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

Annotations already mark the tool read-only and idempotent. The description adds the key behavioral fact that no auth is needed and all receipts are accessible, which is not present in the annotations and is important for an agent to know before calling.

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 sentences convey the action, the lookup key, and the access model with no filler. The essential operation is front-loaded.

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 single-parameter, read-only lookup, the description is nearly complete: it specifies the input, the access policy, and the object class. No output schema exists, so a brief note about the returned receipt object could add value, but the tool is simple enough that 'fetch a receipt' is likely sufficient.

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 'serial' as 'The receipt serial number.' The description merely says 'by serial number,' adding no additional format, constraints, or nuance beyond the schema.

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 action ('Fetch') and resource ('a receipt'), keyed by serial number. This distinguishes it from siblings like vigil_verify_receipt and vigil_journal_get without needing to open the schema.

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 'Public — anyone can read any receipt' sentence gives clear context about access and when no authorization is required. It doesn't explicitly compare to alternatives, but the verb and resource are unique enough among siblings that the intended use is clear.

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