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

Annotations already cover safety (readOnlyHint=true, destructiveHint=false, openWorldHint=true), so the bar is lower. The description does add real value: it discloses the 10-credit cost and that address/branch/replacement-number data is gated behind paid plans, which is behavior an agent cannot get from structured fields. However, it says nothing about failure behavior for failed check digits.

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 body sentence is dense and front-loaded with the most useful facts, and the cost tag is well placed. But the opening 'Routing Number Lookup: Routing Number Lookup is a...' repeats the title twice, wasting the most prominent position on zero-information text.

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?

An output schema exists, yet the description still usefully characterizes result tiers (free vs. paid fields), which helps an agent set expectations. For a one-parameter read-only lookup this is nearly complete; only error/invalid-input handling is unaddressed.

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 a single required parameter, so the baseline is 3. The description adds only the format context (US ABA number) beyond the schema's 'routing number to lookup information about'; the schema already supplies the parameter's meaning and an example value.

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+resource (validate/look up US ABA bank routing numbers) and enumerates what is returned: check-digit validation, bank name and state, routing type, and Federal Reserve district. The 'US ABA' scope implicitly separates it from SWIFTCodeLookup or IBANValidator siblings, but no sibling is named explicitly.

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?

Usage is only implied — an agent can infer it should call this when it has a 9-digit US routing number. There is no explicit when-to-use statement, no exclusion of near-neighbors like IBANValidator or SWIFTCodeLookup, and no note about what happens with an invalid or non-US number.

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