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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the description correctly spends its words elsewhere: the validation algorithm used, the partial result payload (institution type and Fed district), the explicit non-capability (no bank-name lookup), and the statement that nothing is stored. It does not say what happens on an invalid number, which is the main remaining gap.

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?

Four short sentences, front-loaded with purpose, then the method, then two boundary statements. Nothing is repeated from the annotations and no sentence is filler.

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 one-parameter validation tool with no output schema, the description supplies what matters: the check performed, the returned fields, and the excluded capability. Input format expectations and the invalid-number behavior are the only omissions.

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 0% and the single parameter has no description, so the description is the only source of meaning — it implies the input is a US ABA routing number but never states the expected 9-digit format, despite the schema permitting up to 32 characters. Useful implied semantics, but format guidance that would prevent bad input is absent.

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 and resource ("Validate a US ABA routing number") and immediately names the concrete mechanism (3-7-1 checksum) plus the outputs (institution type, Federal Reserve district). It is unmistakably distinct from siblings like validate_iban or validate_bic, which cover other numbering schemes.

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 scopes usage by exclusion — "It does not look up the bank's name" — telling the agent this is a format/checksum validator rather than a bank-directory lookup, and "US" rules out non-US identifiers. It stops short of naming a sibling alternative or stating prerequisites, so context is clear but not exhaustively routed.

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