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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so the safety profile is covered; the description still adds real value beyond that by disclosing the GR→EL prefix normalization rule, the deterministic-vs-LIVE execution distinction, the circuit-breaker hold behavior, and the statelessEphemeralAudit flag. It does not mention authentication requirements or rate/latency characteristics of the LIVE VIES call, which keeps it short of a 5.

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?

Content is front-loaded: identity and normalization come first, then eligibility, then modes, then return shape, then sibling routing. The second sentence is a single very long clause chain dense with parenthetical field lists, which costs some readability, but every sentence carries information and nothing is padded.

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?

With no output schema, the description usefully names the structuredContent keys (ok, mode, normalizedVat, reverseChargeEligible, viesDecision), which the agent would otherwise have to discover at runtime. Error semantics for a failed LIVE lookup and any auth prerequisites for the European Commission endpoint are the remaining gaps, but for a 5-parameter read-only tool this is close to 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 description coverage is 100%, so the schema already documents every parameter, including the mode enum and its default. The description only partially restates the enum ('VALID', 'INVALID', 'MS_UNAVAILABLE', 'LIVE') and omits FORMAT_ONLY and GLOBAL_MAX_CONCURRENT_REQ, adding no syntax or format detail beyond what the schema provides — 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?

The description opens with a specific verb pair (normalize/evaluate) and a precisely bounded resource (EU-27 and XI B2B VAT identifiers, reverse-charge eligibility). It explicitly names the sibling it is not (revclamp_reconcile_b2b_net_terms) and its domain, so an agent can distinguish it from all eight siblings without opening a 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?

It states the use case directly ('Use this tool for EU/XI VAT tax compliance') and supplies the alternative plus the condition that selects it (Net-15/30/60 deposit and credit-note accounting). The mode enumeration also tells the agent which evaluation path to pick for simulation vs. live lookup.

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