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

With no annotations and no output schema, the description carries the full behavioral burden, and it does disclose meaningful traits: the 9-digit ABA scope, the 3-7-1 weighted checksum, and the non-obvious preprocessing step of stripping non-digits before validating. It still omits what is returned (boolean vs. details) and error behavior, but the core behavior is well disclosed.

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 functional sentence is front-loaded and efficient, but the description redundantly restates the tool name ('Is Routing Number:') and appends a marketing line with an emoji about free trial calls that adds nothing for tool selection or invocation.

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 validator with no annotations or output schema, the description covers the input and validation logic adequately. It is incomplete on return-value semantics and failure cases, and it never addresses the sibling 'x402-routing-number', so an agent lacks enough to be fully confident.

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 single 'value' parameter has 0% schema description coverage, so the description must compensate. It does partially, by stating the expected input is a 9-digit ABA routing number and that non-digits are stripped, but it does not clarify whether the input can include separators, spaces, or partial numbers.

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 gives a specific verb and resource ('Checks a 9-digit ABA routing number') and even names the exact validation algorithm (weighted 3-7-1 checksum), which tells an agent precisely what the tool does. It does not, however, differentiate itself from the sibling 'x402-routing-number', leaving ambiguity about which routing-number tool to pick.

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?

There is no when-to-use guidance, no when-not-to-use, and no mention of the closely named alternative 'x402-routing-number'. Usage can only be inferred from the validator phrasing, and the only extra text is a pricing promo, which is not usage guidance.

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