Description

Fixes a bug in AnthropicPromptCachingMiddleware where cache_control parameters remain in the request after ModelFallbackMiddleware switches to non-Anthropic models at runtime, causing TypeError in other providers.

When using AnthropicPromptCachingMiddleware with ModelFallbackMiddleware:

1. Primary Anthropic model fails
2. ModelFallbackMiddleware switches request.model to OpenAI (or other provider)
3. Anthropic-specific cache_control parameters remain in request.model_settings
4. OpenAI API rejects the request: TypeError: Completions.create() got an unexpected keyword argument 'cache_control'

Solution: Add cleanup logic to remove cache_control parameters when the middleware detects a non-Anthropic model. This enables graceful fallback to any provider without Anthropic-specific settings causing errors. *The error is raised in the Prompt Caching Middleware. The solution should also be made in the same Middleware.

Changes

• Add _remove_cache_control() method to safely remove cache settings
• Update wrap_model_call() to clean up for non-Anthropic models
• Update awrap_model_call() with same logic (async version)
• Add comprehensive unit tests for cleanup behavior
• Add integration tests for fallback scenarios (both sync and async)

Example Usage

from langchain.agents import create_agent
from langchain.agents.middleware import ModelFallbackMiddleware
from langchain_anthropic import ChatAnthropic
from langchain_anthropic.middleware import AnthropicPromptCachingMiddleware
from langchain_openai import ChatOpenAI

# Now works correctly with cross-provider fallback
agent = create_agent(
    model="claude-haiku-4-5-20251001",
    middleware=[
        ModelFallbackMiddleware(
            "claude-3-5-haiku-20241022",
            "gpt-4o-mini",  # ✅ No longer causes TypeError!
        ),
        AnthropicPromptCachingMiddleware(ttl="5m"),
    ],
)

Issue

Fixes #33709

Dependencies

None - no new dependencies added.

Testing

All tests pass:

• ✅ make format - passed
• ✅ make lint - passed
• ✅ make test - unit tests pass
• ✅ Integration tests added (were not present before)