fix(anthropic): preserve web_fetch_tool_result in multi-turn conversations

[Chesars]

Relevant issues

Fixes #18137

Pre-Submission checklist

• I have Added testing in the tests/litellm/ directory
• My PR passes all unit tests on make test-unit
• My PR's scope is as isolated as possible, it only solves 1 specific problem

Type

🐛 Bug Fix

Changes

Preserve web_fetch_tool_result in multi-turn conversations (same fix as #17746 for web_search).

Example

from litellm import completion

# First turn - Claude fetches a URL
response = completion(
    model="anthropic/claude-sonnet-4-20250514",
    tools=[{"type": "web_fetch_20250910", "name": "web_fetch", "max_uses": 5}],
    messages=[{"role": "user", "content": "Fetch https://example.com and summarize it"}],
)

# Second turn - follow up question (fails)
messages = [
    {"role": "user", "content": "Fetch https://example.com and summarize it"},
    response.choices[0].message.model_dump(),
    {"role": "user", "content": "What was the title?"},
]

response2 = completion(
    model="anthropic/claude-sonnet-4-20250514",
    tools=[{"type": "web_fetch_20250910", "name": "web_fetch", "max_uses": 5}],
    messages=messages,
)

Solution

When Anthropic returns a web_fetch response, it includes both a server_tool_use block and a web_fetch_tool_result block. LiteLLM was only preserving the server_tool_use but dropping the web_fetch_tool_result. This fix captures and stores web_fetch_tool_result in provider_specific_fields so it can be reconstructed in subsequent turns.

Files changed

• litellm/llms/anthropic/chat/transformation.py - handle web_fetch_tool_result (non-streaming)
• litellm/llms/anthropic/chat/handler.py - capture web_fetch_tool_result (streaming)
• tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_handler.py - add 2 unit tests

Tests

Tests Added

• test_web_fetch_tool_result_captured_in_provider_specific_fields
• test_web_fetch_tool_result_no_extra_tool_calls

tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_handler.py - 22 passed ✅

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Project	Deployment	Review	Updated (UTC)
litellm	Ready	Preview, Comment	Dec 17, 2025 6:59pm