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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint=false. The description adds the external source (Open Food Facts), specific return contents, and the cost signal ('Paid per call'). This goes beyond the annotations with useful behavioral context, and there is no contradiction.

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 two efficient sentences: the first front-loads the primary purpose and return value, the second packs read-only, context, and cost. No wasted words.

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 simple one-parameter lookup with safety annotations present, the description covers purpose, returned fields, external source, and cost. It omits not-found behavior or output format, but with no output schema and low complexity, these gaps are minor.

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 input schema already documents the only parameter (gtin) as 'Barcode (8-14 digits)', giving 100% schema coverage. The tool description does not add any further parameter-specific detail, so the baseline of 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?

The description states a specific verb ('lookup') and resource ('barcode via Open Food Facts'), and lists the returned fields (product name, brand, quantity, Nutri-Score). The sibling list contains no other barcode/product lookup tool, so it clearly distinguishes itself.

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?

Usage is implied: if you need product data from an EAN/UPC/GTIN barcode, you would use this tool. However, it does not explicitly state when to use it vs. alternatives, nor does it name any alternative. 'Read-only, no local context' provides context but not explicit selection guidance.

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