diff --git a/litellm/completion_extras/litellm_responses_transformation/transformation.py b/litellm/completion_extras/litellm_responses_transformation/transformation.py
index 31af5a144eb..71d3f1e900e 100644
--- a/litellm/completion_extras/litellm_responses_transformation/transformation.py
+++ b/litellm/completion_extras/litellm_responses_transformation/transformation.py
@@ -1334,6 +1334,7 @@ class OpenAiResponsesToChatCompletionStreamIterator(BaseModelResponseIterator):
     ):
         super().__init__(streaming_response, sync_stream, json_mode)
         self._chat_completion_id: str | None = None
+        self._served_service_tier: str | None = None
         self._tool_call_index_map: dict[int, int] = {}  # mutable-ok: per-stream accumulator state
 
     def _handle_string_chunk(
@@ -1598,6 +1599,7 @@ class OpenAiResponsesToChatCompletionStreamIterator(BaseModelResponseIterator):
 
                 usage = ResponseAPILoggingUtils._transform_response_api_usage_to_chat_usage(response_data.get("usage"))
             provider_metadata: Final = _provider_metadata(response_data)
+            served_service_tier: Final = response_data.get("service_tier")
             return ModelResponseStream(
                 choices=[
                     StreamingChoices(
@@ -1611,6 +1613,11 @@ class OpenAiResponsesToChatCompletionStreamIterator(BaseModelResponseIterator):
                 ],
                 usage=usage,
                 provider_specific_fields=dict(provider_metadata) or None,  # mutable-ok: field is typed dict
+                **(
+                    MappingProxyType({"service_tier": served_service_tier})
+                    if isinstance(served_service_tier, str)
+                    else MappingProxyType({})
+                ),
             )
         else:
             pass
@@ -1639,12 +1646,28 @@ class OpenAiResponsesToChatCompletionStreamIterator(BaseModelResponseIterator):
             ModelResponseStream: OpenAI-formatted streaming chunk
         """
         verbose_logger.debug("Chat provider: transform_streaming_response called with chunk: %s", chunk)
-        return self._with_stream_scoped_id(
-            OpenAiResponsesToChatCompletionStreamIterator.translate_responses_chunk_to_openai_stream(
-                chunk, tool_call_index_map=self._tool_call_index_map
+        self._remember_served_service_tier(chunk)
+        return self._with_served_service_tier(
+            self._with_stream_scoped_id(
+                OpenAiResponsesToChatCompletionStreamIterator.translate_responses_chunk_to_openai_stream(
+                    chunk, tool_call_index_map=self._tool_call_index_map
+                )
             )
         )
 
+    def _remember_served_service_tier(self, chunk: dict[str, object]) -> None:
+        response_payload: Final = chunk.get("response")
+        if not isinstance(response_payload, dict):
+            return
+        served_tier: Final = response_payload.get("service_tier")
+        if isinstance(served_tier, str) and served_tier:
+            self._served_service_tier = served_tier
+
+    def _with_served_service_tier(self, chunk: "ModelResponseStream") -> "ModelResponseStream":
+        if self._served_service_tier is not None and chunk.model_dump().get("service_tier") is None:
+            setattr(chunk, "service_tier", self._served_service_tier)  # noqa: B010  # pydantic extra, not a declared field
+        return chunk
+
     def _with_stream_scoped_id(self, chunk: "ModelResponseStream") -> "ModelResponseStream":
         if self._chat_completion_id is None:
             self._chat_completion_id = chunk.id
diff --git a/litellm/cost_calculator.py b/litellm/cost_calculator.py
index 2f76b84f5e3..6b3e739ac4f 100644
--- a/litellm/cost_calculator.py
+++ b/litellm/cost_calculator.py
@@ -26,6 +26,7 @@ from litellm.litellm_core_utils.llm_cost_calc.usage_object_transformation import
     TranscriptionUsageObjectTransformation,
 )
 from litellm.litellm_core_utils.llm_cost_calc.utils import (
+    _SERVICE_TIER_TO_COST_KEY_SUFFIX,
     BilledTokenRates,
     CostCalculatorUtils,
     _generic_cost_per_character,
@@ -704,7 +705,7 @@ def cost_per_token(
             data_residency=data_residency,
         )
     elif custom_llm_provider == "databricks":
-        return databricks_cost_per_token(model=model, usage=usage_block)
+        return databricks_cost_per_token(model=model, usage=usage_block, service_tier=service_tier)
     elif custom_llm_provider == "fireworks_ai":
         return fireworks_ai_cost_per_token(model=model, usage=usage_block)
     elif custom_llm_provider == "azure":
@@ -969,6 +970,37 @@ def _normalize_service_tier(service_tier: object) -> str | None:
     return service_tier
 
 
+_BASE_PRICING_SERVICE_TIERS: Final[frozenset[str]] = frozenset({"default", "standard"})
+
+
+def _resolve_billable_service_tier(requested: object, served: object) -> str | None:
+    """Served tier wins when it names a priced tier or explicitly says base pricing; otherwise the request decides."""
+    served_lower: Final = served.lower() if isinstance(served, str) else None
+    if served_lower is not None and served_lower in _SERVICE_TIER_TO_COST_KEY_SUFFIX:
+        return served_lower
+    if served_lower in _BASE_PRICING_SERVICE_TIERS:
+        return None
+    return _normalize_service_tier(requested)
+
+
+def _served_service_tier(completion_response: object, usage_object: Usage | None) -> str | None:
+    """Find the tier the provider actually served: response, then usage, then Gemini trafficType."""
+    response_tier: Final = _extract_service_tier(completion_response)
+    if isinstance(response_tier, str):
+        return response_tier
+    usage_tier: Final = _extract_service_tier(usage_object)
+    if isinstance(usage_tier, str):
+        return usage_tier
+    hidden_params: Final = getattr(completion_response, "_hidden_params", None)
+    if hidden_params is None:
+        return None
+    provider_specific: Final = hidden_params.get("provider_specific_fields") or {}
+    raw_traffic_type: Final = provider_specific.get("traffic_type")
+    if not raw_traffic_type:
+        return None
+    return _map_traffic_type_to_service_tier(raw_traffic_type) or "default"
+
+
 def _extract_service_tier(source: object) -> str | None:
     """Read a raw ``service_tier`` off a response body or usage object, dict or pydantic model alike."""
     if isinstance(source, BaseModel):
@@ -1388,23 +1420,14 @@ def completion_cost(
         )
         rerank_billed_units: RerankBilledUnits | None = None
 
-        # Extract service_tier from optional_params if not provided directly
-        if service_tier is None and optional_params is not None:
-            service_tier = optional_params.get("service_tier")
-
-        service_tier = _normalize_service_tier(service_tier)
-
-        # Extract service_tier from completion_response if not provided
-        if service_tier is None and completion_response is not None:
-            service_tier = _extract_service_tier(completion_response)
-
-        service_tier = _normalize_service_tier(service_tier)
-
-        # Extract service_tier from usage object if not provided
-        if service_tier is None and cost_per_token_usage_object is not None:
-            service_tier = _extract_service_tier(cost_per_token_usage_object)
-
-        service_tier = _normalize_service_tier(service_tier)
+        explicit_tier: Final = _normalize_service_tier(service_tier)
+        if explicit_tier is not None:
+            service_tier = explicit_tier
+        else:
+            service_tier = _resolve_billable_service_tier(  # rebind-ok: resolved from request then response
+                requested=optional_params.get("service_tier") if optional_params is not None else None,
+                served=_served_service_tier(completion_response, cost_per_token_usage_object),
+            )
 
         explicit_pricing: Final = custom_pricing is True or base_model is not None
         selected_model: Final = _select_model_name_for_cost_calc(
@@ -1494,15 +1517,6 @@ def completion_cost(
                         custom_llm_provider = hidden_params.get("custom_llm_provider", custom_llm_provider or None)
                         region_name = hidden_params.get("region_name", region_name)
 
-                        # For Gemini/Vertex AI responses, trafficType is stored in
-                        # provider_specific_fields.  Map it to the service_tier used
-                        # by the cost key lookup (_priority / _flex suffixes) so that
-                        # ON_DEMAND_PRIORITY requests are billed at priority prices.
-                        if service_tier is None:
-                            provider_specific = hidden_params.get("provider_specific_fields") or {}
-                            raw_traffic_type = provider_specific.get("traffic_type")
-                            if raw_traffic_type:
-                                service_tier = _map_traffic_type_to_service_tier(raw_traffic_type)
                 else:
                     if model is None:
                         raise ValueError(
diff --git a/litellm/litellm_core_utils/litellm_logging.py b/litellm/litellm_core_utils/litellm_logging.py
index 06cbfd4fc04..c393fa3caef 100644
--- a/litellm/litellm_core_utils/litellm_logging.py
+++ b/litellm/litellm_core_utils/litellm_logging.py
@@ -1884,7 +1884,6 @@ class Logging(LiteLLMLoggingBaseClass):
                 "standard_built_in_tools_params": self.standard_built_in_tools_params,
                 "router_model_id": router_model_id,
                 "litellm_logging_obj": self,
-                "service_tier": (self.optional_params.get("service_tier") if self.optional_params else None),
                 "data_residency": (
                     self.litellm_params.get("data_residency")
                     if hasattr(self, "litellm_params") and self.litellm_params
diff --git a/litellm/litellm_core_utils/streaming_chunk_builder_utils.py b/litellm/litellm_core_utils/streaming_chunk_builder_utils.py
index 67684a230e3..bdf53013224 100644
--- a/litellm/litellm_core_utils/streaming_chunk_builder_utils.py
+++ b/litellm/litellm_core_utils/streaming_chunk_builder_utils.py
@@ -109,6 +109,7 @@ class _BaseChunk(TypedDict, total=False):
     created: ReadOnly[int]
     model: ReadOnly[str]
     system_fingerprint: ReadOnly[str | None]
+    service_tier: ReadOnly[str | None]
     choices: ReadOnly[Required[Sequence[StreamingChoices]]]
     _hidden_params: ReadOnly[_ChunkHiddenParams]
 
@@ -369,6 +370,13 @@ class ChunkProcessor:
         # Fall back to first chunk's model if no different model found
         return first_chunk_model
 
+    @staticmethod
+    def _get_service_tier_from_chunks(chunks: Sequence["_BaseChunk"]) -> str | None:
+        return next(
+            (tier for chunk in reversed(chunks) if isinstance(tier := chunk.get("service_tier"), str) and tier),
+            None,
+        )
+
     def build_base_response(self, chunks: Sequence["_BaseChunk"]) -> ModelResponse:
         chunk = self.first_chunk
         id: Final = ChunkProcessor._get_chunk_id(chunks)
@@ -378,6 +386,7 @@ class ChunkProcessor:
         # Get the actual model - for Azure Model Router, this finds the real model from later chunks
         model: Final = ChunkProcessor._get_model_from_chunks(chunks, first_chunk_model)
         system_fingerprint: Final = chunk.get("system_fingerprint", None)
+        service_tier: Final = ChunkProcessor._get_service_tier_from_chunks(chunks)
 
         role: Final = ChunkProcessor._get_role_from_chunks(chunks)
         finish_reason = "stop"
@@ -399,6 +408,11 @@ class ChunkProcessor:
                 "created": created,
                 "model": model,
                 "system_fingerprint": system_fingerprint,
+                **(
+                    MappingProxyType({"service_tier": service_tier})
+                    if service_tier is not None
+                    else MappingProxyType({})
+                ),
                 "choices": [
                     {
                         "index": 0,
diff --git a/litellm/litellm_core_utils/streaming_handler.py b/litellm/litellm_core_utils/streaming_handler.py
index 946d19c028f..d2853a625c9 100644
--- a/litellm/litellm_core_utils/streaming_handler.py
+++ b/litellm/litellm_core_utils/streaming_handler.py
@@ -72,6 +72,12 @@ def _next_sync_or_exhausted(it: Any) -> object:
         return _SYNC_ITER_EXHAUSTED
 
 
+def _stamp_served_service_tier(response: ModelResponseStream, complete_streaming_response: ModelResponse) -> None:
+    served_tier: Final = complete_streaming_response.model_dump().get("service_tier")
+    if isinstance(served_tier, str) and served_tier:
+        setattr(response, "service_tier", served_tier)  # noqa: B010  # pydantic extra, not a declared field
+
+
 def is_async_iterable(obj: object) -> bool:
     """
     Check if an object is an async iterable (can be used with 'async for').
@@ -1876,6 +1882,7 @@ class CustomStreamWrapper:
                         "usage",
                         getattr(complete_streaming_response, "usage"),
                     )
+                    _stamp_served_service_tier(response, complete_streaming_response)
                     try:
                         _cache_copy = complete_streaming_response.model_copy(deep=True)
                         _log_copy = complete_streaming_response.model_copy(deep=True)
@@ -2127,6 +2134,7 @@ class CustomStreamWrapper:
                     "usage",
                     getattr(complete_streaming_response, "usage"),
                 )
+                _stamp_served_service_tier(response, complete_streaming_response)
                 try:
                     _copy = complete_streaming_response.model_copy(deep=True)
                 except RuntimeError:
diff --git a/litellm/llms/anthropic/pass_through/adapters/streaming_iterator.py b/litellm/llms/anthropic/pass_through/adapters/streaming_iterator.py
index 12eee663ca5..38380cc056d 100644
--- a/litellm/llms/anthropic/pass_through/adapters/streaming_iterator.py
+++ b/litellm/llms/anthropic/pass_through/adapters/streaming_iterator.py
@@ -11,6 +11,7 @@ from typing import (
     Final,
     Literal,
     Protocol,
+    cast,
     get_args,
 )
 
@@ -35,6 +36,7 @@ from litellm.types.utils import AdapterCompletionStreamWrapper, Delta
 
 if TYPE_CHECKING:
     from litellm.litellm_core_utils.litellm_logging import Logging as LiteLLMLoggingObject
+    from litellm.types.llms.openai import AllMessageValues
     from litellm.types.utils import ModelResponseStream
 
 
@@ -115,6 +117,18 @@ class _CombinedChunkSplitter:
         self._async_iter: AsyncIterator[ModelResponseStream] | None = None
         self._buffer: deque[ModelResponseStream] = deque()
 
+    @property
+    def chunks(self) -> "list[ModelResponseStream] | None":
+        return cast(  # cast-ok: chunks is a list of ModelResponseStream on the inner stream
+            "list[ModelResponseStream] | None", getattr(self._stream, "chunks", None)
+        )
+
+    @property
+    def messages(self) -> "list[AllMessageValues] | None":
+        return cast(  # cast-ok: messages is a list of AllMessageValues on the inner stream
+            "list[AllMessageValues] | None", getattr(self._stream, "messages", None)
+        )
+
     @staticmethod
     def _is_combined(chunk: "ModelResponseStream") -> bool:
         """True if ``chunk`` carries response content AND a finish_reason."""
@@ -351,6 +365,18 @@ class AnthropicStreamWrapper(AdapterCompletionStreamWrapper):
             text="",
         )
 
+    @property
+    def chunks(self) -> "list[ModelResponseStream] | None":
+        return cast(  # cast-ok: chunks is a list of ModelResponseStream on the inner stream
+            "list[ModelResponseStream] | None", getattr(self.completion_stream, "chunks", None)
+        )
+
+    @property
+    def messages(self) -> "list[AllMessageValues] | None":
+        return cast(  # cast-ok: messages is a list of AllMessageValues on the inner stream
+            "list[AllMessageValues] | None", getattr(self.completion_stream, "messages", None)
+        )
+
     def _merge_usage_into_held_stop_reason_chunk(self, chunk: Any) -> MessageBlockDelta:
         """Merge usage data from ``chunk`` into the held ``message_delta`` chunk.
 
@@ -1173,3 +1199,37 @@ class AnthropicStreamWrapper(AdapterCompletionStreamWrapper):
                 return True
 
         return False
+
+
+class AnthropicSSEStream(AsyncIterator[bytes]):
+    """
+    AsyncIterator[bytes] view of AnthropicStreamWrapper returned to callers of
+    translate_completion_output_params_streaming. Keeps the wrapper reachable so
+    the proxy's disconnect-time partial billing can read the inner chat stream's
+    collected chunks, messages, and model; a bare async generator would hide them.
+    """
+
+    def __init__(self, anthropic_wrapper: AnthropicStreamWrapper) -> None:
+        self._anthropic_wrapper = anthropic_wrapper
+        self._byte_stream: Final[AsyncIterator[bytes]] = anthropic_wrapper.async_anthropic_sse_wrapper()
+        self._hidden_params: dict[
+            str, object
+        ] = {}  # mutable-ok: the proxy merges provider headers onto _hidden_params in place
+
+    @property
+    def chunks(self) -> "list[ModelResponseStream] | None":
+        return self._anthropic_wrapper.chunks
+
+    @property
+    def messages(self) -> "list[AllMessageValues] | None":
+        return self._anthropic_wrapper.messages
+
+    @property
+    def model(self) -> str:
+        return self._anthropic_wrapper.model
+
+    async def __anext__(self) -> bytes:
+        return await self._byte_stream.__anext__()
+
+    async def aclose(self) -> None:
+        await self._byte_stream.aclose()
diff --git a/litellm/llms/anthropic/pass_through/adapters/transformation.py b/litellm/llms/anthropic/pass_through/adapters/transformation.py
index 2bb081bd0a4..040c8f0e170 100644
--- a/litellm/llms/anthropic/pass_through/adapters/transformation.py
+++ b/litellm/llms/anthropic/pass_through/adapters/transformation.py
@@ -201,7 +201,7 @@ from litellm.types.llms.openai import (
 from litellm.types.utils import Choices, ModelResponse, StreamingChoices, Usage
 from litellm.utils import supports_mid_conversation_system
 
-from .streaming_iterator import AnthropicStreamWrapper
+from .streaming_iterator import AnthropicSSEStream, AnthropicStreamWrapper
 
 if TYPE_CHECKING:
     from litellm.litellm_core_utils.litellm_logging import Logging as LiteLLMLoggingObject
@@ -341,7 +341,7 @@ class AnthropicAdapter:
         )
         # Return the SSE-wrapped version for proper event formatting.
         if is_async:
-            return anthropic_wrapper.async_anthropic_sse_wrapper()
+            return AnthropicSSEStream(anthropic_wrapper)
         return anthropic_wrapper.anthropic_sse_wrapper()
 
 
diff --git a/litellm/llms/anthropic/pass_through/messages/response_cache.py b/litellm/llms/anthropic/pass_through/messages/response_cache.py
index 1a8b041e674..7b9435d45ac 100644
--- a/litellm/llms/anthropic/pass_through/messages/response_cache.py
+++ b/litellm/llms/anthropic/pass_through/messages/response_cache.py
@@ -1,7 +1,7 @@
 import re
 from collections.abc import AsyncIterator, Mapping, Sequence
 from types import MappingProxyType
-from typing import TYPE_CHECKING, Final
+from typing import TYPE_CHECKING, Final, cast
 
 import litellm
 from litellm._logging import verbose_logger
@@ -17,6 +17,8 @@ from litellm.llms.anthropic.pass_through.messages.streaming_iterator import (
 if TYPE_CHECKING:
     from litellm.caching.caching_handler import LLMCachingHandler
     from litellm.litellm_core_utils.litellm_logging import Logging as LiteLLMLoggingObj
+    from litellm.types.llms.openai import AllMessageValues
+    from litellm.types.utils import ModelResponseStream
 
 CACHED_STREAM_EVENTS_KEY: Final = "litellm_cached_anthropic_sse_events"
 
@@ -51,6 +53,24 @@ class AnthropicMessagesStreamCacheWriter:
     def has_buffered_provider_output(self) -> bool:
         return getattr(self.stream, "has_buffered_provider_output", False) is True
 
+    @property
+    def chunks(self) -> "list[ModelResponseStream] | None":
+        return cast(  # cast-ok: chunks is a list of ModelResponseStream on the inner stream
+            "list[ModelResponseStream] | None", getattr(self.stream, "chunks", None)
+        )
+
+    @property
+    def messages(self) -> "list[AllMessageValues] | None":
+        return cast(  # cast-ok: messages is a list of AllMessageValues on the inner stream
+            "list[AllMessageValues] | None", getattr(self.stream, "messages", None)
+        )
+
+    @property
+    def model(self) -> str | None:
+        return cast(  # cast-ok: model is a str on the inner stream
+            "str | None", getattr(self.stream, "model", None)
+        )
+
     def __aiter__(self) -> "AnthropicMessagesStreamCacheWriter":
         return self
 
diff --git a/litellm/llms/databricks/chat/transformation.py b/litellm/llms/databricks/chat/transformation.py
index 538904b34e6..d669f2acc6d 100644
--- a/litellm/llms/databricks/chat/transformation.py
+++ b/litellm/llms/databricks/chat/transformation.py
@@ -778,6 +778,17 @@ class DatabricksChatResponseIterator(BaseModelResponseIterator):
                 )
                 choice["delta"]["thinking_blocks"] = thinking_blocks
                 translated_choices.append(choice)
+            service_tier: Final = chunk.get("service_tier")
+            if isinstance(service_tier, str) and service_tier:
+                return ModelResponseStream(
+                    id=chunk["id"],
+                    object="chat.completion.chunk",
+                    created=chunk["created"],
+                    model=chunk["model"],
+                    choices=translated_choices,
+                    usage=chunk.get("usage"),
+                    service_tier=service_tier,
+                )
             return ModelResponseStream(
                 id=chunk["id"],
                 object="chat.completion.chunk",
diff --git a/litellm/llms/databricks/cost_calculator.py b/litellm/llms/databricks/cost_calculator.py
index 64166e6fc11..2bb5b99f0ad 100644
--- a/litellm/llms/databricks/cost_calculator.py
+++ b/litellm/llms/databricks/cost_calculator.py
@@ -30,7 +30,7 @@ def _registry_key(model: str) -> str:
     )
 
 
-def cost_per_token(model: str, usage: Usage) -> tuple[float, float]:
+def cost_per_token(model: str, usage: Usage, service_tier: str | None = None) -> tuple[float, float]:
     """
     Calculates the cost per token for a given model, prompt tokens, and completion tokens.
 
@@ -45,4 +45,5 @@ def cost_per_token(model: str, usage: Usage) -> tuple[float, float]:
         model=_registry_key(model),
         usage=usage,
         custom_llm_provider="databricks",
+        service_tier=service_tier,
     )
diff --git a/litellm/llms/openai/chat/gpt_transformation.py b/litellm/llms/openai/chat/gpt_transformation.py
index 62351d8e39a..3b38825c83d 100644
--- a/litellm/llms/openai/chat/gpt_transformation.py
+++ b/litellm/llms/openai/chat/gpt_transformation.py
@@ -890,6 +890,9 @@ class OpenAIChatCompletionStreamingHandler(BaseModelResponseIterator):
             }
             if "usage" in chunk and chunk["usage"] is not None:
                 kwargs["usage"] = chunk["usage"]
+            service_tier: Final = chunk.get("service_tier")
+            if isinstance(service_tier, str) and service_tier:
+                kwargs["service_tier"] = service_tier
             return ModelResponseStream(**kwargs)
         except Exception as e:
             raise e
diff --git a/litellm/proxy/proxy_server.py b/litellm/proxy/proxy_server.py
index 7cd116c23d7..6a221f2bfed 100644
--- a/litellm/proxy/proxy_server.py
+++ b/litellm/proxy/proxy_server.py
@@ -9296,9 +9296,10 @@ def _fast_serialize_simple_model_response_stream(
         "object": getattr(chunk, "object", None),
         "created": getattr(chunk, "created", None),
         "model": model,
+        "service_tier": getattr(chunk, "service_tier", None),
         "choices": [choice_dict],
     }
-    for top_level_key in ("id", "object", "created"):
+    for top_level_key in ("id", "object", "created", "service_tier"):
         if payload[top_level_key] is None:
             payload.pop(top_level_key)
     return orjson.dumps(payload)
diff --git a/litellm/router.py b/litellm/router.py
index 842cd9de378..86a67a8d5ca 100644
--- a/litellm/router.py
+++ b/litellm/router.py
@@ -638,6 +638,24 @@ class FallbackAwareAnthropicMessagesStream:
     def has_buffered_provider_output(self) -> bool:
         return getattr(self._source_iterator, "has_buffered_provider_output", False) is True
 
+    @property
+    def chunks(self) -> list[ModelResponseStream] | None:
+        return cast(  # cast-ok: chunks is a list of ModelResponseStream on the inner stream
+            "list[ModelResponseStream] | None", getattr(self._source_iterator, "chunks", None)
+        )
+
+    @property
+    def messages(self) -> list[AllMessageValues] | None:
+        return cast(  # cast-ok: messages is a list of AllMessageValues on the inner stream
+            "list[AllMessageValues] | None", getattr(self._source_iterator, "messages", None)
+        )
+
+    @property
+    def model(self) -> str | None:
+        return cast(  # cast-ok: model is a str on the inner stream
+            "str | None", getattr(self._source_iterator, "model", None)
+        )
+
     def adopt_fallback_source(self, fallback_response: object) -> None:
         self._source_iterator = fallback_response
         self.fallback_headers_adopted = True
diff --git a/tests/code_coverage_tests/router_code_coverage.py b/tests/code_coverage_tests/router_code_coverage.py
index 06e5b020836..7c247ae3303 100644
--- a/tests/code_coverage_tests/router_code_coverage.py
+++ b/tests/code_coverage_tests/router_code_coverage.py
@@ -81,6 +81,9 @@ ignored_function_names = [
     "_merge_tools_from_deployment",  # Tested indirectly via _update_kwargs_with_deployment (test files lack "router" in name)
     "_invalidate_access_groups_cache",  # Tested indirectly via set_model_list, upsert_model etc. (test files lack "router" in name)
     "has_buffered_provider_output",  # Property, so its reads in test_router.py are never an ast.Call
+    "chunks",  # Property on FallbackAwareAnthropicMessagesStream, so its reads in tests are never an ast.Call
+    "messages",  # Property on FallbackAwareAnthropicMessagesStream, so its reads in tests are never an ast.Call
+    "model",  # Property on FallbackAwareAnthropicMessagesStream, so its reads in tests are never an ast.Call
     "_request_header",  # Tested through Claude Code session routing in test_router.py
     "_claude_code_session_router_cache_key",  # Tested through Claude Code session routing in test_router.py
     "_delete_claude_code_session_router_binding",  # Tested through Redis cleanup failure in test_router.py
diff --git a/tests/e2e/coverage_registry/llm_conversational.yaml b/tests/e2e/coverage_registry/llm_conversational.yaml
index cd52e563d69..8de8d5875b4 100644
--- a/tests/e2e/coverage_registry/llm_conversational.yaml
+++ b/tests/e2e/coverage_registry/llm_conversational.yaml
@@ -13,6 +13,7 @@
 - {id: llm.chat_completions.openai.vision.nonstream.works, module: llm, tier: P0, subject_endpoint: chat_completions, route: openai, capability: vision, streaming: nonstream, assertions: [works], source: "model_prices json", rationale: "gpt-4o vision; high usage"}
 - {id: llm.chat_completions.openai.prompt_cache_5m.nonstream.works, module: llm, tier: P0, subject_endpoint: chat_completions, route: openai, capability: prompt_cache_5m, streaming: nonstream, assertions: [works], source: "model_prices json", rationale: "Prompt caching cost optimization"}
 - {id: llm.chat_completions.openai.service_tier.nonstream.works, module: llm, tier: P1, subject_endpoint: chat_completions, route: openai, capability: service_tier, streaming: nonstream, assertions: [works], source: "OpenAI service_tier param", rationale: "OpenAI scale-tier request option is forwarded and echoed"}
+- {id: llm.chat_completions.openai.service_tier.stream.echoes_served_tier, module: llm, tier: P1, subject_endpoint: chat_completions, route: openai, capability: service_tier, streaming: stream, assertions: [works], source: "litellm_core_utils/streaming_handler.py", fail_before_fix: proven, rationale: "Every relayed stream chunk carries the service_tier OpenAI stamped on it, so a streaming caller can see which tier served the request"}
 - {id: llm.chat_completions.openai.thinking.nonstream.works, module: llm, tier: P1, subject_endpoint: chat_completions, route: openai, capability: thinking, streaming: nonstream, assertions: [works], source: "model_prices json", rationale: "o-series reasoning; emerging"}
 - {id: llm.chat_completions.openai.structured_output.nonstream.works, module: llm, tier: P1, subject_endpoint: chat_completions, route: openai, capability: structured_output, streaming: nonstream, assertions: [works], source: "model_prices json", rationale: "response_schema extraction"}
 - {id: llm.chat_completions.anthropic.basic.nonstream.works, module: llm, tier: P0, subject_endpoint: chat_completions, route: anthropic, capability: basic, streaming: nonstream, assertions: [works], source: "proxy_server.py:8455", rationale: "P0 route translated to Anthropic"}
diff --git a/tests/e2e/coverage_registry/quota_management.yaml b/tests/e2e/coverage_registry/quota_management.yaml
index 1051bf0bda9..163de67fc41 100644
--- a/tests/e2e/coverage_registry/quota_management.yaml
+++ b/tests/e2e/coverage_registry/quota_management.yaml
@@ -61,6 +61,9 @@
 - {id: quota_management.spend_tracking.stream_cache_read.bills_cache_read_rate, module: quota_management, tier: P1, behavior: spend_tracking, variant: stream_cache_read, assertions: [bills_cache_read_rate], exercised_on: [chat_completions], source: "litellm_core_utils/streaming_chunk_builder_utils.py", rationale: "A streamed call's reassembled usage keeps the cached-token detail so cache reads bill at the cache-read discount, not full input price (#34812)"}
 - {id: quota_management.spend_tracking.messages_bridge.keeps_cache_tokens, module: quota_management, tier: P1, behavior: spend_tracking, variant: messages_bridge, assertions: [keeps_cache_tokens], exercised_on: [messages], source: "llms/anthropic/pass_through/responses_adapters/handler.py", rationale: "A /v1/messages request served by a Responses-only OpenAI model keeps its cache-read tokens and their discounted billing across the bridge (#34957)"}
 - {id: quota_management.spend_tracking.service_tier.bills_tier_rates, module: quota_management, tier: P1, behavior: spend_tracking, variant: service_tier, assertions: [bills_tier_rates], exercised_on: [chat_completions], source: "cost_calculator.py", rationale: "A priority service_tier call bills input, output, and reasoning at the deployment's *_priority rates and records the tier on the row (#35923, #35925)"}
+- {id: quota_management.spend_tracking.service_tier_stream.records_served_tier, module: quota_management, tier: P1, behavior: spend_tracking, variant: service_tier_stream, assertions: [records_served_tier], exercised_on: [chat_completions], source: "litellm_core_utils/streaming_chunk_builder_utils.py", fail_before_fix: proven, rationale: "A streamed call with no service_tier requested bills at the rates of the tier OpenAI stamps on its chunks and records that served tier on the row; the reassembled stream dropped the provider tier so the row recorded none and priced at the default rates"}
+- {id: quota_management.spend_tracking.service_tier_stream.responses_records_served_tier, module: quota_management, tier: P1, behavior: spend_tracking, variant: service_tier_stream, assertions: [records_served_tier], exercised_on: [responses], source: "responses/streaming_iterator.py", rationale: "A streamed /v1/responses call bills at the tier carried on the response.completed event's inner response and records that served tier on the spend row"}
+- {id: quota_management.spend_tracking.service_tier_stream.messages_records_served_tier, module: quota_management, tier: P1, behavior: spend_tracking, variant: service_tier_stream, assertions: [records_served_tier], exercised_on: [messages], source: "llms/anthropic/pass_through/adapters/streaming_iterator.py", rationale: "A streamed /v1/messages call on an OpenAI-backed deployment bills at the tier OpenAI served; the Anthropic wire format has no tier field, so the spend row is the only record of it"}
 - {id: quota_management.spend_tracking.cost_headers.additive_components, module: quota_management, tier: P1, behavior: spend_tracking, variant: cost_headers, assertions: [additive_components], exercised_on: [chat_completions], source: "proxy/common_request_processing.py", rationale: "The x-litellm-response-cost-* component headers sum to the total, input covers only fresh tokens, and reasoning stays a subset of output (#36965)"}
 - {id: quota_management.spend_tracking.passthrough_stream.injects_usage_cost, module: quota_management, tier: P1, behavior: spend_tracking, variant: passthrough_stream, assertions: [injects_usage_cost], exercised_on: [openai_passthrough], source: "proxy/pass_through_endpoints/streaming_handler.py", rationale: "With include_cost_in_streaming_usage on, the /openai passthrough's final streaming usage frame carries the proxy-computed cost (#36503). Uncovered: the flag is only settable in litellm_settings, and the shared e2e stack does not turn it on yet"}
 - {id: quota_management.spend_tracking.websearch_interception.bills_under_request_session, module: quota_management, tier: P1, behavior: spend_tracking, variant: websearch_interception, assertions: [bills_under_request_session], exercised_on: [messages], source: "integrations/websearch_interception/handler.py", fail_before_fix: proven, rationale: "A web_search server tool the proxy intercepts into litellm.asearch writes its own asearch spend row, and that row carries the parent request's session_id so the session view counts the search and its cost next to the turn that triggered it (LIT-8063)"}
diff --git a/tests/e2e/models.py b/tests/e2e/models.py
index 6e66529ec8f..0383da48c43 100644
--- a/tests/e2e/models.py
+++ b/tests/e2e/models.py
@@ -568,6 +568,17 @@ class AnthropicMessagesBody(BaseModel):
     cache: dict[str, bool] | None = {"no-cache": True}
 
 
+class ResponsesStreamBody(BaseModel):
+    """POST /v1/responses body in the subset the spend tests stream with.
+    `input` stays a plain string: the tests only drive single-turn prompts."""
+
+    model: str
+    input: str
+    stream: bool = True
+    max_output_tokens: int | None = None
+    cache: dict[str, bool] | None = {"no-cache": True}
+
+
 class CountTokensBody(BaseModel):
     """POST /v1/messages/count_tokens body: the /v1/messages shape minus
     max_tokens (the endpoint only counts the prompt)."""
diff --git a/tests/e2e/proxy_client.py b/tests/e2e/proxy_client.py
index 4ea83e4b0d3..bd87828db2e 100644
--- a/tests/e2e/proxy_client.py
+++ b/tests/e2e/proxy_client.py
@@ -84,6 +84,7 @@ from models import (
     OcrResponse,
     RerankBody,
     RerankResponse,
+    ResponsesStreamBody,
     RouterCurrentValues,
     RouterSettingsResponse,
     SearchToolCreateBody,
@@ -969,6 +970,9 @@ class ProxyClient:
     def messages_stream(self, key: str, body: AnthropicMessagesBody) -> StreamingResponse:
         return self.transport.stream("/v1/messages", headers=self.transport.bearer(key), json=body)
 
+    def responses_stream(self, key: str, body: ResponsesStreamBody) -> StreamingResponse:
+        return self.transport.stream("/v1/responses", headers=self.transport.bearer(key), json=body)
+
     def embed(self, key: str, body: EmbedBody) -> Result[EmbedResponse]:
         return self.transport.post(
             "/embeddings",
diff --git a/tests/e2e/quota_management/spend_tracking/test_service_tier_pricing_e2e.py b/tests/e2e/quota_management/spend_tracking/test_service_tier_pricing_e2e.py
index 770c5699b4e..bf68fb68a60 100644
--- a/tests/e2e/quota_management/spend_tracking/test_service_tier_pricing_e2e.py
+++ b/tests/e2e/quota_management/spend_tracking/test_service_tier_pricing_e2e.py
@@ -13,27 +13,45 @@ priority processing and served the default tier, the test fails there instead of
 producing a vacuous rate comparison. Reasoning is requested explicitly with
 `reasoning_effort`, so the reasoning-rate assertion rests on a parameter the test
 sets rather than on whatever the model happens to do by default.
+
+The streaming cases pin the served-tier contract: OpenAI stamps the tier it actually
+used on every stream chunk, and that echo is what the caller sees and what the bill
+must be computed on. The request sets no service_tier, so the only place the tier
+can come from is the provider's response. The spend row must record the served tier
+and price input at that tier's rate, and every chunk the proxy relays must carry the
+same service_tier the provider sent.
 """
 
-import pytest
+import json
 
+import pytest
 from cost_rows import (
     approx_equal,
     assert_fresh_tokens_billed_at,
     assert_total_is_sum_of_components,
     poll_cost_row,
+    poll_cost_row_where,
     register_priced_model,
 )
-from e2e_config import unique_marker
+from e2e_config import CHEAP_OPENAI_MODEL, unique_marker
 from e2e_http import unwrap
 from lifecycle import ResourceManager
-from models import ChatBody, ChatMessage, LiteLLMParamsBody
+from models import (
+    AnthropicMessagesBody,
+    ChatBody,
+    ChatMessage,
+    ChatStreamOptions,
+    LiteLLMParamsBody,
+    ResponsesStreamBody,
+)
+from pydantic import BaseModel
 from spend_e2e_client import SpendClient
 
 pytestmark = pytest.mark.e2e
 
 BACKEND = "openai/gpt-5.6-luna"
 OPENAI_API_KEY = "os.environ/OPENAI_API_KEY"
+STREAM_BACKEND = f"openai/{CHEAP_OPENAI_MODEL}"
 
 INPUT_RATE = 4e-05
 OUTPUT_RATE = 8e-05
@@ -42,6 +60,40 @@ PRIORITY_OUTPUT_RATE = 1.6e-04
 
 REASONING_EFFORT = "high"
 
+TIER_INPUT_RATES = {"default": INPUT_RATE, "priority": PRIORITY_INPUT_RATE}
+
+
+class _StreamChunk(BaseModel):
+    id: str | None = None
+    service_tier: str | None = None
+
+
+class _CompletedResponseObject(BaseModel):
+    id: str | None = None
+    service_tier: str | None = None
+
+
+class _ResponsesStreamEvent(BaseModel):
+    type: str | None = None
+    response: _CompletedResponseObject | None = None
+
+
+class _MessagesStreamEvent(BaseModel):
+    type: str | None = None
+
+
+def _stream_chunks(events: list[str]) -> list[_StreamChunk]:
+    return [_StreamChunk.model_validate_json(event) for event in events if event.strip() != "[DONE]"]
+
+
+def _served_tier(chunks: list[_StreamChunk]) -> str:
+    tiers = {chunk.service_tier for chunk in chunks if chunk.service_tier}
+    assert len(tiers) == 1, (
+        f"the relayed stream carried {tiers or 'no'} service tier(s) across {len(chunks)} chunks; OpenAI stamps "
+        "the served tier on every chat chunk, so exactly one tier must reach the caller"
+    )
+    return tiers.pop()
+
 
 class TestServiceTierPricing:
     @pytest.mark.covers("quota_management.spend_tracking.service_tier.bills_tier_rates")
@@ -83,8 +135,7 @@ class TestServiceTierPricing:
             )
         )
         assert chat.service_tier == "priority", (
-            f"OpenAI served tier {chat.service_tier!r} instead of priority; "
-            "tier billing was never exercised"
+            f"OpenAI served tier {chat.service_tier!r} instead of priority; tier billing was never exercised"
         )
         assert chat.id, f"chat response carried no id: {chat}"
 
@@ -119,3 +170,150 @@ class TestServiceTierPricing:
         )
 
         assert_total_is_sum_of_components(row)
+
+    @pytest.mark.covers("quota_management.spend_tracking.service_tier_stream.records_served_tier")
+    def test_streamed_call_records_and_bills_the_served_tier(
+        self, client: SpendClient, resources: ResourceManager, scoped_key: str
+    ) -> None:
+        model = register_priced_model(
+            client.proxy,
+            resources,
+            "tier-priced-stream",
+            LiteLLMParamsBody(
+                model=BACKEND,
+                api_key=OPENAI_API_KEY,
+                input_cost_per_token=INPUT_RATE,
+                output_cost_per_token=OUTPUT_RATE,
+                input_cost_per_token_priority=PRIORITY_INPUT_RATE,
+                output_cost_per_token_priority=PRIORITY_OUTPUT_RATE,
+            ),
+        )
+
+        result = client.proxy.chat_stream(
+            scoped_key,
+            ChatBody(
+                model=model,
+                messages=[ChatMessage(role="user", content=f"{unique_marker()} reply with one word")],
+                max_completion_tokens=64,
+                stream=True,
+            ),
+        )
+        assert result.ok and result.stream_events, (
+            f"streamed chat failed (status {result.status_code}): {result.body[:300]}"
+        )
+        chunks = _stream_chunks(result.stream_events)
+        served_tier = _served_tier(chunks)
+        assert served_tier in TIER_INPUT_RATES, f"no custom rate registered for served tier {served_tier!r}"
+        stream_id = chunks[0].id
+        assert stream_id, f"first stream chunk carried no id: {result.stream_events[0][:200]}"
+
+        row = poll_cost_row(client.proxy, stream_id)
+        assert row is not None, f"no spend row with a cost breakdown landed for {stream_id}"
+        assert row.breakdown.service_tier == served_tier, (
+            f"the provider served tier {served_tier!r} on every chunk but the bill records "
+            f"pricing basis {row.breakdown.service_tier!r}"
+        )
+        assert_fresh_tokens_billed_at(row, TIER_INPUT_RATES[served_tier])
+        assert_total_is_sum_of_components(row)
+
+    @pytest.mark.covers("llm.chat_completions.openai.service_tier.stream.echoes_served_tier")
+    def test_every_streamed_chunk_carries_the_served_tier(
+        self, client: SpendClient, resources: ResourceManager, scoped_key: str
+    ) -> None:
+        model = register_priced_model(
+            client.proxy, resources, "tier-echo-stream", LiteLLMParamsBody(model=STREAM_BACKEND, api_key=OPENAI_API_KEY)
+        )
+        result = client.proxy.chat_stream(
+            scoped_key,
+            ChatBody(
+                model=model,
+                messages=[ChatMessage(role="user", content=f"{unique_marker()} reply with one word")],
+                max_completion_tokens=64,
+                stream=True,
+                stream_options=ChatStreamOptions(include_usage=True),
+            ),
+        )
+        assert result.ok and result.stream_events, (
+            f"streamed chat failed (status {result.status_code}): {result.body[:300]}"
+        )
+        chunks = _stream_chunks(result.stream_events)
+        served_tier = _served_tier(chunks)
+        missing = [
+            json.loads(event) for event, chunk in zip(result.stream_events, chunks) if chunk.service_tier is None
+        ]
+        assert not missing, (
+            f"{len(missing)} of {len(chunks)} relayed chunks dropped the provider's service_tier "
+            f"{served_tier!r}: {missing}"
+        )
+
+    @pytest.mark.covers("quota_management.spend_tracking.service_tier_stream.responses_records_served_tier")
+    def test_responses_stream_records_the_served_tier(
+        self, client: SpendClient, resources: ResourceManager, scoped_key: str
+    ) -> None:
+        model = register_priced_model(
+            client.proxy,
+            resources,
+            "tier-responses-stream",
+            LiteLLMParamsBody(model=STREAM_BACKEND, api_key=OPENAI_API_KEY),
+        )
+
+        result = client.proxy.responses_stream(
+            scoped_key,
+            ResponsesStreamBody(model=model, input=f"{unique_marker()} reply with one word"),
+        )
+        assert result.ok and result.stream_events, (
+            f"streamed responses call failed (status {result.status_code}): {result.body[:300]}"
+        )
+
+        events = [_ResponsesStreamEvent.model_validate_json(event) for event in result.stream_events]
+        completed = next((event for event in reversed(events) if event.type == "response.completed"), None)
+        assert completed is not None and completed.response is not None, (
+            f"no response.completed event in the stream: {[e.type for e in events]}"
+        )
+        served_tier = completed.response.service_tier
+        assert served_tier, f"response.completed carried no service_tier: {completed.response}"
+        assert served_tier in TIER_INPUT_RATES, f"no custom rate registered for served tier {served_tier!r}"
+
+        row = poll_cost_row_where(client.proxy, scoped_key, lambda r: r.spend is not None and r.spend > 0)
+        assert row is not None, f"no spend row with a cost breakdown landed for the streamed responses call on {model}"
+        assert row.breakdown.service_tier == served_tier, (
+            f"response.completed served tier {served_tier!r} but the bill records "
+            f"pricing basis {row.breakdown.service_tier!r}"
+        )
+
+    @pytest.mark.covers("quota_management.spend_tracking.service_tier_stream.messages_records_served_tier")
+    def test_messages_stream_records_the_served_tier(
+        self, client: SpendClient, resources: ResourceManager, scoped_key: str
+    ) -> None:
+        model = register_priced_model(
+            client.proxy,
+            resources,
+            "tier-messages-stream",
+            LiteLLMParamsBody(model=STREAM_BACKEND, api_key=OPENAI_API_KEY),
+        )
+
+        result = client.proxy.messages_stream(
+            scoped_key,
+            AnthropicMessagesBody(
+                model=model,
+                messages=[ChatMessage(role="user", content=f"{unique_marker()} reply with one word")],
+                max_tokens=64,
+                stream=True,
+            ),
+        )
+        assert result.ok and result.stream_events, (
+            f"streamed messages call failed (status {result.status_code}): {result.body[:300]}"
+        )
+
+        events = [_MessagesStreamEvent.model_validate_json(event) for event in result.stream_events]
+        assert any(event.type == "message_delta" for event in events), (
+            f"the anthropic stream emitted no message_delta: {[e.type for e in events]}"
+        )
+
+        row = poll_cost_row_where(client.proxy, scoped_key, lambda r: r.spend is not None and r.spend > 0)
+        assert row is not None, f"no spend row with a cost breakdown landed for the streamed messages call on {model}"
+        served_tier = row.breakdown.service_tier
+        assert served_tier in TIER_INPUT_RATES and served_tier is not None, (
+            "the anthropic wire format carries no service_tier, so the bill is the only record of "
+            f"the tier OpenAI served; the row recorded pricing basis {served_tier!r}"
+        )
diff --git a/tests/integration/spend/test_service_tier_stream_billing.py b/tests/integration/spend/test_service_tier_stream_billing.py
new file mode 100644
index 00000000000..4791f941d61
--- /dev/null
+++ b/tests/integration/spend/test_service_tier_stream_billing.py
@@ -0,0 +1,660 @@
+"""Served service_tier drives billing on streamed calls, complete and disconnected.
+
+The scripted upstream answers OpenAI-compatible /chat/completions with SSE chunks
+that carry service_tier "priority" and terminal usage. The deployment registers
+distinct default and *_priority rates, so a bill computed on the wrong tier cannot
+match the hand-computed expectation. /v1/messages deployments on hosted_vllm have
+no anthropic-messages provider config, so they take the chat adapter: the
+streamed response is an AnthropicStreamWrapper under AnthropicSSEStream, wrapped
+by AnthropicMessagesStreamCacheWriter when litellm.cache is on and then by the
+router's FallbackAwareAnthropicMessagesStream; each layer must delegate the
+inner stream's chunks for disconnect billing to find them.
+
+Azure streams run the same OpenAI chunk path against /openai/deployments, so the
+served tier must reach the spend row there too (LIT-2850). Databricks streams go
+through DatabricksChatResponseIterator.chunk_parser and the databricks branch of
+cost_per_token (LIT-8121). The responses bridge relays Responses API SSE as chat
+chunks, so the served tier remembered from response.created must land on both
+the chunks and the row. Gemini reports capacity as usageMetadata.trafficType, which maps to
+service_tier "flex" and the *_flex rates (LIT-6287, LIT-6292).
+"""
+
+import json
+from collections.abc import Callable
+from hashlib import sha256
+from typing import Final
+from uuid import uuid4
+
+import pytest
+from integration._support.client import Gateway, Scenario, eventually, object_value
+from integration._support.database import read_rows
+from integration._support.wire import Reply, Request, Wire, wire_server
+from pydantic import JsonValue
+
+PROMPT_TOKENS: Final = 30
+COMPLETION_TOKENS: Final = 40
+INPUT_RATE: Final = 0.001
+OUTPUT_RATE: Final = 0.002
+PRIORITY_INPUT_RATE: Final = 0.01
+PRIORITY_OUTPUT_RATE: Final = 0.02
+EXPECTED_FULL_SPEND: Final = PROMPT_TOKENS * PRIORITY_INPUT_RATE + COMPLETION_TOKENS * PRIORITY_OUTPUT_RATE
+FLEX_INPUT_RATE: Final = 0.0005
+FLEX_OUTPUT_RATE: Final = 0.001
+EXPECTED_FLEX_SPEND: Final = PROMPT_TOKENS * FLEX_INPUT_RATE + COMPLETION_TOKENS * FLEX_OUTPUT_RATE
+
+
+def _sse_frame(payload: dict[str, JsonValue]) -> bytes:
+    return f"data: {json.dumps(payload, separators=(',', ':'))}\n\n".encode()
+
+
+def _chat_chunk(request_id: str, upstream_model: str, content: str, served_tier: str) -> dict[str, JsonValue]:
+    return {
+        "id": request_id,
+        "object": "chat.completion.chunk",
+        "created": 1,
+        "model": upstream_model,
+        "service_tier": served_tier,
+        "choices": [{"index": 0, "delta": {"role": "assistant", "content": content}, "finish_reason": None}],
+    }
+
+
+def _respond_for(
+    request_id: str,
+    prompt: str,
+    *,
+    expected_target: str = "/v1/chat/completions",
+    pause: float = 0.4,
+    served_tier: str = "priority",
+    expected_requested_tier: str | None = None,
+) -> Callable[[Request], Reply]:
+    def respond(request: Request) -> Reply:
+        if request.target == "/v1/models":
+            return Reply(
+                body=json.dumps({"object": "list", "data": [{"id": "gpt-4o-mini", "object": "model"}]}).encode()
+            )
+        assert request.target.startswith(expected_target), request.target
+        body: Final = json.loads(request.body)
+        assert body["messages"] == [{"role": "user", "content": prompt}], body
+        if expected_requested_tier is not None:
+            assert body.get("service_tier") == expected_requested_tier, body
+        upstream_model: Final = str(body["model"])
+        terminal: Final[dict[str, JsonValue]] = {
+            "id": request_id,
+            "object": "chat.completion.chunk",
+            "created": 1,
+            "model": upstream_model,
+            "service_tier": served_tier,
+            "choices": [{"index": 0, "delta": {}, "finish_reason": "stop"}],
+            "usage": {
+                "prompt_tokens": PROMPT_TOKENS,
+                "completion_tokens": COMPLETION_TOKENS,
+                "total_tokens": PROMPT_TOKENS + COMPLETION_TOKENS,
+            },
+        }
+        return Reply(
+            content_type="text/event-stream",
+            chunks=(
+                _sse_frame(_chat_chunk(request_id, upstream_model, "first", served_tier)),
+                _sse_frame(_chat_chunk(request_id, upstream_model, "second", served_tier)),
+                _sse_frame(_chat_chunk(request_id, upstream_model, "third", served_tier)),
+                _sse_frame(terminal),
+                b"data: [DONE]\n\n",
+            ),
+            pause_between_chunks=pause,
+        )
+
+    return respond
+
+
+def _tiered_model(
+    scenario: Scenario,
+    wire: Wire,
+    *,
+    litellm_model: str,
+    api_base: str | None = None,
+    **extra: JsonValue,
+) -> str:
+    return scenario.model(
+        model=litellm_model,
+        api_base=api_base or f"{wire.url}/v1",
+        input_cost_per_token=INPUT_RATE,
+        output_cost_per_token=OUTPUT_RATE,
+        input_cost_per_token_priority=PRIORITY_INPUT_RATE,
+        output_cost_per_token_priority=PRIORITY_OUTPUT_RATE,
+        input_cost_per_token_flex=FLEX_INPUT_RATE,
+        output_cost_per_token_flex=FLEX_OUTPUT_RATE,
+        **extra,
+    )
+
+
+def _events(lines: list[str]) -> list[dict[str, JsonValue]]:
+    return [
+        object_value(json.loads(line.removeprefix("data:")))
+        for line in lines
+        if line.startswith("data:") and line.removeprefix("data:").strip() != "[DONE]"
+    ]
+
+
+def _rows_for_key(key: str) -> list[dict[str, JsonValue]]:
+    return read_rows(
+        'SELECT request_id, status, prompt_tokens, completion_tokens, spend, metadata FROM "LiteLLM_SpendLogs" '
+        "WHERE api_key=%s",
+        (sha256(key.encode()).hexdigest(),),
+    )
+
+
+def _single_spend_row(key: str) -> dict[str, JsonValue]:
+    rows: Final = eventually(lambda: _rows_for_key(key), lambda values: len(values) == 1, seconds=70)
+    return rows[0]
+
+
+def _cost_breakdown(row: dict[str, JsonValue]) -> dict[str, JsonValue]:
+    metadata: Final = row["metadata"]
+    parsed: Final = json.loads(metadata) if isinstance(metadata, str) else object_value(metadata)
+    return object_value(parsed["cost_breakdown"])
+
+
+@pytest.mark.timeout(120)
+def test_completed_chat_stream_bills_the_served_tier(gateway: Gateway) -> None:
+    prompt: Final = f"tier control {uuid4().hex[:8]}"
+    request_id: Final = f"chatcmpl-{uuid4().hex[:8]}"
+    with (
+        wire_server(_respond_for(request_id, prompt)) as wire,
+        gateway.scenario() as scenario,
+    ):
+        model: Final = _tiered_model(scenario, wire, litellm_model="openai/gpt-4o-mini")
+        key: Final = scenario.key(models=[model])
+        response: Final = gateway.request(
+            "POST",
+            "/v1/chat/completions",
+            {"model": model, "messages": [{"role": "user", "content": prompt}], "stream": True},
+            key=key,
+        )
+        assert response.status_code == 200, response.text
+        chunks: Final = _events(list(response.iter_lines()))
+
+        assert len(chunks) == 4, chunks
+        tiers: Final = {chunk.get("service_tier") for chunk in chunks}
+        assert tiers == {"priority"}, f"every relayed chunk must carry the served tier: {tiers}"
+
+        row: Final = _single_spend_row(key)
+        assert row["status"] == "success", row
+        assert row["request_id"] == request_id, row
+        assert row["prompt_tokens"] == PROMPT_TOKENS, row
+        assert row["completion_tokens"] == COMPLETION_TOKENS, row
+        assert float(str(row["spend"])) == pytest.approx(EXPECTED_FULL_SPEND), row
+        breakdown: Final = _cost_breakdown(row)
+        assert breakdown["service_tier"] == "priority", breakdown
+        assert len(wire.drain()) == 1
+
+
+@pytest.mark.timeout(120)
+def test_disconnected_chat_stream_bills_partial_usage_at_the_served_tier(gateway: Gateway) -> None:
+    prompt: Final = f"tier control {uuid4().hex[:8]}"
+    request_id: Final = f"chatcmpl-{uuid4().hex[:8]}"
+    with (
+        wire_server(_respond_for(request_id, prompt, pause=2.0)) as wire,
+        gateway.scenario() as scenario,
+    ):
+        model: Final = _tiered_model(scenario, wire, litellm_model="openai/gpt-4o-mini")
+        key: Final = scenario.key(models=[model])
+        with gateway.client.stream(
+            "POST",
+            "/v1/chat/completions",
+            json={
+                "model": model,
+                "messages": [{"role": "user", "content": prompt}],
+                "stream": True,
+            },
+            headers={"Authorization": f"Bearer {key}"},
+        ) as response:
+            assert response.status_code == 200, response.read().decode()
+            first_event: Final = next(line for line in response.iter_lines() if line.startswith("data:"))
+            assert object_value(json.loads(first_event.removeprefix("data:")))["id"] == request_id
+
+        row: Final = _single_spend_row(key)
+        assert row["status"] == "success", row
+        assert int(row["prompt_tokens"]) > 0, row
+        assert int(row["completion_tokens"]) == 1, row
+        assert float(str(row["spend"])) == pytest.approx(
+            int(row["prompt_tokens"]) * PRIORITY_INPUT_RATE + PRIORITY_OUTPUT_RATE
+        ), row
+        breakdown: Final = _cost_breakdown(row)
+        assert breakdown["service_tier"] == "priority", breakdown
+        assert len(wire.drain()) == 1
+
+
+@pytest.mark.timeout(120)
+def test_completed_messages_stream_bills_the_served_tier(gateway: Gateway) -> None:
+    prompt: Final = f"tier control {uuid4().hex[:8]}"
+    with (
+        wire_server(_respond_for(f"chatcmpl-{uuid4().hex[:8]}", prompt)) as wire,
+        gateway.scenario() as scenario,
+    ):
+        model: Final = _tiered_model(scenario, wire, litellm_model="hosted_vllm/gpt-4o-mini")
+        key: Final = scenario.key(models=[model])
+        with gateway.client.stream(
+            "POST",
+            "/v1/messages",
+            json={
+                "model": model,
+                "messages": [{"role": "user", "content": prompt}],
+                "max_tokens": COMPLETION_TOKENS,
+                "stream": True,
+            },
+            headers={"Authorization": f"Bearer {key}"},
+        ) as response:
+            assert response.status_code == 200, response.read().decode()
+            events: Final = _events(list(response.iter_lines()))
+
+        assert events[0]["type"] == "message_start", events
+        assert any(event["type"] == "message_delta" for event in events), events
+
+        row: Final = _single_spend_row(key)
+        assert row["status"] == "success", row
+        assert row["prompt_tokens"] == PROMPT_TOKENS, row
+        assert row["completion_tokens"] == COMPLETION_TOKENS, row
+        assert float(str(row["spend"])) == pytest.approx(EXPECTED_FULL_SPEND), row
+        breakdown: Final = _cost_breakdown(row)
+        assert breakdown["service_tier"] == "priority", breakdown
+        assert len(wire.drain()) == 1
+
+
+@pytest.mark.timeout(120)
+def test_disconnected_messages_stream_bills_partial_usage_at_the_served_tier(gateway: Gateway) -> None:
+    prompt: Final = f"tier control {uuid4().hex[:8]}"
+    with (
+        wire_server(_respond_for(f"chatcmpl-{uuid4().hex[:8]}", prompt, pause=2.0)) as wire,
+        gateway.scenario() as scenario,
+    ):
+        model: Final = _tiered_model(scenario, wire, litellm_model="hosted_vllm/gpt-4o-mini")
+        key: Final = scenario.key(models=[model])
+        with gateway.client.stream(
+            "POST",
+            "/v1/messages",
+            json={
+                "model": model,
+                "messages": [{"role": "user", "content": prompt}],
+                "max_tokens": COMPLETION_TOKENS,
+                "stream": True,
+            },
+            headers={"Authorization": f"Bearer {key}"},
+        ) as response:
+            assert response.status_code == 200, response.read().decode()
+            first_event: Final = next(line for line in response.iter_lines() if line.startswith("data:"))
+            assert object_value(json.loads(first_event.removeprefix("data:")))["type"] == "message_start", first_event
+
+        row: Final = _single_spend_row(key)
+        assert row["status"] == "success", row
+        assert float(str(row["spend"])) > 0, row
+        assert int(row["completion_tokens"]) < COMPLETION_TOKENS, row
+        breakdown: Final = _cost_breakdown(row)
+        assert breakdown["service_tier"] == "priority", breakdown
+        assert len(wire.drain()) == 1
+
+
+def _responses_frame(event: str, payload: dict[str, JsonValue]) -> bytes:
+    return f"event: {event}\ndata: {json.dumps(payload, separators=(',', ':'))}\n\n".encode()
+
+
+def _respond_responses_for(response_id: str, prompt: str) -> Callable[[Request], Reply]:
+    def respond(request: Request) -> Reply:
+        assert request.target == "/v1/responses", request.target
+        body: Final = json.loads(request.body)
+        assert prompt in json.dumps(body["input"]), body["input"]
+        assert body["stream"] is True, body
+        upstream_model: Final = str(body["model"])
+        text: Final = "firstsecondthird"
+        response_payload: Final[dict[str, JsonValue]] = {
+            "id": response_id,
+            "object": "response",
+            "model": upstream_model,
+            "status": "in_progress",
+            "service_tier": "priority",
+            "output": [],
+        }
+        message_item: Final[dict[str, JsonValue]] = {
+            "type": "message",
+            "id": "msg_1",
+            "status": "completed",
+            "role": "assistant",
+            "content": [{"type": "output_text", "text": text, "annotations": []}],
+        }
+        return Reply(
+            content_type="text/event-stream",
+            chunks=(
+                _responses_frame("response.created", {"type": "response.created", "response": response_payload}),
+                _responses_frame(
+                    "response.output_item.added",
+                    {
+                        "type": "response.output_item.added",
+                        "output_index": 0,
+                        "item": {
+                            "type": "message",
+                            "id": "msg_1",
+                            "status": "in_progress",
+                            "role": "assistant",
+                            "content": [],
+                        },
+                    },
+                ),
+                _responses_frame(
+                    "response.content_part.added",
+                    {
+                        "type": "response.content_part.added",
+                        "item_id": "msg_1",
+                        "output_index": 0,
+                        "content_index": 0,
+                        "part": {"type": "output_text", "text": ""},
+                    },
+                ),
+                *(
+                    _responses_frame(
+                        "response.output_text.delta",
+                        {
+                            "type": "response.output_text.delta",
+                            "item_id": "msg_1",
+                            "output_index": 0,
+                            "content_index": 0,
+                            "delta": delta,
+                        },
+                    )
+                    for delta in ("first", "second", "third")
+                ),
+                _responses_frame(
+                    "response.output_text.done",
+                    {
+                        "type": "response.output_text.done",
+                        "item_id": "msg_1",
+                        "output_index": 0,
+                        "content_index": 0,
+                        "text": text,
+                    },
+                ),
+                _responses_frame(
+                    "response.content_part.done",
+                    {
+                        "type": "response.content_part.done",
+                        "item_id": "msg_1",
+                        "output_index": 0,
+                        "content_index": 0,
+                        "part": {"type": "output_text", "text": text},
+                    },
+                ),
+                _responses_frame(
+                    "response.output_item.done",
+                    {"type": "response.output_item.done", "output_index": 0, "item": message_item},
+                ),
+                _responses_frame(
+                    "response.completed",
+                    {
+                        "type": "response.completed",
+                        "response": {
+                            **response_payload,
+                            "status": "completed",
+                            "output": [message_item],
+                            "usage": {
+                                "input_tokens": PROMPT_TOKENS,
+                                "output_tokens": COMPLETION_TOKENS,
+                                "total_tokens": PROMPT_TOKENS + COMPLETION_TOKENS,
+                            },
+                        },
+                    },
+                ),
+            ),
+        )
+
+    return respond
+
+
+def _gemini_chunk(text: str) -> dict[str, JsonValue]:
+    return {"candidates": [{"index": 0, "content": {"role": "model", "parts": [{"text": text}]}}]}
+
+
+def _respond_gemini_for(prompt: str) -> Callable[[Request], Reply]:
+    def respond(request: Request) -> Reply:
+        assert request.target.startswith("/models/gemini-2.5-flash:streamGenerateContent"), request.target
+        body: Final = json.loads(request.body)
+        assert prompt in json.dumps(body["contents"]), body["contents"]
+        terminal: Final[dict[str, JsonValue]] = {
+            "candidates": [{"index": 0, "content": {"role": "model", "parts": [{"text": ""}]}, "finishReason": "STOP"}],
+            "usageMetadata": {
+                "promptTokenCount": PROMPT_TOKENS,
+                "candidatesTokenCount": COMPLETION_TOKENS,
+                "totalTokenCount": PROMPT_TOKENS + COMPLETION_TOKENS,
+                "trafficType": "ON_DEMAND_FLEX",
+            },
+        }
+        return Reply(
+            content_type="text/event-stream",
+            chunks=(
+                _sse_frame(_gemini_chunk("first")),
+                _sse_frame(_gemini_chunk("second")),
+                _sse_frame(_gemini_chunk("third")),
+                _sse_frame(terminal),
+            ),
+        )
+
+    return respond
+
+
+@pytest.mark.timeout(120)
+def test_azure_chat_stream_bills_the_served_tier(gateway: Gateway) -> None:
+    prompt: Final = f"tier control {uuid4().hex[:8]}"
+    request_id: Final = f"chatcmpl-{uuid4().hex[:8]}"
+    with (
+        wire_server(
+            _respond_for(request_id, prompt, expected_target="/openai/deployments/gpt-4o-mini/chat/completions")
+        ) as wire,
+        gateway.scenario() as scenario,
+    ):
+        model: Final = _tiered_model(
+            scenario,
+            wire,
+            litellm_model="azure/gpt-4o-mini",
+            api_base=wire.url,
+            api_version="2024-10-21",
+        )
+        key: Final = scenario.key(models=[model])
+        response: Final = gateway.request(
+            "POST",
+            "/v1/chat/completions",
+            {"model": model, "messages": [{"role": "user", "content": prompt}], "stream": True},
+            key=key,
+        )
+        assert response.status_code == 200, response.text
+        chunks: Final = _events(list(response.iter_lines()))
+
+        assert len(chunks) == 4, chunks
+        tiers: Final = {chunk.get("service_tier") for chunk in chunks}
+        assert tiers == {"priority"}, f"every relayed chunk must carry the served tier: {tiers}"
+
+        row: Final = _single_spend_row(key)
+        assert row["status"] == "success", row
+        assert row["prompt_tokens"] == PROMPT_TOKENS, row
+        assert row["completion_tokens"] == COMPLETION_TOKENS, row
+        assert float(str(row["spend"])) == pytest.approx(EXPECTED_FULL_SPEND), row
+        breakdown: Final = _cost_breakdown(row)
+        assert breakdown["service_tier"] == "priority", breakdown
+        assert len(wire.drain()) == 1
+
+
+@pytest.mark.timeout(120)
+def test_databricks_chat_stream_bills_the_served_tier(gateway: Gateway) -> None:
+    prompt: Final = f"tier control {uuid4().hex[:8]}"
+    request_id: Final = f"chatcmpl-{uuid4().hex[:8]}"
+    with (
+        wire_server(_respond_for(request_id, prompt, expected_target="/serving-endpoints/chat/completions")) as wire,
+        gateway.scenario() as scenario,
+    ):
+        model: Final = _tiered_model(
+            scenario,
+            wire,
+            litellm_model="databricks/dbrx-instruct",
+            api_base=f"{wire.url}/serving-endpoints",
+        )
+        key: Final = scenario.key(models=[model])
+        response: Final = gateway.request(
+            "POST",
+            "/v1/chat/completions",
+            {"model": model, "messages": [{"role": "user", "content": prompt}], "stream": True},
+            key=key,
+        )
+        assert response.status_code == 200, response.text
+        chunks: Final = _events(list(response.iter_lines()))
+
+        assert len(chunks) == 4, chunks
+        tiers: Final = {chunk.get("service_tier") for chunk in chunks}
+        assert tiers == {"priority"}, f"every relayed chunk must carry the served tier: {tiers}"
+
+        row: Final = _single_spend_row(key)
+        assert row["status"] == "success", row
+        assert row["prompt_tokens"] == PROMPT_TOKENS, row
+        assert row["completion_tokens"] == COMPLETION_TOKENS, row
+        assert float(str(row["spend"])) == pytest.approx(EXPECTED_FULL_SPEND), row
+        breakdown: Final = _cost_breakdown(row)
+        assert breakdown["service_tier"] == "priority", breakdown
+        assert len(wire.drain()) == 1
+
+
+@pytest.mark.timeout(120)
+def test_responses_bridge_stream_bills_the_served_tier(gateway: Gateway) -> None:
+    prompt: Final = f"tier control {uuid4().hex[:8]}"
+    with (
+        wire_server(_respond_responses_for(f"resp_{uuid4().hex[:8]}", prompt)) as wire,
+        gateway.scenario() as scenario,
+    ):
+        model: Final = _tiered_model(scenario, wire, litellm_model="openai/responses/gpt-4o-mini")
+        key: Final = scenario.key(models=[model])
+        response: Final = gateway.request(
+            "POST",
+            "/v1/chat/completions",
+            {"model": model, "messages": [{"role": "user", "content": prompt}], "stream": True},
+            key=key,
+        )
+        assert response.status_code == 200, response.text
+        chunks: Final = _events(list(response.iter_lines()))
+
+        assert len(chunks) >= 4, chunks
+        tiers: Final = {chunk.get("service_tier") for chunk in chunks}
+        assert tiers == {"priority"}, f"every relayed chunk must carry the served tier: {tiers}"
+
+        row: Final = _single_spend_row(key)
+        assert row["status"] == "success", row
+        assert row["prompt_tokens"] == PROMPT_TOKENS, row
+        assert row["completion_tokens"] == COMPLETION_TOKENS, row
+        assert float(str(row["spend"])) == pytest.approx(EXPECTED_FULL_SPEND), row
+        breakdown: Final = _cost_breakdown(row)
+        assert breakdown["service_tier"] == "priority", breakdown
+        assert len(wire.drain()) == 1
+
+
+@pytest.mark.timeout(120)
+def test_gemini_chat_stream_bills_the_flex_tier(gateway: Gateway) -> None:
+    prompt: Final = f"tier control {uuid4().hex[:8]}"
+    with (
+        wire_server(_respond_gemini_for(prompt)) as wire,
+        gateway.scenario() as scenario,
+    ):
+        model: Final = _tiered_model(
+            scenario,
+            wire,
+            litellm_model="gemini/gemini-2.5-flash",
+            api_base=wire.url,
+        )
+        key: Final = scenario.key(models=[model])
+        response: Final = gateway.request(
+            "POST",
+            "/v1/chat/completions",
+            {"model": model, "messages": [{"role": "user", "content": prompt}], "stream": True},
+            key=key,
+        )
+        assert response.status_code == 200, response.text
+        chunks: Final = _events(list(response.iter_lines()))
+        assert len(chunks) >= 2, chunks
+
+        row: Final = _single_spend_row(key)
+        assert row["status"] == "success", row
+        assert row["prompt_tokens"] == PROMPT_TOKENS, row
+        assert row["completion_tokens"] == COMPLETION_TOKENS, row
+        assert float(str(row["spend"])) == pytest.approx(EXPECTED_FLEX_SPEND), row
+        breakdown: Final = _cost_breakdown(row)
+        assert breakdown["service_tier"] == "flex", breakdown
+        assert len(wire.drain()) == 1
+
+
+@pytest.mark.timeout(120)
+def test_requested_priority_downgraded_to_default_bills_base_rates(gateway: Gateway) -> None:
+    prompt: Final = f"tier control {uuid4().hex[:8]}"
+    request_id: Final = f"chatcmpl-{uuid4().hex[:8]}"
+    with (
+        wire_server(
+            _respond_for(request_id, prompt, served_tier="default", expected_requested_tier="priority")
+        ) as wire,
+        gateway.scenario() as scenario,
+    ):
+        model: Final = _tiered_model(scenario, wire, litellm_model="openai/gpt-4o-mini")
+        key: Final = scenario.key(models=[model])
+        response: Final = gateway.request(
+            "POST",
+            "/v1/chat/completions",
+            {
+                "model": model,
+                "messages": [{"role": "user", "content": prompt}],
+                "stream": True,
+                "service_tier": "priority",
+            },
+            key=key,
+        )
+        assert response.status_code == 200, response.text
+        chunks: Final = _events(list(response.iter_lines()))
+
+        assert len(chunks) == 4, chunks
+        tiers: Final = {chunk.get("service_tier") for chunk in chunks}
+        assert tiers == {"default"}, f"every relayed chunk must carry the served tier: {tiers}"
+
+        row: Final = _single_spend_row(key)
+        assert row["status"] == "success", row
+        assert row["request_id"] == request_id, row
+        assert row["prompt_tokens"] == PROMPT_TOKENS, row
+        assert row["completion_tokens"] == COMPLETION_TOKENS, row
+        assert float(str(row["spend"])) == pytest.approx(
+            PROMPT_TOKENS * INPUT_RATE + COMPLETION_TOKENS * OUTPUT_RATE
+        ), row
+        breakdown: Final = _cost_breakdown(row)
+        assert breakdown.get("service_tier") != "priority", breakdown
+        assert len(wire.drain()) == 1
+
+
+@pytest.mark.timeout(120)
+def test_requested_priority_with_auto_echo_bills_priority(gateway: Gateway) -> None:
+    prompt: Final = f"tier control {uuid4().hex[:8]}"
+    request_id: Final = f"chatcmpl-{uuid4().hex[:8]}"
+    with (
+        wire_server(_respond_for(request_id, prompt, served_tier="auto")) as wire,
+        gateway.scenario() as scenario,
+    ):
+        model: Final = _tiered_model(scenario, wire, litellm_model="openai/gpt-4o-mini")
+        key: Final = scenario.key(models=[model])
+        response: Final = gateway.request(
+            "POST",
+            "/v1/chat/completions",
+            {
+                "model": model,
+                "messages": [{"role": "user", "content": prompt}],
+                "stream": True,
+                "service_tier": "priority",
+            },
+            key=key,
+        )
+        assert response.status_code == 200, response.text
+        chunks: Final = _events(list(response.iter_lines()))
+        assert len(chunks) == 4, chunks
+
+        row: Final = _single_spend_row(key)
+        assert row["status"] == "success", row
+        assert row["request_id"] == request_id, row
+        assert float(str(row["spend"])) == pytest.approx(EXPECTED_FULL_SPEND), row
+        breakdown: Final = _cost_breakdown(row)
+        assert breakdown["service_tier"] == "priority", breakdown
+        assert len(wire.drain()) == 1
diff --git a/tests/test_litellm/proxy/proxy_server/test_streaming_helpers.py b/tests/test_litellm/proxy/proxy_server/test_streaming_helpers.py
index 86dd356e5f5..92de00a4a3f 100644
--- a/tests/test_litellm/proxy/proxy_server/test_streaming_helpers.py
+++ b/tests/test_litellm/proxy/proxy_server/test_streaming_helpers.py
@@ -2058,3 +2058,13 @@ async def test_queue_request_stream_is_untouched_while_keepalives_are_unconfigur
 
     assert not any(chunk.startswith(b": ping") for chunk in chunks)
     assert chunks[-1] == b"data: [DONE]\n\n"
+
+
+def test_fast_serialize_simple_model_response_stream_keeps_served_service_tier():
+    chunk = _simple_chunk()
+    chunk.service_tier = "priority"
+
+    result = _fast_serialize_simple_model_response_stream(chunk)
+
+    assert result is not None
+    assert json.loads(result)["service_tier"] == "priority"
diff --git a/tests/test_litellm/proxy/test_common_request_processing.py b/tests/test_litellm/proxy/test_common_request_processing.py
index c17f41a8b8f..8485c286a30 100644
--- a/tests/test_litellm/proxy/test_common_request_processing.py
+++ b/tests/test_litellm/proxy/test_common_request_processing.py
@@ -64,6 +64,7 @@ from litellm.proxy._types import ProxyErrorTypes, ProxyException
 from litellm.proxy._types import UserAPIKeyAuth as ProxyUserAPIKeyAuth
 from litellm.proxy.utils import ProxyLogging
 from litellm.router import Router
+from litellm.router_utils.add_retry_fallback_headers import prepare_response_for_header_attachment
 
 
 def test_attach_guardrail_information_copies_recorded_entries_onto_model_response():
@@ -7337,6 +7338,61 @@ class TestStreamingClientDisconnectBilling:
         assert standard_logging_object["total_tokens"] > 0
         assert standard_logging_object["response_cost"] >= 0.002
 
+    @pytest.mark.asyncio
+    async def test_disconnect_bills_partial_spend_for_anthropic_adapter_stream(self):
+        """
+        The proxy's cleanup gets the FallbackAwareAnthropicMessagesStream the
+        router returns for /v1/messages; its chunks/messages must delegate
+        through the translate_completion_output_params_streaming result to the
+        inner chat stream's collected chunks or a disconnect bills nothing.
+        """
+        from litellm.llms.anthropic.pass_through.adapters.streaming_iterator import (
+            AnthropicSSEStream,
+        )
+        from litellm.llms.anthropic.pass_through.adapters.transformation import (
+            AnthropicAdapter,
+        )
+        from litellm.router import FallbackAwareAnthropicMessagesStream
+
+        async def _sse_frames() -> AsyncGenerator[bytes, None]:
+            yield b"event: message_start\n\n"
+
+        recorder = _RecordingSuccessLogger()
+        original_callbacks = litellm.callbacks
+        litellm.callbacks = [recorder]
+        try:
+            response = await self._start_partial_stream()
+            setattr(response.chunks[-1], "service_tier", "priority")  # noqa: B010  # pydantic extra, not a declared field
+            source_iterator: Final = AnthropicAdapter().translate_completion_output_params_streaming(
+                response,
+                model=response.model or "gpt-4o-mini",
+                is_async=True,
+                litellm_logging_obj=response.logging_obj,
+            )
+            assert isinstance(source_iterator, AnthropicSSEStream)
+            streamed: Final = prepare_response_for_header_attachment(
+                FallbackAwareAnthropicMessagesStream(_sse_frames(), source_iterator)
+            )
+
+            billed: Final = await _bill_partial_streamed_spend_on_disconnect(
+                {"litellm_logging_obj": response.logging_obj},
+                streamed,
+            )
+
+            for _ in range(50):
+                if recorder.success_events:
+                    break
+                await asyncio.sleep(0.1)
+            await asyncio.sleep(0.5)
+        finally:
+            litellm.callbacks = original_callbacks
+
+        assert billed is True
+        assert len(recorder.success_events) == 1
+        partial_response: Final = recorder.success_events[0]["response_obj"]
+        assert getattr(partial_response, "service_tier") == "priority"
+        assert partial_response.usage.total_tokens > 0
+
     @pytest.mark.asyncio
     async def test_completed_stream_does_not_double_bill_on_late_disconnect(self):
         recorder = _RecordingSuccessLogger()
diff --git a/tests/unit/completion_extras/litellm_responses_transformation/test_completion_extras_litellm_responses_transformation_transformation.py b/tests/unit/completion_extras/litellm_responses_transformation/test_completion_extras_litellm_responses_transformation_transformation.py
index d0f9bad795d..282b84104a6 100644
--- a/tests/unit/completion_extras/litellm_responses_transformation/test_completion_extras_litellm_responses_transformation_transformation.py
+++ b/tests/unit/completion_extras/litellm_responses_transformation/test_completion_extras_litellm_responses_transformation_transformation.py
@@ -4352,3 +4352,39 @@ def test_map_optional_params_verbosity_merges_into_text():
         verbosity_only_request,
     )
     assert verbosity_only_request["text"] == {"verbosity": "low"}
+
+
+def test_response_completed_carries_the_served_service_tier():
+    from litellm.completion_extras.litellm_responses_transformation.transformation import (
+        OpenAiResponsesToChatCompletionStreamIterator,
+    )
+
+    iterator = OpenAiResponsesToChatCompletionStreamIterator(streaming_response=None, sync_stream=True)
+
+    result = iterator.chunk_parser(
+        {
+            "type": "response.completed",
+            "response": {"id": "resp_1", "status": "completed", "output": [], "service_tier": "default"},
+        }
+    )
+
+    assert result.model_dump()["service_tier"] == "default"
+
+
+def test_every_bridged_chunk_after_response_created_carries_the_served_service_tier():
+    from litellm.completion_extras.litellm_responses_transformation.transformation import (
+        OpenAiResponsesToChatCompletionStreamIterator,
+    )
+
+    iterator = OpenAiResponsesToChatCompletionStreamIterator(streaming_response=None, sync_stream=True)
+    events = [
+        {"type": "response.created", "response": {"id": "resp_1", "status": "in_progress", "service_tier": "default"}},
+        {"type": "response.output_item.added", "output_index": 0, "item": {"type": "message"}},
+        {"type": "response.output_text.delta", "output_index": 0, "delta": "Hi"},
+        {"type": "response.output_item.done", "output_index": 0, "item": {"type": "message"}},
+        {"type": "response.completed", "response": {"id": "resp_1", "status": "completed", "output": []}},
+    ]
+
+    relayed = [iterator.chunk_parser(event).model_dump().get("service_tier") for event in events]
+
+    assert relayed == ["default"] * len(events), relayed
diff --git a/tests/unit/litellm_core_utils/test_litellm_logging.py b/tests/unit/litellm_core_utils/test_litellm_logging.py
index ed614b93a77..60b7ed32399 100644
--- a/tests/unit/litellm_core_utils/test_litellm_logging.py
+++ b/tests/unit/litellm_core_utils/test_litellm_logging.py
@@ -8811,6 +8811,58 @@ async def test_async_failure_handler_delivers_failure_payload_to_custom_logger()
     assert events.empty()
 
 
+def test_responses_completed_event_bills_the_served_service_tier():
+    """The served service_tier on response.completed's inner ResponsesAPIResponse
+    must reach the cost calculator, so a priority-served stream prices at the
+    priority rates instead of the default tier's."""
+    logging_obj: Final = LitellmLogging(
+        model="openai/gpt-5.1",
+        messages=[{"role": "user", "content": "hi"}],
+        stream=True,
+        call_type="aresponses",
+        start_time=time.time(),
+        litellm_call_id="resp-served-tier",
+        function_id="resp-served-tier",
+    )
+    logging_obj.update_environment_variables(
+        model="openai/gpt-5.1",
+        user="",
+        optional_params={},
+        litellm_params={},
+        custom_llm_provider="openai",
+    )
+    inner: Final = ResponsesAPIResponse(
+        id="resp-served-tier",
+        created_at=1,
+        object="response",
+        status="completed",
+        model="gpt-5.1",
+        output=[],
+        usage=ResponseAPIUsage(input_tokens=10, output_tokens=20, total_tokens=30),
+        service_tier="priority",
+    )
+    event: Final = ResponseCompletedEvent(type="response.completed", response=inner)
+
+    cost: Final = logging_obj._response_cost_calculator(result=event)  # pyright: ignore[reportPrivateUsage]  # parity with the suite's own direct calls
+
+    billed_response: Final = ModelResponse(
+        model="gpt-5.1",
+        usage=litellm.Usage(prompt_tokens=10, completion_tokens=20, total_tokens=30),
+    )
+    tier_cost: Final = litellm.completion_cost(
+        completion_response=billed_response,
+        model="openai/gpt-5.1",
+        service_tier="priority",
+    )
+    default_cost: Final = litellm.completion_cost(
+        completion_response=billed_response,
+        model="openai/gpt-5.1",
+    )
+
+    assert cost == pytest.approx(tier_cost)
+    assert cost > default_cost
+
+
 def _image_logging_obj() -> LitellmLogging:
     logging_obj = LitellmLogging(
         model="gpt-image-2",
diff --git a/tests/unit/litellm_core_utils/test_streaming_chunk_builder_utils.py b/tests/unit/litellm_core_utils/test_streaming_chunk_builder_utils.py
index aaf877df364..83e53b2d80a 100644
--- a/tests/unit/litellm_core_utils/test_streaming_chunk_builder_utils.py
+++ b/tests/unit/litellm_core_utils/test_streaming_chunk_builder_utils.py
@@ -1820,3 +1820,34 @@ def test_calculate_usage_keeps_a_reported_count_over_a_later_chunks_zero() -> No
     )
 
     assert (usage.prompt_tokens, usage.completion_tokens, usage.total_tokens) == (5, 17, 22)
+
+
+def _tier_chunk(content: str, service_tier: str | None, finish_reason: str | None = None) -> ModelResponseStream:
+    return ModelResponseStream(
+        id="chatcmpl-tier",
+        created=1,
+        model="gpt-4.1-mini",
+        object="chat.completion.chunk",
+        choices=[StreamingChoices(finish_reason=finish_reason, index=0, delta=Delta(content=content, role=None))],
+        **({"service_tier": service_tier} if service_tier is not None else {}),
+    )
+
+
+def test_stream_chunk_builder_records_the_last_service_tier_the_provider_stamped():
+    chunks = [
+        _tier_chunk("Hel", "auto"),
+        _tier_chunk("lo", None),
+        _tier_chunk("", "default", finish_reason="stop"),
+    ]
+
+    response = stream_chunk_builder(chunks=chunks)
+
+    assert response is not None
+    assert response.model_dump()["service_tier"] == "default"
+
+
+def test_stream_chunk_builder_omits_service_tier_when_no_chunk_carried_one():
+    response = stream_chunk_builder(chunks=[_tier_chunk("Hi", None, finish_reason="stop")])
+
+    assert response is not None
+    assert "service_tier" not in response.model_dump()
diff --git a/tests/unit/litellm_core_utils/test_streaming_handler.py b/tests/unit/litellm_core_utils/test_streaming_handler.py
index 62d8b0e203f..d07e8822eb0 100644
--- a/tests/unit/litellm_core_utils/test_streaming_handler.py
+++ b/tests/unit/litellm_core_utils/test_streaming_handler.py
@@ -4983,3 +4983,50 @@ async def test_async_stream_without_usage_counts_tokens_off_the_event_loop():
     assert chunks[-1].usage.prompt_tokens > 100_000
     assert chunks[-1].usage.completion_tokens > 100_000
     assert_loop_stayed_free(took, lags)
+
+
+@pytest.mark.parametrize("sync_mode", [True, False])
+@pytest.mark.asyncio
+async def test_openai_stream_relays_the_served_service_tier_on_every_chunk_including_usage(
+    logging_obj: Logging, sync_mode: bool
+):
+    from litellm.utils import ModelResponseListIterator
+
+    def _chunk(content: str, finish_reason: str | None, usage: Usage | None, choices: bool = True):
+        return ModelResponseStream(
+            id="chatcmpl-tier",
+            created=1742056047,
+            model="gpt-4.1-mini",
+            choices=[StreamingChoices(finish_reason=finish_reason, index=0, delta=Delta(content=content))]
+            if choices
+            else [],
+            usage=usage,
+            service_tier="default",
+        )
+
+    logging_obj.update_environment_variables(
+        model="gpt-4.1-mini",
+        optional_params={"stream_options": {"include_usage": True}},
+        litellm_params={},
+        custom_llm_provider="openai",
+    )
+    wrapper = CustomStreamWrapper(
+        completion_stream=ModelResponseListIterator(
+            model_responses=[
+                _chunk("Hi", None, None),
+                _chunk("", "stop", None),
+                _chunk("", None, Usage(prompt_tokens=10, completion_tokens=1, total_tokens=11), choices=False),
+            ]
+        ),
+        model="gpt-4.1-mini",
+        custom_llm_provider="openai",
+        logging_obj=logging_obj,
+        stream_options={"include_usage": True},
+    )
+
+    relayed = (
+        [chunk.model_dump() for chunk in wrapper] if sync_mode else [chunk.model_dump() async for chunk in wrapper]
+    )
+
+    assert [chunk.get("service_tier") for chunk in relayed] == ["default"] * len(relayed), relayed
+    assert relayed[-1]["usage"]["total_tokens"] == 11
diff --git a/tests/unit/llms/anthropic/pass_through/adapters/test_streaming_iterator_sse_stream.py b/tests/unit/llms/anthropic/pass_through/adapters/test_streaming_iterator_sse_stream.py
new file mode 100644
index 00000000000..fbbbc579d94
--- /dev/null
+++ b/tests/unit/llms/anthropic/pass_through/adapters/test_streaming_iterator_sse_stream.py
@@ -0,0 +1,87 @@
+"""
+Tests for AnthropicSSEStream, the object translate_completion_output_params_streaming
+hands to the proxy for /v1/messages streaming. It must emit the same SSE bytes as
+the wrapper's async_anthropic_sse_wrapper, propagate aclose into it, and expose the
+wrapper's chunks/messages/model so disconnect-time partial billing can read them.
+"""
+
+from typing import Final
+from unittest.mock import MagicMock
+
+import pytest
+
+from litellm.llms.anthropic.pass_through.adapters.streaming_iterator import (
+    AnthropicSSEStream,
+    AnthropicStreamWrapper,
+)
+from litellm.types.utils import Delta, StreamingChoices
+
+
+def _make_chunk(delta: Delta, finish_reason: str | None = None) -> MagicMock:
+    chunk = MagicMock()
+    chunk.choices = [StreamingChoices(finish_reason=finish_reason, index=0, delta=delta, logprobs=None)]
+    chunk.usage = None
+    chunk._hidden_params = {}
+    return chunk
+
+
+class _AsyncStream:
+    def __init__(self, items: list[MagicMock]):
+        self._it = iter(items)
+        self.chunks = list(items)
+        self.messages: list[dict] = [{"role": "user", "content": "hi"}]
+
+    def __aiter__(self):
+        return self
+
+    async def __anext__(self):
+        try:
+            return next(self._it)
+        except StopIteration:
+            raise StopAsyncIteration
+
+
+def _streamed_events() -> AnthropicSSEStream:
+    upstream: Final = _AsyncStream(
+        [
+            _make_chunk(Delta(content="Once")),
+            _make_chunk(Delta(content=" upon"), finish_reason="stop"),
+        ]
+    )
+    wrapper: Final = AnthropicStreamWrapper(completion_stream=upstream, model="gpt-4o-mini")
+    wrapper._message_id = "msg_test"
+    return AnthropicSSEStream(wrapper)
+
+
+@pytest.mark.asyncio
+async def test_sse_stream_yields_identical_bytes_to_the_wrappers_sse_wrapper():
+    upstream_a: Final = _AsyncStream(
+        [_make_chunk(Delta(content="Once")), _make_chunk(Delta(content=" upon"), finish_reason="stop")]
+    )
+    wrapper_a: Final = AnthropicStreamWrapper(completion_stream=upstream_a, model="gpt-4o-mini")
+    wrapper_a._message_id = "msg_test"
+    expected: Final = [event async for event in wrapper_a.async_anthropic_sse_wrapper()]
+
+    actual: Final = [event async for event in _streamed_events()]
+
+    assert actual == expected
+
+
+@pytest.mark.asyncio
+async def test_sse_stream_aclose_ends_the_wrapped_stream():
+    stream: Final = _streamed_events()
+
+    first: Final = await stream.__anext__()
+    assert first.startswith(b"event: message_start")
+    await stream.aclose()
+    with pytest.raises(StopAsyncIteration):
+        await stream.__anext__()
+
+
+def test_sse_stream_exposes_chunks_messages_and_model():
+    stream: Final = _streamed_events()
+
+    assert stream.model == "gpt-4o-mini"
+    assert stream.messages == [{"role": "user", "content": "hi"}]
+    chunks: Final = stream.chunks
+    assert isinstance(chunks, list) and len(chunks) == 2
diff --git a/tests/unit/llms/anthropic/pass_through/messages/test_response_cache.py b/tests/unit/llms/anthropic/pass_through/messages/test_response_cache.py
index e55e73ed43f..a8a8eba0bf7 100644
--- a/tests/unit/llms/anthropic/pass_through/messages/test_response_cache.py
+++ b/tests/unit/llms/anthropic/pass_through/messages/test_response_cache.py
@@ -10,6 +10,7 @@ import litellm
 from litellm._internal_context import in_post_response_phase
 from litellm.caching.caching import Cache, LiteLLMCacheType
 from litellm.caching.caching_handler import LLMCachingHandler
+from litellm.litellm_core_utils.logging_worker import GLOBAL_LOGGING_WORKER
 from litellm.llms.anthropic.pass_through.messages import handler
 from litellm.llms.anthropic.pass_through.messages.response_cache import (
     AnthropicMessagesStreamCacheWriter,
@@ -63,6 +64,12 @@ async def _collect(stream: AsyncIterator[bytes]) -> List[bytes]:
     return [chunk async for chunk in stream]
 
 
+@pytest.fixture(autouse=True)
+async def _drain_logging_worker():
+    yield
+    await GLOBAL_LOGGING_WORKER.flush()
+
+
 @pytest.fixture
 def local_cache():
     previous_cache = litellm.cache
@@ -282,6 +289,40 @@ class _HeldBackStream:
         raise StopAsyncIteration
 
 
+class _AttributedStream:
+    """Stream stub carrying the billing attributes the disconnect helper reads."""
+
+    def __init__(self, chunks: list) -> None:
+        self.chunks = [object()]
+        self.messages = [{"role": "user", "content": "hi"}]
+        self.model = "gpt-4o-mini"
+        self._pending = list(chunks)
+
+    def __aiter__(self) -> "_AttributedStream":
+        return self
+
+    async def __anext__(self) -> bytes:
+        if not self._pending:
+            raise StopAsyncIteration
+        return self._pending.pop(0)
+
+
+@pytest.mark.asyncio
+async def test_cache_writer_exposes_inner_stream_billing_attributes(request_kwargs):
+    caching_handler = LLMCachingHandler(
+        original_function=handler.anthropic_messages,
+        request_kwargs=dict(request_kwargs),
+        start_time=datetime.datetime.now(),
+    )
+    inner = _AttributedStream(STREAM_EVENTS)
+    writer = AnthropicMessagesStreamCacheWriter(stream=inner, caching_handler=caching_handler)
+
+    assert writer.chunks is inner.chunks
+    assert writer.messages is inner.messages
+    assert writer.model == "gpt-4o-mini"
+    assert await _collect(writer) == STREAM_EVENTS
+
+
 @pytest.mark.asyncio
 async def test_stream_cache_write_runs_in_post_response_phase(request_kwargs, monkeypatch):
     """Every event, message_stop included, is already with the client when the stream write
diff --git a/tests/unit/llms/databricks/chat/test_databricks_chat_transformation.py b/tests/unit/llms/databricks/chat/test_databricks_chat_transformation.py
index 9cd17bd3580..1cbc9eeb897 100644
--- a/tests/unit/llms/databricks/chat/test_databricks_chat_transformation.py
+++ b/tests/unit/llms/databricks/chat/test_databricks_chat_transformation.py
@@ -883,3 +883,13 @@ def test_completion_merges_system_messages_when_one_has_empty_content(respx_mock
         {"role": "system", "content": "You are terse."},
         {"role": "user", "content": "Hello"},
     ]
+
+
+def test_chunk_parser_relays_the_served_service_tier():
+    iterator = DatabricksChatResponseIterator(streaming_response=None, sync_stream=True)
+
+    with_tier: Final = iterator.chunk_parser({**_streaming_chunk(), "service_tier": "priority"})
+    assert with_tier.model_dump()["service_tier"] == "priority"
+
+    without_tier: Final = iterator.chunk_parser(_streaming_chunk())
+    assert getattr(without_tier, "service_tier", None) is None
diff --git a/tests/unit/llms/databricks/test_databricks_cost_calculator.py b/tests/unit/llms/databricks/test_databricks_cost_calculator.py
index 494b99c1d11..7120a130462 100644
--- a/tests/unit/llms/databricks/test_databricks_cost_calculator.py
+++ b/tests/unit/llms/databricks/test_databricks_cost_calculator.py
@@ -156,8 +156,6 @@ def test_uncached_request_bills_every_prompt_token_at_the_input_rate(local_model
     assert completion_cost == pytest.approx(200 * info["output_cost_per_token"])
 
 
-
-
 @pytest.mark.parametrize("model", NEW_MODELS)
 def test_new_models_carry_cache_pricing(local_model_cost_map: None, model: str) -> None:
     info: Final = _model_info(model)
@@ -232,3 +230,28 @@ def test_sonnet_5_ships_standard_rates_not_introductory(local_model_cost_map: No
 
     for field in PRICE_FIELDS:
         assert sonnet_5[field] == pytest.approx(sonnet_4_6[field]), field
+
+
+def test_cost_per_token_bills_the_served_priority_tier(
+    local_model_cost_map: None, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    rates: Final = {
+        "input_cost_per_token": 0.001,
+        "output_cost_per_token": 0.002,
+        "input_cost_per_token_priority": 0.01,
+        "output_cost_per_token_priority": 0.02,
+        "litellm_provider": "databricks",
+        "mode": "chat",
+    }
+    monkeypatch.setitem(litellm.model_cost, "databricks/dbrx-tiered-test", rates)
+    usage: Final = Usage(prompt_tokens=30, completion_tokens=40, total_tokens=70)
+
+    prompt_cost, completion_cost = cost_per_token(
+        model="databricks/dbrx-tiered-test", usage=usage, service_tier="priority"
+    )
+    assert prompt_cost == pytest.approx(30 * 0.01)
+    assert completion_cost == pytest.approx(40 * 0.02)
+
+    prompt_cost, completion_cost = cost_per_token(model="databricks/dbrx-tiered-test", usage=usage)
+    assert prompt_cost == pytest.approx(30 * 0.001)
+    assert completion_cost == pytest.approx(40 * 0.002)
diff --git a/tests/unit/llms/openai/chat/test_openai_gpt_transformation.py b/tests/unit/llms/openai/chat/test_openai_gpt_transformation.py
index 53c5b9d7cbc..85a04778e5c 100644
--- a/tests/unit/llms/openai/chat/test_openai_gpt_transformation.py
+++ b/tests/unit/llms/openai/chat/test_openai_gpt_transformation.py
@@ -248,6 +248,33 @@ class TestOpenAIChatCompletionStreamingHandler:
         assert result.usage.completion_tokens == 350
         assert result.usage.total_tokens == 14147
 
+    def test_chunk_parser_preserves_service_tier(self):
+        """OpenAI-compatible upstreams serve a service_tier on every streamed
+        chunk; chunk_parser must keep it on the emitted ModelResponseStream so
+        disconnect billing and the reassembled response see the served tier."""
+        handler = OpenAIChatCompletionStreamingHandler(
+            streaming_response=None, sync_stream=True
+        )
+
+        tiered_chunk = {
+            "id": "gen-123",
+            "created": 1234567890,
+            "model": "openai/gpt-4o-mini",
+            "object": "chat.completion.chunk",
+            "choices": [
+                {
+                    "index": 0,
+                    "delta": {"role": "assistant", "content": ""},
+                    "finish_reason": None,
+                }
+            ],
+            "service_tier": "priority",
+        }
+        plain_chunk = {key: value for key, value in tiered_chunk.items() if key != "service_tier"}
+
+        assert handler.chunk_parser(tiered_chunk).model_dump().get("service_tier") == "priority"
+        assert handler.chunk_parser(plain_chunk).model_dump().get("service_tier") is None
+
     def test_chunk_parser_raises_on_in_body_error_payload(self):
         """vLLM/sglang return HTTP 200 streams whose body carries the error,
         e.g. data: {"error": {..., "code": 400}}. chunk_parser must surface it
diff --git a/tests/unit/test_cost_calculator.py b/tests/unit/test_cost_calculator.py
index c1939a23e74..2fe09e6de98 100644
--- a/tests/unit/test_cost_calculator.py
+++ b/tests/unit/test_cost_calculator.py
@@ -1948,7 +1948,7 @@ def test_completion_cost_extracts_service_tier_from_usage(_local_model_cost_map)
 
 
 def test_completion_cost_service_tier_priority(_local_model_cost_map):
-    """Test that service_tier extraction follows priority: optional_params > completion_response > usage."""
+    """Test that the served tier wins over the requested tier: response > usage > request."""
     from litellm import completion_cost
 
     # Test with gpt-5-nano which has flex pricing
@@ -1965,7 +1965,7 @@ def test_completion_cost_service_tier_priority(_local_model_cost_map):
     )
     setattr(response, "service_tier", "priority")
 
-    # Test that optional_params takes priority over response and usage
+    # A request-level tier loses to the tier the response actually served
     cost_from_params = completion_cost(
         completion_response=response,
         model=model,
@@ -1973,20 +1973,18 @@ def test_completion_cost_service_tier_priority(_local_model_cost_map):
         optional_params={"service_tier": "flex"},
     )
 
-    # Test that response takes priority over usage when optional_params is not provided
-    completion_cost(
+    # Response takes priority over usage
+    cost_served_priority = completion_cost(
         completion_response=response,
         model=model,
         custom_llm_provider="openai",
     )
 
-    # Test that usage is used when neither optional_params nor response have service_tier
-    # Create a new response without service_tier attribute
+    # Create a new response without service_tier attribute so it falls back to usage
     response_no_tier = ModelResponse(
         usage=usage,
         model=model,
     )
-    # Don't set service_tier on response, so it will fall back to usage
 
     cost_from_usage = completion_cost(
         completion_response=response_no_tier,
@@ -1994,12 +1992,13 @@ def test_completion_cost_service_tier_priority(_local_model_cost_map):
         custom_llm_provider="openai",
     )
 
-    # All should use flex pricing (from different sources)
     assert cost_from_params > 0, "Cost from params should be greater than 0"
     assert cost_from_usage > 0, "Cost from usage should be greater than 0"
 
-    # Costs should be similar (all using flex)
-    assert abs(cost_from_params - cost_from_usage) < 1e-6, "Costs from params and usage should be similar (both flex)"
+    # Requested flex is ignored once the response reports served priority
+    assert cost_from_params == pytest.approx(cost_served_priority), (
+        "request-level service_tier must defer to the served tier on the response"
+    )
 
 
 def test_completion_cost_service_tier_for_bedrock(_local_model_cost_map):
@@ -5468,3 +5467,100 @@ def test_completion_cost_is_zero_when_explicit_rates_are_zero(monkeypatch: pytes
     )
 
     assert cost == 0.0
+
+
+@pytest.mark.parametrize(
+    ("requested", "served", "expected"),
+    [
+        (None, "priority", "priority"),
+        ("priority", "flex", "flex"),
+        ("priority", "default", None),
+        ("priority", "standard", None),
+        ("priority", "auto", "priority"),
+        ("priority", "scale", "priority"),
+        ("priority", None, "priority"),
+        ("auto", None, None),
+        (None, "Priority", "priority"),
+        ("flex", "on_demand", "flex"),
+    ],
+)
+def test_resolve_billable_service_tier(requested: object, served: object, expected: str | None) -> None:
+    from litellm.cost_calculator import _resolve_billable_service_tier
+
+    assert _resolve_billable_service_tier(requested=requested, served=served) == expected
+
+
+def _served_tier_cost_model(monkeypatch: pytest.MonkeyPatch) -> str:
+    model: Final = "served-tier-cost-model"
+    monkeypatch.setitem(
+        litellm.model_cost,
+        model,
+        {
+            "input_cost_per_token": 0.001,
+            "output_cost_per_token": 0.002,
+            "input_cost_per_token_priority": 0.01,
+            "output_cost_per_token_priority": 0.02,
+            "litellm_provider": "openai",
+            "mode": "chat",
+        },
+    )
+    return model
+
+
+def test_completion_cost_bills_base_when_served_default_overrides_requested_priority(
+    _local_model_cost_map: None, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    model: Final = _served_tier_cost_model(monkeypatch)
+    response: Final = ModelResponse(
+        model=model,
+        usage=Usage(prompt_tokens=100, completion_tokens=50, total_tokens=150),
+    )
+    setattr(response, "service_tier", "default")
+
+    cost: Final = completion_cost(
+        completion_response=response,
+        model=model,
+        custom_llm_provider="openai",
+        optional_params={"service_tier": "priority"},
+    )
+
+    assert cost == pytest.approx(100 * 0.001 + 50 * 0.002)
+
+
+def test_completion_cost_bills_priority_when_served_tier_overrides_missing_request(
+    _local_model_cost_map: None, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    model: Final = _served_tier_cost_model(monkeypatch)
+    response: Final = ModelResponse(
+        model=model,
+        usage=Usage(prompt_tokens=100, completion_tokens=50, total_tokens=150),
+    )
+    setattr(response, "service_tier", "priority")
+
+    cost: Final = completion_cost(
+        completion_response=response,
+        model=model,
+        custom_llm_provider="openai",
+    )
+
+    assert cost == pytest.approx(100 * 0.01 + 50 * 0.02)
+
+
+def test_completion_cost_bills_base_when_gemini_serves_on_demand(
+    _local_model_cost_map: None, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    model: Final = _served_tier_cost_model(monkeypatch)
+    response: Final = ModelResponse(
+        model=model,
+        usage=Usage(prompt_tokens=100, completion_tokens=50, total_tokens=150),
+    )
+    response._hidden_params["provider_specific_fields"] = {"traffic_type": "ON_DEMAND"}
+
+    cost: Final = completion_cost(
+        completion_response=response,
+        model=model,
+        custom_llm_provider="openai",
+        optional_params={"service_tier": "priority"},
+    )
+
+    assert cost == pytest.approx(100 * 0.001 + 50 * 0.002)