Bug Report: aws_role_name in proxy config YAML silently ignored — cross-account Bedrock AssumeRole never fires

Summary

When configuring aws_role_name in litellm_params via the proxy config YAML for cross-account Bedrock access, the parameter is silently dropped during optional params processing. The STS AssumeRole call never fires, and Bedrock calls land in the host account's IAM identity instead of the target role's account.

Use Case

We're a government organization (City and County of San Francisco) deploying LiteLLM as a centralized LLM Gateway on ECS Fargate. We have a central platform account running LiteLLM and multiple department accounts with Bedrock model access enabled. Each department has its own AWS account with a LiteLLMBedrockAccess IAM role that the LiteLLM ECS task role can assume.

Our config uses tag-based routing so each department's models route to their own Bedrock account for billing isolation:

model_list:
  - model_name: claude-sonnet-4-6
    litellm_params:
      model: bedrock/us.anthropic.claude-sonnet-4-6
      aws_region_name: us-west-2
      aws_role_name: arn:aws:iam::244910800867:role/LiteLLMBedrockAccess
      aws_session_name: litellm-dept-a
    model_info:
      tags:
        - dept-a
  - model_name: claude-sonnet-4-6
    litellm_params:
      model: bedrock/us.anthropic.claude-sonnet-4-6
      aws_region_name: us-west-2
      aws_role_name: arn:aws:iam::987654321012:role/LiteLLMBedrockAccess
      aws_session_name: litellm-dept-b
    model_info:
      tags:
        - dept-b

litellm_settings:
  enable_tag_filtering: true
  drop_params: true

This config format matches the LiteLLM documentation at https://docs.litellm.ai/docs/providers/bedrock (cross-account section).

Expected behavior: LiteLLM task role calls STS AssumeRole into the department role, then invokes Bedrock with the assumed credentials.

Actual behavior: AssumeRole never fires. Bedrock is called with the ECS task role's own credentials (central account), which doesn't have Bedrock model access enabled. Error: "Model use case details have not been submitted."

Root Cause (traced through source code)

The parameter flows correctly through the router but gets dropped inside get_optional_params() before reaching the Bedrock handler. Here is the exact code path:

1. Router passes aws_role_name correctly

litellm/router.py — The router spreads litellm_params from the deployment config into litellm.completion(**kwargs). At this point, aws_role_name is a top-level kwarg. ✅

2. completion() includes it in non-default params

litellm/main.py — get_non_default_completion_params(kwargs) returns aws_role_name because it is not in all_litellm_params or OPENAI_CHAT_COMPLETION_PARAMS. ✅

3. get_optional_params() receives it via **kwargs

litellm/utils.py:3934 — aws_role_name lands in **kwargs (the catch-all). Then:

passed_params = locals().copy()
special_params = passed_params.pop("kwargs")  # aws_role_name is here

✅

4. base_pre_process_non_default_params() merges it, then filters it out

litellm/utils.py:3688-3723

Line 3688-3702: aws_role_name is correctly merged from special_params into passed_params (the aws_ prefix check correctly allows it for bedrock):

for k, v in special_params.items():
    if k.startswith("aws_") and (
        custom_llm_provider != "bedrock"  # Correctly NOT skipped for bedrock
        ...
    ):
        continue
    passed_params[k] = v  # aws_role_name merged into passed_params ✅

Line 3705-3723 — THE BUG: The comprehension that builds non_default_params requires k in default_param_values:

non_default_params = {
    k: v
    for k, v in passed_params.items()
    if (
        ...
        and k in default_param_values   # ← DEFAULT_CHAT_COMPLETION_PARAM_VALUES
        and v != default_param_values[k]
        ...
    )
}

DEFAULT_CHAT_COMPLETION_PARAM_VALUES (litellm/constants.py:668) contains only OpenAI chat completion params (temperature, top_p, tools, etc.). aws_role_name is not in this dict, so it is filtered out. ❌

5. map_special_auth_params() doesn't help

litellm/utils.py:3851-3856 → litellm/llms/bedrock/common_utils.py:77-88

map_special_auth_params() only maps region_name → aws_region_name. It does not handle aws_role_name, aws_session_name, or any other AWS credential params. ❌

6. Handler gets None

litellm/llms/bedrock/chat/converse_handler.py:324:

aws_role_name = optional_params.pop("aws_role_name", None)  # Returns None

get_credentials() is called with aws_role_name=None → no AssumeRole → ambient credentials used. ❌

Meanwhile, litellm_params has it

litellm/litellm_core_utils/get_litellm_params.py:160-162 correctly stores aws_role_name in litellm_params via _OPTIONAL_KWARGS_KEYS. The handler receives litellm_params as a parameter, but reads aws_role_name from optional_params instead. Nobody transfers it.

Reproduction

# config.yaml
model_list:
  - model_name: test-model
    litellm_params:
      model: bedrock/us.anthropic.claude-sonnet-4-6
      aws_region_name: us-west-2
      aws_role_name: arn:aws:iam::TARGET_ACCOUNT:role/SomeRole
      aws_session_name: litellm-test

litellm_settings:
  drop_params: true

litellm --config config.yaml --detailed_debug
# Send a chat completion request
# Observe: no STS AssumeRole call in logs, Bedrock called with host credentials

Suggested Fix

Option A (minimal): In converse_handler.py:completion(), fall back to litellm_params when optional_params doesn't have AWS credential fields:

aws_role_name = optional_params.pop("aws_role_name", None) or litellm_params.get("aws_role_name")
aws_session_name = optional_params.pop("aws_session_name", None) or litellm_params.get("aws_session_name")
# ... same for other aws_ params

Option B (proper): Add aws_role_name, aws_session_name, aws_access_key_id, aws_secret_access_key, aws_session_token, aws_profile_name, aws_web_identity_token, aws_sts_endpoint, aws_external_id, and aws_bedrock_runtime_endpoint to DEFAULT_CHAT_COMPLETION_PARAM_VALUES in litellm/constants.py so they survive the filter in base_pre_process_non_default_params().

Option C: Expand map_special_auth_params() in bedrock/common_utils.py to copy all aws_* params from passed_params into optional_params, not just region_name.

Environment

• LiteLLM version: main-v1.82.3-stable (also tested main-latest)
• Deployment: ECS Fargate with task role that has sts:AssumeRole permission to target roles
• IAM trust policies verified (direct aws sts assume-role works from the task role)
• Direct Bedrock invocation from target account works

Impact

This blocks any proxy deployment that uses per-model cross-account Bedrock routing via aws_role_name in the config YAML. The parameter is documented, accepted by the config parser, and stored correctly in litellm_params, but never reaches the handler that needs it.