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

Annotations already cover read-only and idempotency, so the lower bar applies, yet the description still adds meaningful behavior: the check is format-only and it returns a signed XDR-1 receipt, which tells the agent a verifiable artifact comes back. It stops short of describing failure behavior (error vs. invalid-result) and does not address why openWorldHint=true for what reads as an offline syntax check.

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?

Three short sentences that front-load the primary action and scope before usage context and return payload. No filler or restated title language beyond what is needed.

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 an output schema present, the description needn't explain return fields, and it correctly spends its words on scope, usage context, and the receipt artifact. The remaining gap is what an invalid EIN produces and whether the receipt is returned on failure — the one thing an agent still has to guess.

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?

Single parameter with 100% schema description coverage ('US EIN, 9 digits; dashes/spaces allowed'), so the schema does the heavy lifting and the baseline is 3. The description's 'NN-NNNNNNN' pattern adds a concrete shape, though it slightly undersells the dashes/spaces tolerance stated in 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?

States a specific verb and resource ('Validate US Employer Identification Number FORMAT') and pins the exact pattern (NN-NNNNNNN), with the capitalized 'FORMAT' making the syntax-only scope explicit. It is distinguishable from heavier counterparty checks, but it never names or contrasts with the many other format validators in the sibling set (pan_format_check, abn_check, e164_check, uscc_check).

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 during US business counterparty KYB verification' gives a usage context, which is more than implied. However, there is no when-not guidance (e.g., that this does not confirm the EIN is assigned or active) and no routing to alternatives such as uscc_check or vendor_onboarding_pack for the broader onboarding flow.

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