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

Annotations already establish read-only, idempotent, and non-destructive behavior, so the lower bar applies; the description still adds substantive context by disclosing that the content hash is recomputed and altered content is rejected. What remains unspecified is the failure surface (e.g., behavior on an unknown receipt ID) and the shape of the returned envelope, though the output schema covers the latter.

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 sentences, front-loaded with the usage trigger and followed by the one behavioral caveat that matters. No filler, no restatement of the title.

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, idempotent read tool with an output schema, the description covers when to use it, what it does mechanically, and its key limitation (integrity, not truth). Return-value explanation is correctly left to the output schema.

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 receipt_id parameter already documents its role and pattern (^fr_[0-9a-f]{64}$). The description only restates that lookup happens 'by its receipt ID' and adds no format or source detail beyond the schema, so the baseline 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?

Names a specific verb pair (re-fetch/audit) and a precise resource (a previously returned FACTRAIL result identified by receipt ID). The clause 'this checks integrity, not truth' cleanly separates it from the verification-oriented siblings (factrail_verify, factrail_assess) without the schema being opened.

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 when you need to re-fetch or audit a previously returned FACTRAIL result by its receipt ID' gives an explicit trigger condition — you must already hold a receipt from an earlier call. The integrity-vs-truth note functions as a partial exclusion, but no alternative tool is named outright for the truth-checking case.

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