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 adds only the underlying REST endpoint (GET /subscriptions/{external_id}), which is documentation mapping rather than behavioral context such as error behavior or auth/permission requirements. With annotations present, a 3 is appropriate.

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, front-loaded with the operation and lookup key. Nothing is bloated; the endpoint sentence is slightly redundant but serves as a useful API mapping.

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?

This is a low-complexity single-object GET with full schema coverage and a readOnly annotation. No output schema exists, so the description could say more about the returned subscription, but the tool's purpose and required input are unambiguous enough for correct invocation.

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% and the single parameter is already documented as the subscription's external unique identifier. The description merely restates 'by external id' with no added format, sourcing, or validity guidance, 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 and resource: 'Retrieve a single subscription by external id.' The word 'single' implicitly distinguishes it from the sibling lago_list_subscriptions, but no sibling is named. An agent can identify the operation, though differentiation is left to inference.

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?

No when-to-use guidance, no alternatives, no prerequisites. 'By external id' hints at the required lookup key but the description never says to prefer this over lago_list_subscriptions or lago_list_customer_subscriptions, nor what to do if the subscription is not found.

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