Fallback Tool

# Fallback Tool

Provide alternative tools when the primary is unavailable.

## Category
Tool Resilience

## Context
Critical operations with backup options.

## Problem
Tool unavailability blocks the entire workflow.

## Solution
Implement fallbacks:
- **Primary tool**: Preferred option
- **Fallback tool**: Alternative when primary fails
- **Automatic**: Switch transparently
- **Or explicit**: Return fallback suggestion

## Example (Python)
```python
@tool
def send_notification(
    user_id: str,
    message: str,
    preferred_channel: str = "slack"
) -> NotifyResult:
    """Send notification with automatic fallback.
    
    Tries preferred channel first, falls back to alternatives
    if unavailable.
    """
    channels = get_fallback_order(preferred_channel)
    # e.g., ["slack", "email", "sms"]
    
    for channel in channels:
        try:
            result = send_via_channel(channel, user_id, message)
            return NotifyResult(
                success=True,
                channel_used=channel,
                was_fallback=channel != preferred_channel
            )
        except ChannelUnavailable:
            continue
    
    raise ToolError(
        "All notification channels unavailable",
        tried_channels=channels
    )
```

## Considerations
- Document the fallback order
- Indicate when fallback was used
- Consider user preferences in fallback order

## Related Patterns
- health-check
- graceful-degradation
- error-classification

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Source: https://arcade.dev/patterns/fallback-tool