Bug: additional_drop_params ignored on POST /v1/messages — Bedrock rejects unsupported fields

Environment

• LiteLLM version: 1.83.9 (also reproduced on 1.83.3)
• Python: 3.11 / RHEL 9 / systemd proxy, 4 workers
• Provider: AWS Bedrock — bedrock/us.anthropic.claude-sonnet-4-6
• Client: Claude Code v2.1.112
• Endpoint: POST /v1/messages?beta=true

Summary

Claude Code v2.1+ enables the context-management-2025-06-27 Anthropic beta, which adds a context_management field to Messages API request bodies. This field is valid against Anthropic's direct API but is rejected by Bedrock with "context_management: Extra inputs are not permitted".

Setting additional_drop_params: ["context_management"] in litellm_settings or in a model's litellm_params has no effect on this code path — the field is forwarded to Bedrock and the 400 is returned to the client.

Config attempted (does not fix the issue)

litellm_settings:
  drop_params: true
  additional_drop_params: ["context_management"]  # no effect on /v1/messages path

model_list:
  - model_name: us.anthropic.claude-sonnet-4-6
    litellm_params:
      model: bedrock/us.anthropic.claude-sonnet-4-6
      drop_params: true
      additional_drop_params: ["context_management"]  # also no effect

Error returned to client

API Error: 400 {"error":{"message":"{\"message\":\"context_management: Extra inputs are not permitted\"}
. Received Model Group=us.anthropic.claude-sonnet-4-6\nAvailable Model Group Fallbacks=None"}}

Key traceback frames

litellm/proxy/anthropic_endpoints/endpoints.py:53  anthropic_response()
litellm/llms/anthropic/experimental_pass_through/messages/handler.py:286  anthropic_messages()
litellm/llms/custom_httpx/llm_http_handler.py:2025  async_anthropic_messages_handler()
litellm/llms/custom_httpx/llm_http_handler.py:1878  _async_post_anthropic_messages_with_http_error_retry()
litellm/llms/custom_httpx/llm_http_handler.py:1840  async_httpx_client.post()

MaskedHTTPStatusError: 400 Bad Request
  url: https://bedrock-runtime.us-west-2.amazonaws.com/model/us.anthropic.claude-sonnet-4-6/invoke-with-response-stream

BaseLLMException: {"message":"context_management: Extra inputs are not permitted"}

Root cause

POST /v1/messages routes through experimental_pass_through/messages/handler.py → async_anthropic_messages_handler. This code path never consults drop_params or additional_drop_params. It passes request_body directly to sign_request(), which serialises and AWS-signs the body including the offending field. The drop_params logic only runs in the /v1/chat/completions path.

Workaround

Insert before the sign_request() call in async_anthropic_messages_handler (llm_http_handler.py):

# Strip Anthropic-only fields Bedrock rejects — additional_drop_params
# is not consulted in this code path.
for _f in ("context_management",):
    request_body.pop(_f, None)

Expected behaviour

additional_drop_params set in litellm_settings or a model's litellm_params should apply to all outbound provider requests regardless of whether they arrived via /v1/chat/completions or /v1/messages.