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

The description explicitly labels the operation '(additive)', which is the MCP-spec wording for destructiveHint=false, while the annotations declare destructiveHint=true. Key behaviors are also undisclosed: what happens if the external_id already exists (upsert vs. error), whether the call overwrites existing customer fields, and whether permissions beyond the API key are needed.

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 compact sentences, front-loaded with the verb+resource and followed by the single most useful implementation detail (endpoint and body envelope). Nothing extraneous.

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 create tool with no output schema, the description adequately covers what is created but omits the duplicate-external_id/upsert behavior, the return payload, and any auth or rate-limit expectations. The schema is rich enough that missing parameter detail is acceptable, but the behavioral gaps leave an agent guessing at edge cases.

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% across all 12 parameters, so the schema already carries the field semantics and the baseline of 3 applies. The description adds only the outer body shape ('{ "customer": { ... } }'), a small formatting hint rather than per-parameter meaning.

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 ('Creates a customer in Lago') and adds the underlying endpoint (POST /customers) plus the body envelope, so the agent can tell exactly what operation this performs. No sibling performs a customer creation, so it is inherently distinct from lago_get_customer and lago_list_customers.

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 description never says when to reach for this tool versus alternatives (lago_get_customer, lago_list_customers) and gives no prerequisite or precondition guidance. The lone qualifier '(additive)' gestures at semantics but is not usage direction.

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