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Message Sanitization for Tool Calling for anthropic models

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Claude Code Knowledge Pack7/10/2026

Overview

Message Sanitization for Tool Calling for anthropic models

Automatically fix common message formatting issues when using tool calling with modify_params=True

LiteLLM can automatically sanitize messages to handle common issues that occur during tool calling workflows, especially when using OpenAI-compatible clients with providers that have strict message format requirements (like Anthropic Claude).

Overview

When litellm.modify_params = True is enabled, LiteLLM automatically sanitizes messages to fix three common issues:

  1. Orphaned Tool Calls - Assistant messages with tool_calls but missing tool results
  2. Orphaned Tool Results - Tool messages that reference non-existent tool_call_ids
  3. Empty Message Content - Messages with empty or whitespace-only text content

This ensures your tool calling workflows work seamlessly across different LLM providers without manual message validation.

Why Message Sanitization?

Different LLM providers have varying requirements for message formats, especially during tool calling:

  • Anthropic Claude requires every tool_call to have a corresponding tool result
  • Some providers reject messages with empty content
  • OpenAI-compatible clients may not always maintain perfect message consistency

Without sanitization, these issues cause API errors that interrupt your workflows. With modify_params=True, LiteLLM handles these edge cases automatically.

Quick Start


# Enable automatic message sanitization
litellm.modify_params = True

# This will work even if messages have formatting issues
response = litellm.completion(
    model="anthropic/claude-3-5-sonnet-20241022",
    messages=[
        {"role": "user", "content": "What's the weather in Boston?"},
        {
            "role": "assistant",
            "tool_calls": [
                {
                    "id": "call_123",
                    "type": "function",
                    "function": {"name": "get_weather", "arguments": '{"city": "Boston"}'}
                }
            ]
            # Missing tool result - LiteLLM will add a dummy result automatically
        },
        {"role": "user", "content": "Thanks!"}
    ],
    tools=[{
        "type": "function",
        "function": {
            "name": "get_weather",
            "description": "Get weather for a city",
            "parameters": {
                "type": "object",
                "properties": {"city": {"type": "string"}},
                "required": ["city"]
            }
        }
    }]
)

litellm_settings:
  modify_params: true  # Enable automatic message sanitization

model_list:
  - model_name: claude-3-5-sonnet
    litellm_params:
      model: anthropic/claude-3-5-sonnet-20241022

Sanitization Cases

Case A: Orphaned Tool Calls (Missing Tool Results)

Problem: An assistant message contains tool_calls, but no corresponding tool result messages follow.

Solution: LiteLLM automatically adds dummy tool result messages for any missing tool results.

Example:


litellm.modify_params = True

# Messages with orphaned tool calls
messages = [
    {"role": "user", "content": "Search for Python tutorials"},
    {
        "role": "assistant",
        "tool_calls": [
            {
                "id": "call_abc123",
                "type": "function",
                "function": {"name": "web_search", "arguments": '{"query": "Python tutorials"}'}
            }
        ]
    },
    # Missing tool result here!
    {"role": "user", "content": "What about JavaScript?"}
]

# LiteLLM automatically adds:
# {
#     "role": "tool",
#     "tool_call_id": "call_abc123",
#     "content": "[System: Tool execution skipped/interrupted by user. No result provided for tool 'web_search'.]"
# }

response = litellm.completion(
    model="anthropic/claude-3-5-sonnet-20241022",
    messages=messages,
    tools=[...]
)

When this happens:

  • User interrupts tool execution
  • Client loses tool results due to network issues
  • Conversation flow changes before tool completes
  • Multi-turn conversations where tools are optional

Case B: Orphaned Tool Results (Invalid tool_call_id)

Problem: A tool message references a tool_call_id that doesn't exist in any previous assistant message.

Solution: LiteLLM automatically removes these orphaned tool result messages.

Example:


litellm.modify_params = True

# Messages with orphaned tool result
messages = [
    {"role": "user", "content": "Hello"},
    {"role": "assistant", "content": "Hi! How can I help?"},
    {
        "role": "tool",
        "tool_call_id": "call_nonexistent",  # This tool_call_id doesn't exist!
        "content": "Some result"
    }
]

# LiteLLM automatically removes the orphaned tool message

response = litellm.completion(
    model="anthropic/claude-3-5-sonnet-20241022",
    messages=messages
)

When this happens:

  • Message history is manually edited
  • Tool results are duplicated or mismatched
  • Conversation state is restored incorrectly
  • Messages are merged from different conversations

Case C: Empty Message Content

Problem: User or assistant messages have empty or whitespace-only content.

Solution: LiteLLM replaces empty content with a system placeholder message.

Example:


litellm.modify_params = True

# Messages with empty content
messages = [
    {"role": "user", "content": ""},  # Empty content
    {"role": "assistant", "content": "   "},  # Whitespace only
]

# LiteLLM automatically replaces with:
# {"role": "user", "content": "[System: Empty message content sanitised to satisfy protocol]"}
# {"role": "assistant", "content": "[System: Empty message content sanitised to satisfy protocol]"}

response = litellm.completion(
    model="anthropic/claude-3-5-sonnet-20241022",
    messages=messages
)

When this happens:

  • UI sends empty messages
  • Content is stripped during preprocessing
  • Placeholder messages in conversation history
  • Edge cases in message construction

Configuration

Enable Globally


# Enable for all completion calls
litellm.modify_params = True

litellm_settings:
  modify_params: true

Enable Per-Request


# Enable only for specific requests
response = litellm.completion(
    model="anthropic/claude-3-5-sonnet-20241022",
    messages=messages,
    modify_params=True  # Override global setting
)

Supported Providers

Message sanitization currently works with:

  • ✅ Anthropic (Claude)

Note: While the sanitization logic is provider-agnostic, it is currently only applied in the Anthropic message transformation pipeline. Support for additional providers may be added in future releases.

Implementation Details

How It Works

The message sanitization process runs before messages are converted to provider-specific formats:

  1. Input: OpenAI-format messages with potential issues
  2. Sanitization: Three helper functions process the messages:
    • _sanitize_empty_text_content() - Fixes empty content
    • _add_missing_tool_results() - Adds dummy tool results
    • _is_orphaned_tool_result() - Identifies orphaned results
  3. Output: Clean, provider-compatible messages

Code Reference

The sanitization logic is implemented in:

  • litellm/litellm_core_utils/prompt_templates/factory.py
  • Function: sanitize_messages_for_tool_calling()

Logging

When sanitization occurs, LiteLLM logs debug messages:


litellm.set_verbose = True  # Enable debug logging

# You'll see logs like:
# "_add_missing_tool_results: Found 1 orphaned tool calls. Adding dummy tool results."
# "_is_orphaned_tool_result: Found orphaned tool result with tool_call_id=call_123"
# "_sanitize_empty_text_content: Replaced empty text content in user message"

Best Practices

1. Enable for Production Workflows

# Recommended for production
litellm.modify_params = True

# Ensures robust handling of edge cases
response = litellm.completion(
    model="anthropic/claude-3-5-sonnet-20241022",
    messages=messages,
    tools=tools
)

2. Preserve Tool Results When Possible

While sanitization handles missing tool results, it's better to provide actual results:

# Good: Provide actual tool results
messages = [
    {"role": "user", "content": "Search for Python"},
    {"role": "assistant", "tool_calls": [...]},
    {"role": "tool", "tool_call_id": "call_123", "content": "Actual search results"}
]

# Fallback: Sanitization adds dummy result if missing
messages = [
    {"role": "user", "content": "Search for Python"},
    {"role": "assistant", "tool_calls": [...]},
    # Missing tool result - sanitization adds dummy
]

3. Monitor Sanitization Events

Use logging to track when sanitization occurs:


# Enable debug logging
litellm.set_verbose = True
logging.basicConfig(level=logging.DEBUG)

# Track sanitization events in your application
response = litellm.completion(
    model="anthropic/claude-3-5-sonnet-20241022",
    messages=messages
)

4. Test Edge Cases

Ensure your application handles sanitized messages correctly:


litellm.modify_params = True

# Test orphaned tool calls
test_messages = [
    {"role": "user", "content": "Test"},
    {"role": "assistant", "tool_calls": [{"id": "call_1", "type": "function", "function": {"name": "test", "arguments": "{}"}}]},
    {"role": "user", "content": "Continue"}  # No tool result
]

response = litellm.completion(
    model="anthropic/claude-3-5-sonnet-20241022",
    messages=test_messages,
    tools=[...]
)

# Verify the response handles the dummy tool result appropriately

Related Features

Troubleshooting

Sanitization Not Working

Issue: Messages still cause errors despite modify_params=True

Solution:

  1. Verify modify_params is enabled:

    
print(litellm.modify_params)  # Should be True

  2. Check if the issue is provider-specific:

    litellm.set_verbose = True  # Enable debug logging

  3. Ensure you're using a recent version of LiteLLM:

    uv add --upgrade-package litellm litellm

Unexpected Dummy Tool Results

Issue: Dummy tool results appear when you expect actual results

Cause: Tool result messages are missing or have incorrect tool_call_id

Solution:

  1. Verify tool result messages have correct tool_call_id:

    # Correct
{"role": "tool", "tool_call_id": "call_123", "content": "result"}

# Incorrect - will be treated as orphaned
{"role": "tool", "tool_call_id": "wrong_id", "content": "result"}

  2. Ensure tool results immediately follow assistant messages with tool_calls

Performance Impact

Issue: Concerned about performance overhead

Details: Message sanitization has minimal performance impact:

  • Runs in O(n) time where n = number of messages
  • Only processes messages when modify_params=True
  • Typically adds < 1ms to request processing time

FAQ

Q: Does sanitization modify my original messages?

A: No, sanitization creates a new list of messages. Your original messages remain unchanged.

Q: Can I disable specific sanitization cases?

A: Currently, all three cases are handled together when modify_params=True. To disable sanitization entirely, set modify_params=False.

Q: What happens to the dummy tool results?

A: Dummy tool results are sent to the LLM provider along with other messages. The model sees them as regular tool results with informative error messages.

Q: Does this work with streaming?

A: Yes, message sanitization works with both streaming and non-streaming requests.

Q: Is this related to drop_params?

A: No, they're separate features:

  • modify_params - Modifies/fixes message content and structure
  • drop_params - Removes unsupported API parameters

Both can be enabled simultaneously.

See Also