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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint and destructiveHint=false, so the safety profile is fully covered. The description adds genuine value by enumerating what the response contains (category group, high-risk flag, Visa and Mastercard definitions), which is the main thing annotations cannot convey. It does not say what happens when no match is found or how ambiguous business descriptions are handled.

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 sentences, zero filler, and the dual mode is front-loaded right after the tool's name so the agent learns the branching behavior before any detail. Examples are embedded inline rather than padding out extra sentences.

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 no output schema, the description carries the return-value burden and does so by naming the four result components. It is slightly incomplete on edge cases — no-match behavior, whether a business description can return multiple candidates, or output format — but for a single-parameter read-only lookup it is nearly sufficient.

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% and already documents the single query parameter, so the baseline is 3. The description earns an extra point by explaining that the one field is polymorphic — it carries either a code or a natural-language business type — and pairs each form with the shape of result it produces, which the schema alone does not make explicit.

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 ("Merchant category code (MCC) lookup") and immediately distinguishes its two operating modes: resolve a code to a definition, or map a business description to a code. Against siblings like lookup_bin and lookup_issuer, the resource is unambiguous.

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?

Tells the agent exactly which input shapes to use and when, with concrete examples ("5411", "grocery store", "software"). It stops short of naming alternatives or exclusions (e.g., when to prefer lookup_bin for card-prefix questions), so the routing guidance is clear but not complete.

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