Single tool (send_email) with well-structured schema and comprehensive parameter descriptions. Tool name is action-oriented and clear. Input schema is complete with proper JSON Schema types, format constraints, and conditional logic (oneOf for attachments). Description is detailed and contextualizes use case. However, no output schema is documented, and error handling guidance is minimal. The tool performs a state-changing operation (send email) but lacks explicit confirmation/dry-run pattern. Natural language scheduling parameter ('tomorrow at 10am') is good UX but may be fragile without validation details or error recovery guidance in the description.

• Add explicit output schema documentation: describe the response object (e.g., email_id, sent_at, status). This enables agents to reference the sent email ID in follow-up operations.
• Add a 'dry_run' boolean parameter (default false) or a separate 'preview_email' tool. Agents can call preview_email first, review the output, then send_email. Prevents accidental sends.
• Clarify scheduledAt parameter: add timezone guidance to description (e.g., 'Uses system timezone or ISO 8601 offset. Example: "tomorrow at 10am EST". If ambiguous, defaults to UTC.'). Document what happens if parsing fails.
• Enhance error messages with actionable guidance. Instead of 'Invalid arguments for send_email tool', return: 'Missing required field: subject. Required fields: to (email), subject (string), content (string).' or 'Invalid email format in to field: got "not-an-email", expected user@example.com'.
• Document attachment constraints in parameter description: max file size, supported formats, timeout for remote URLs, expected base64 response size limits.
• Add permission/scope declaration in tool description: 'Requires: send:email scope. Sent emails are recorded in audit logs.'
• Consider wrapping attachment validation errors with suggestions: 'Attachment file not found at /path/to/file. Check the path and ensure the file exists and is readable.'