

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 schema only provides the parameter name and type, so the description must supply meaning. It does by indicating the receipt carries a signature and an `iss` claim that determines which key validates it. It does not specify the serialization format, but for a single string parameter this is a moderate gap, not a blocking one.

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 opens with a specific action and object: 'Check a receipt's signature against the key its issuer publishes.' This clearly identifies the tool's core function. It does not explicitly name sibling tools, but the usage note about receipts 'from anywhere, including ones this server did not produce' gives some scope differentiation.

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 an explicit usage instruction: 'Use this on receipts from anywhere, including ones this server did not produce.' This tells an agent the tool is not limited to receipts the server generated. It does not name alternatives or list when not to use it, so it stops short of full routing guidance.

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