Summary

SyncFallbackStreamWrapper and FallbackStreamWrapper (introduced in PR #22375) subclass CustomStreamWrapper but hide attributes that third-party instrumentation libraries add to the original stream object. This causes AttributeError at runtime when those libraries access their injected attributes on the returned wrapper.

Concrete case: ddtrace (Datadog's APM library) wraps CustomStreamWrapper in a TracedStream (wrapt.ObjectProxy) that adds a .handler attribute. When the Router wraps this in SyncFallbackStreamWrapper, .handler is no longer accessible.

Steps to Reproduce

import litellm

router = litellm.Router(
    model_list=[
        {"model_name": "gemini", "litellm_params": {"model": "vertex_ai/gemini-2.0-flash", "vertex_project": "...", "vertex_location": "..."}},
    ],
    fallbacks=[{"gemini": ["gemini"]}],
)

# With ddtrace LLM Observability enabled (DD_LLMOBS_ENABLED=1, ddtrace-run):
response = router.completion(model="gemini", messages=[{"role": "user", "content": "hi"}], stream=True)
# → AttributeError: 'SyncFallbackStreamWrapper' object has no attribute 'handler'

Expected: Streaming works — attributes from the original stream are accessible through the wrapper. Actual: AttributeError on first access of .handler by ddtrace's traced_router_completion.

Root Cause

The call chain with ddtrace enabled:

1. Router.completion() calls internal litellm.completion() → returns CustomStreamWrapper
2. ddtrace patches litellm.completion and wraps the result in TracedStream(wrapt.ObjectProxy) — this adds .handler
3. Router wraps this in SyncFallbackStreamWrapper(CustomStreamWrapper) — the new outer wrapper
4. traced_router_completion (ddtrace's Router.completion patch) does resp.handler → AttributeError

Step 4 fails because SyncFallbackStreamWrapper inherits from CustomStreamWrapper which has its own __init__ that doesn't know about .handler. The TracedStream is buried inside _wrapped_response / model_response and its attributes are hidden.

Same issue exists for the async FallbackStreamWrapper (unreachable in current ddtrace since Router.acompletion is not yet patched, but will break when it is).

Suggested Fix

Add _wrapped_response + __getattr__ to both wrapper classes to proxy unknown attribute lookups to the original stream:

class SyncFallbackStreamWrapper(CustomStreamWrapper):
    def __init__(self, sync_generator: Generator):
        self._wrapped_response = model_response
        super().__init__(...)
        ...

    def __getattr__(self, name: str):
        return getattr(self._wrapped_response, name)

This is transparent to existing behavior — __getattr__ is only called when normal attribute lookup fails, so it doesn't interfere with CustomStreamWrapper's own attributes.

Impact

Any user running LiteLLM Router with streaming + ddtrace LLM Observability enabled (DD_LLMOBS_ENABLED=1) hits this on every streaming call. Non-streaming and non-ddtrace usage is unaffected.

What part of LiteLLM is this about?

SDK (litellm Python package)

What LiteLLM version are you on?

1.82.1+ (any version with PR #22375 merged)