❌ "You entered an invalid email"
✅ "Please enter a valid email address"

❌ "You don't have permission"
✅ "This action requires additional permissions"

❌ "An error occurred"
✅ "Unable to save task. Please check your connection and try again."

❌ "Invalid input"
✅ "Title must be less than 200 characters (currently 215)"

❌ "Error 0x80004005: Unspecified error"
✅ "Something went wrong. Please try again."

❌ "ECONNREFUSED 127.0.0.1:5432"
✅ "Unable to connect to the database. Please try again later."

❌ Silent failure (no feedback)
✅ "We couldn't complete that action. Your data is safe."

❌ Technical stack trace
✅ "Something unexpected happened. Our team has been notified."

# Standardized error response
{
    "error": {
        "code": "TASK_NOT_FOUND",      # Machine-readable
        "message": "Task not found",    # Human-readable
        "details": {}                   # Optional context
    }
}

def get_task(task_id: int, user_id: str):
    task = db.get_task(task_id)

    if task is None:
        # Task doesn't exist at all
        raise NotFoundError("Task not found")

    if task.user_id != user_id:
        # Task exists but belongs to someone else
        raise ForbiddenError("You don't have access to this task")

    return task

// Toast notification pattern
addToast("Task created successfully", "success");
addToast("Unable to save changes", "error");
addToast("Are you sure?", "warning");

Primary action fails
    ↓
Try recovery (retry, alternative)
    ↓
If still fails → Show friendly error
    ↓
Preserve user's work/state
    ↓
Offer clear next steps

// Bad: Silent failure
async function saveTask(task) {
    await api.save(task);  // If fails, data lost
}

// Good: Preserve on failure
async function saveTask(task) {
    try {
        await api.save(task);
        showSuccess("Task saved");
    } catch (error) {
        // Keep form data intact
        showError("Couldn't save. Your changes are preserved.");
        // Optionally save to localStorage as backup
    }
}

User input not understood
    ↓
"I didn't quite catch that. Could you rephrase?"
    ↓
Still unclear after 2 attempts
    ↓
"I'm having trouble understanding. Here's what I can help with: [options]"
    ↓
Offer human escalation if available

const ERROR_MESSAGES = {
    // Network
    NETWORK_ERROR: "Unable to connect. Please check your internet connection.",
    TIMEOUT: "Request timed out. Please try again.",

    // Auth
    SESSION_EXPIRED: "Your session has expired. Please sign in again.",
    UNAUTHORIZED: "Please sign in to continue.",
    FORBIDDEN: "You don't have permission to do this.",

    // Resources
    NOT_FOUND: "{resource} not found.",
    ALREADY_EXISTS: "A {resource} with this name already exists.",

    // Validation
    REQUIRED_FIELD: "{field} is required.",
    TOO_LONG: "{field} must be less than {max} characters.",
    INVALID_FORMAT: "Please enter a valid {field}.",

    // Server
    SERVER_ERROR: "Something went wrong. Please try again.",
    SERVICE_UNAVAILABLE: "Service temporarily unavailable. Please try again later.",

    // Generic fallback
    UNKNOWN: "An unexpected error occurred. Please try again."
};

const SUCCESS_MESSAGES = {
    // CRUD
    CREATED: "{resource} created successfully.",
    UPDATED: "Changes saved.",
    DELETED: "{resource} deleted.",

    // Actions
    COMPLETED: "Nice work! {resource} completed.",
    SAVED: "Saved.",
    SENT: "Sent successfully.",

    // Bulk
    BULK_UPDATE: "{count} items updated.",
    BULK_DELETE: "{count} items deleted."
};

# Bad: Leaks information
"User [email protected] not found"  # Confirms email doesn't exist

# Good: Generic auth messages
"Invalid email or password"  # Same message for both cases