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

Annotations already declare readOnlyHint=true, so the safety profile is covered structurally. The description adds that the response tells you which payment rails the bank supports, which is useful return-value context, but says nothing about not-found behavior, errors, or rate limits.

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?

Two tight sentences: the behavioral payload ('which real-time services the bank supports') leads, followed by the API endpoint for reference. No filler or repetition.

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 parameterless-in-practice lookup with no output schema, the description conveys what the caller gets back and the endpoint reference, and annotations cover the read-only nature. Only the error/not-found case is unaddressed, a minor gap.

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 all three parameters (routing_number, limit, cursor) are already documented, including the 9-digit pattern and pagination semantics. The description's mention of '9-digit ABA routing number' merely restates the schema, so the 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?

States a specific verb ('look up') and resource ('9-digit ABA routing number') and goes further by describing what the lookup reveals: which real-time services (ACH, Real-Time Payments, wire) the bank supports. No sibling tool overlaps with routing-number lookup, so no disambiguation is needed.

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 implied by the nature of a lookup endpoint, but the description never states when to reach for this tool versus alternatives, nor any preconditions (e.g., needing a valid 9-digit number before calling). No exclusions or sibling routing are offered.

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