diff --git a/litellm/proxy/common_utils/http_parsing_utils.py b/litellm/proxy/common_utils/http_parsing_utils.py
index ea33bc763d4..f1e409781ef 100644
--- a/litellm/proxy/common_utils/http_parsing_utils.py
+++ b/litellm/proxy/common_utils/http_parsing_utils.py
@@ -191,6 +191,23 @@ def is_otlp_trace_request(request: HTTPConnection) -> bool:
     return request.scope.get("method") == "POST" and get_route_path(request.scope) in {"/v1/traces", "/v1/logs"}
 
 
+def _parse_form_metadata(raw_metadata: str) -> object:
+    """The JSON body path rejects a lone surrogate escape (``"\\ud800"``) with a
+    400 because no provider or database can encode it. ``json.loads`` accepts
+    the escape, so the multipart ``metadata`` field gets the same check."""
+    parsed: Final[object] = json.loads(raw_metadata)
+    try:
+        json.dumps(parsed, ensure_ascii=False).encode("utf-8")
+    except UnicodeEncodeError as e:
+        raise ProxyException(
+            message=f"Invalid form payload: metadata {e}",
+            type="invalid_request_error",
+            param="metadata",
+            code=status.HTTP_400_BAD_REQUEST,
+        ) from e
+    return parsed
+
+
 async def read_request_body(request: Request | None) -> dict:
     """
     Safely read the request body and parse it as JSON.
@@ -238,9 +255,13 @@ async def read_request_body(request: Request | None) -> dict:
                     param="request_body",
                     code=status.HTTP_400_BAD_REQUEST,
                 )
-            parsed_body = dict(form_data)
-            if "metadata" in parsed_body and isinstance(parsed_body["metadata"], str):
-                parsed_body["metadata"] = json.loads(parsed_body["metadata"])
+            form_body: Final = dict(form_data)
+            raw_metadata: Final = form_body.get("metadata")
+            parsed_body = (
+                {**form_body, "metadata": _parse_form_metadata(raw_metadata)}
+                if isinstance(raw_metadata, str)
+                else form_body
+            )
         else:
             # Read the request body
             body: Final = await request.body()
diff --git a/litellm/proxy/litellm_pre_call_utils.py b/litellm/proxy/litellm_pre_call_utils.py
index 1f0ec0a405a..4c7cae223d0 100644
--- a/litellm/proxy/litellm_pre_call_utils.py
+++ b/litellm/proxy/litellm_pre_call_utils.py
@@ -407,8 +407,9 @@ _ALLOW_CLIENT_MESSAGE_REDACTION_OPT_OUT_METADATA_KEY: Final = "allow_client_mess
 # ``litellm.model_cost`` map. Both effects belong to deployment configuration,
 # not to user-supplied request bodies, so the proxy strips them before they
 # reach the call path. Built from the Pydantic model so newly-added pricing
-# fields are covered automatically.
-_CLIENT_PRICING_CONTROL_FIELDS: Final = frozenset(CustomPricingLiteLLMParams.model_fields.keys())
+# fields are covered automatically. ``base_model`` selects which cost-map entry
+# prices the row, so a caller naming a cheaper model lowers their own spend.
+_CLIENT_PRICING_CONTROL_FIELDS: Final = frozenset(CustomPricingLiteLLMParams.model_fields.keys()) | {"base_model"}
 # ``model_info`` carries the same pricing fields when read by
 # ``use_custom_pricing_for_model``; strip from metadata for the same reason.
 # ``standard_logging_guardrail_information`` is proxy-written telemetry summed
diff --git a/litellm/proxy/utils.py b/litellm/proxy/utils.py
index b056ac6690a..e0f8ecf83a6 100644
--- a/litellm/proxy/utils.py
+++ b/litellm/proxy/utils.py
@@ -8525,7 +8525,7 @@ async def _create_spend_logs_with_poison_isolation(
         await repo.table.create_many(data=rows, skip_duplicates=True)
         return failure_budget
     except Exception as e:
-        if not PrismaDBExceptionHandler.is_prisma_data_error(e):
+        if not (PrismaDBExceptionHandler.is_prisma_data_error(e) or isinstance(e, UnicodeEncodeError)):
             raise
         if PrismaDBExceptionHandler.is_database_service_unavailable_error(e):
             raise
diff --git a/tests/unit/proxy/common_utils/test_http_parsing_utils.py b/tests/unit/proxy/common_utils/test_http_parsing_utils.py
index 88ae5e0d84e..cd5820fa2fd 100644
--- a/tests/unit/proxy/common_utils/test_http_parsing_utils.py
+++ b/tests/unit/proxy/common_utils/test_http_parsing_utils.py
@@ -1556,3 +1556,39 @@ def test_is_otlp_trace_request_matches_only_http_posts_to_the_otlp_routes(
     )
 
     assert http_parsing_utils.is_otlp_trace_request(connection) is expected
+
+
+@pytest.mark.asyncio
+async def test_form_data_metadata_with_lone_surrogate_escape_is_rejected_like_a_json_body():
+    """``json.loads`` accepts ``"\\ud800"`` and yields a lone surrogate that no
+    provider or database can utf-8 encode. The JSON body path already answers
+    400, so the multipart ``metadata`` field must too, instead of letting the
+    value through to the spend log writers."""
+    mock_request = MagicMock()
+    mock_request.form = AsyncMock(
+        return_value=FormData({"model": "whisper-1", "metadata": '{"tags": ["poison-\\ud800-tag"]}'})
+    )
+    mock_request.headers = {"content-type": "multipart/form-data"}
+    mock_request.scope = {}
+    mock_request.state._cached_headers = None
+
+    with pytest.raises(ProxyException) as exc_info:
+        await read_request_body(mock_request)
+
+    assert exc_info.value.code == "400"
+    assert "metadata" in exc_info.value.message
+
+
+@pytest.mark.asyncio
+async def test_form_data_metadata_keeps_non_ascii_and_valid_surrogate_pairs():
+    mock_request = MagicMock()
+    mock_request.form = AsyncMock(
+        return_value=FormData({"model": "whisper-1", "metadata": '{"tags": ["caf\\u00e9", "\\ud83d\\ude00"]}'})
+    )
+    mock_request.headers = {"content-type": "multipart/form-data"}
+    mock_request.scope = {}
+    mock_request.state._cached_headers = None
+
+    result = await read_request_body(mock_request)
+
+    assert result["metadata"]["tags"] == ["café", "😀"]
diff --git a/tests/unit/proxy/test_pricing_field_strip.py b/tests/unit/proxy/test_pricing_field_strip.py
index 395fdc3a16e..3e9bee9217d 100644
--- a/tests/unit/proxy/test_pricing_field_strip.py
+++ b/tests/unit/proxy/test_pricing_field_strip.py
@@ -57,7 +57,7 @@ class TestStripClientPricingOverrides:
         # The strip set is built from the model so additions are picked up
         # automatically — this test guards against the model and the strip
         # set drifting apart if someone replaces the auto-derivation later.
-        assert _CLIENT_PRICING_CONTROL_FIELDS == frozenset(CustomPricingLiteLLMParams.model_fields.keys())
+        assert _CLIENT_PRICING_CONTROL_FIELDS >= frozenset(CustomPricingLiteLLMParams.model_fields.keys())
         # Sanity: the obvious top-level pricing fields are in the set.
         for field in (
             "input_cost_per_token",
@@ -82,6 +82,22 @@ class TestStripClientPricingOverrides:
             "messages": [{"role": "user", "content": "hi"}],
         }
 
+    def test_root_base_model_dropped(self):
+        # ``base_model`` picks the cost-map entry a spend row is priced with
+        # (success rows via the standard logging payload, billed failure rows
+        # via the failure lift), so a caller naming a cheaper model lowers
+        # their own recorded spend.
+        data = {
+            "model": "gpt-4o",
+            "messages": [{"role": "user", "content": "hi"}],
+            "base_model": "gpt-4o-mini",
+        }
+        _strip_client_pricing_overrides(data)
+        assert data == {
+            "model": "gpt-4o",
+            "messages": [{"role": "user", "content": "hi"}],
+        }
+
     def test_metadata_model_info_dropped(self):
         data = {
             "model": "gpt-4",
@@ -280,6 +296,7 @@ async def test_add_litellm_data_to_request_skips_strip_with_key_opt_in():
         "model": "gpt-4",
         "messages": [{"role": "user", "content": "hi"}],
         "input_cost_per_token": 0.0001,
+        "base_model": "gpt-4o-mini",
         "metadata": {"model_info": {"output_cost_per_token": 0.0002}},
     }
 
@@ -295,6 +312,7 @@ async def test_add_litellm_data_to_request_skips_strip_with_key_opt_in():
 
     assert updated["input_cost_per_token"] == 0.0001
     assert updated["metadata"]["model_info"] == {"output_cost_per_token": 0.0002}
+    assert updated["base_model"] == "gpt-4o-mini"
 
 
 @pytest.mark.asyncio
diff --git a/tests/unit/proxy/test_update_spend.py b/tests/unit/proxy/test_update_spend.py
index 62adb7303e2..571341e85e7 100644
--- a/tests/unit/proxy/test_update_spend.py
+++ b/tests/unit/proxy/test_update_spend.py
@@ -323,3 +323,31 @@ async def test_update_spend_logs_multiple_batches_with_failure():
 
     # Verify all logs were cleared from transactions
     assert len(prisma_client.spend_log_transactions) == 0
+
+
+@pytest.mark.asyncio
+async def test_update_spend_logs_drops_row_prisma_cannot_encode_and_keeps_the_rest():
+    """A row the Prisma engine cannot utf-8 encode (a lone surrogate in a string
+    column) is permanent, so it must be dropped like any other poison row instead
+    of being requeued at the head of the queue where it would block every tenant."""
+    prisma_client = MockPrismaClient()
+    proxy_logging_obj = create_mock_proxy_logging()
+    prisma_client.spend_log_transactions = [
+        {"request_id": "good-1", "request_tags": '["a"]'},
+        {"request_id": "poison", "request_tags": "\ud800"},
+        {"request_id": "good-2", "request_tags": '["b"]'},
+    ]
+    written = []
+
+    async def create_many(data, skip_duplicates):
+        if any("\ud800" in row["request_tags"] for row in data):
+            raise UnicodeEncodeError("utf-8", "\ud800", 0, 1, "surrogates not allowed")
+        written.extend(row["request_id"] for row in data)
+
+    prisma_client.db.litellm_spendlogs.create_many = AsyncMock(side_effect=create_many)
+
+    await update_spend(prisma_client, None, proxy_logging_obj)
+
+    assert sorted(written) == ["good-1", "good-2"]
+    assert prisma_client.spend_log_transactions == []
+    proxy_logging_obj.failure_handler.assert_not_called()