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

With no annotations, the description carries the whole burden. It usefully discloses that the call is free and that it is a retrieval (non-mutating), but it stays silent on what happens when neither key is supplied (both parameters are optional), which key wins if both are given, and what the result looks like when no receipt exists. Those are real behavioral gaps for a lookup tool.

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 tight sentences: the first front-loads the operation and cost, the second front-loads the situation that calls for it. Every clause carries information and nothing is repeated from 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?

For a two-parameter read with no output schema and no annotations, the description covers the essentials: what it does, that it costs nothing, and when to reach for it. The main residual gaps are the both-keys/neither-key behavior and not-found handling, which keep it just under fully 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 description coverage is 0%, so the description has to compensate and only partially does: the phrase "by receipt_id or idempotency_key" conveys that the two are alternative lookup modes, which is marginally more than the bare schema. It adds no format, precedence, or both-supplied guidance, and the schema marks neither parameter as required.

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 gives a specific verb and resource ("fetch a stored receipt") plus the two lookup keys, so the agent knows exactly what the tool returns. It never needs to name a sibling because check_payment_recipient, list_capabilities and ping are clearly different operations, but that sibling differentiation is left implicit rather than stated.

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?

"Use it to reconcile an uncertain payment before ever paying again" is a concrete trigger condition and even implies the ordering constraint relative to a pay action. There is no explicit when-not-to-use or named alternative, which keeps it short of a 5.

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