Actual behavior

LiteLLM detects litellm_web_search, executes the search successfully, then fails during the follow-up request.

Observed log sequence:

WebSearchInterception: Search completed for 'latest developments ongoing event 2025 2026', got 7257 chars WebSearchInterception: Making follow-up request with search results TypeError: litellm.anthropic_interface.messages.acreate() got multiple values for keyword argument 'context_management'

After locally hotfixing that duplication, the next failure becomes:

TypeError: litellm.anthropic_interface.messages.acreate() got multiple values for keyword argument 'output_config'

Claude Code then falls back to saying web search is unavailable.

Expected behavior

LiteLLM should complete the full agentic loop:

1. intercept native web search
2. execute configured search provider
3. make Anthropic follow-up request with search results
4. return final answer to Claude Code

No duplicate keyword arguments should be passed to anthropic_messages.acreate().

Root cause

In litellm/integrations/websearch_interception/handler.py, _execute_agentic_loop() builds the follow-up call like this:

final_response = await anthropic_messages.acreate( max_tokens=max_tokens, messages=follow_up_messages, model=full_model_name, **optional_params_without_max_tokens, **kwargs_for_followup, )

But _prepare_followup_kwargs() currently only filters:

_internal_keys = {"litellm_logging_obj"}

Claude Code requests include fields such as context_management and output_config, and these can exist in both:

• anthropic_messages_optional_request_params
• kwargs

So the follow-up call receives duplicate kwargs and crashes.

TL;DR

Filter out duplicate keyword arguments in _prepare_followup_kwargs() by checking against anthropic_messages_optional_request_params to prevent crashes in the Anthropic follow-up request.

Guidance

• Identify the source of duplicate keyword arguments in the follow-up request, specifically context_management and output_config, which are present in both anthropic_messages_optional_request_params and kwargs.
• Modify _prepare_followup_kwargs() to exclude any key already present in anthropic_messages_optional_request_params before expanding kwargs_for_followup.
• Use a dictionary comprehension to filter out duplicate kwargs, such as: kwargs_for_followup = {k: v for k, v in kwargs.items() if k not in anthropic_messages_optional_request_params}.
• Verify that the modified _prepare_followup_kwargs() function correctly filters out duplicate keyword arguments and prevents crashes in the Anthropic follow-up request.

Example

kwargs_for_followup = {
    k: v for k, v in kwargs.items()
    if k not in anthropic_messages_optional_request_params
}

Notes

• The provided local hotfix suggests that filtering out duplicate keyword arguments can resolve the issue, but a more generic solution is needed to prevent future crashes.
• The anthropic_messages_optional_request_params dictionary should be consulted to determine which keyword arguments are optional and can be safely excluded from the follow-up request.

Recommendation

Apply the suggested fix by modifying _prepare_followup_kwargs() to filter out duplicate keyword arguments against anthropic_messages_optional_request_params, as this will prevent crashes in the Anthropic follow-up request and ensure the correct functioning of the web search interception feature.