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

Annotations already declare readOnlyHint=true, so the safety profile is covered. The description only reinforces this via the 'GET' path and adds no further behavioral context such as permission requirements, error behavior for unknown ids, or what the returned payment method exposes. With annotations carrying the burden, this is an adequate baseline.

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 short sentences, zero filler, with the purpose front-loaded and the endpoint as supporting detail. Nothing is repeated or wasted.

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 two-parameter read operation with full schema coverage and readOnlyHint, the definition is sufficient to invoke correctly. It is not sufficient to fully set expectations on the response, since there is no output schema and the description never characterizes the returned payment method.

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%, with both parameters documented (entityId/foreign id, paymentMethodId e.g. pm_...). The path template '/entity/{entityId}/paymentMethod/{paymentMethodId}' maps parameters to their roles but adds no format or validation detail beyond the schema, so 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+resource ('Get a single payment method') and scopes it to an entity, so the agent knows it returns one record rather than a collection. However, it never names the sibling it is distinguished from (mercoa_list_entity_payment_methods), leaving the list-vs-single distinction for the agent to infer from 'a single'.

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?

The word 'single' implicitly signals this is for fetching one known payment method id rather than enumerating an entity's methods, which implies the alternative. But there is no explicit 'use this when you have a paymentMethodId; use list_entity_payment_methods otherwise' guidance, and no prerequisites or exclusions.

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