All 5 tools have explicit schemas with type definitions and descriptions. Naming follows verb_noun pattern (list_*, get_*) and is clear. However, descriptions are generic and lack LLM-optimization guidance. Parameters lack enums for constrained values (status, customer_id filters). No output schema documentation. Critical security issue: _apiKey and _site exposed as tool parameters instead of server-side injection. No error handling guidance or recovery paths documented.

• Move _apiKey and _site to server-side environment variables or a configuration file. Inject them inside callTool() without exposing them as parameters. This prevents credential leakage in logs and traces.
• Document output schemas for each tool. Add a comment block showing the structure of returned objects, e.g., 'Returns: { list: [{ subscription: { id, customer_id, status, ... } }], next_offset?: string }'.
• Convert status and customer_id parameters to enum types in inputSchema. For chargebee_list_subscriptions, add 'enum: ["active", "cancelled", "non_renewing", "future", "in_trial", "paused"]' to the status property.
• Expand tool descriptions to 120-180 characters. Include WHEN to use (e.g., 'Use to retrieve a single subscription by ID when you have the subscription_id. Returns full subscription details including billing cycle and status.') and any prerequisites.
• Enhance error messages in callTool() to include recovery hints. E.g., 'Subscription not found (404). Verify subscription_id is correct. Use chargebee_list_subscriptions to find valid IDs.' Include the invalid value and valid constraint.