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

Annotations cover safety (readOnly, idempotent, openWorld), and the description goes well beyond them: no authentication required, a concrete rate limit of 20 checks/hour per connection, and the full result taxonomy (receivable under both, only 0208, only 9925, neither, unknown) plus access point, risk and fix per ID. This is exactly the operational context an agent needs 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?

Front-loaded with the most decision-relevant facts ('Free, no sign-in needed') followed by outcome taxonomy and the rate limit/alternative. The single long sentence packing the return categories is dense but every clause earns its place; only slight tightening would improve scannability.

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?

There is no output schema, so the description carries the return-value burden and does so fully — enumerating every possible status, the recommended ID, the access point, and the localized risk/fix. Combined with auth and rate-limit disclosure, nothing an agent needs to call or interpret results is missing.

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% with an enum and worked examples, so the schema does the heavy lifting. The description still adds meaning beyond it: that the id resolves against two distinct Belgian ID schemes (0208 enterprise and 9925 VAT) and that lang localizes the returned risk/fix text, which clarifies the parameter's downstream effect rather than just its type.

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?

Specific verb and resource: checks whether one Belgian company can receive Peppol e-invoices, and uniquely qualifies the scope as two Belgian Peppol IDs (0208 KBO/BCE and 9925 VAT). An agent can distinguish this from check_belgian_peppol_list and lookup_peppol_participant without opening any 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?

Explicitly states the access conditions ('free, no sign-in needed'), the volume constraint (20 checks/hour per connection), and routes the alternative case: 'for client lists use check_belgian_peppol_list (sign-in)'. Both when-to-use and when-to-use-something-else are named with the selecting condition.

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