diff --git a/litellm/litellm_core_utils/litellm_logging.py b/litellm/litellm_core_utils/litellm_logging.py
index a43574b1a04..ad7bdcb91ed 100644
--- a/litellm/litellm_core_utils/litellm_logging.py
+++ b/litellm/litellm_core_utils/litellm_logging.py
@@ -6274,7 +6274,7 @@ class StandardLoggingPayloadSetup:
             return None
         user_agent_tags: list[str] | None = None
         headers: Final = proxy_server_request.get("headers", {})
-        if headers is not None and isinstance(headers, dict):
+        if headers is not None and isinstance(headers, Mapping):
             if "user-agent" in headers:
                 user_agent: Final = headers["user-agent"]
                 if user_agent is not None:
@@ -6299,7 +6299,7 @@ class StandardLoggingPayloadSetup:
             return None
 
         headers: Final = proxy_server_request.get("headers", {})
-        if not isinstance(headers, dict):
+        if not isinstance(headers, Mapping):
             return None
 
         header_tags: Final = []
diff --git a/tests/integration/spend/test_passthrough_spend_tags.py b/tests/integration/spend/test_passthrough_spend_tags.py
new file mode 100644
index 00000000000..9d89a866cc8
--- /dev/null
+++ b/tests/integration/spend/test_passthrough_spend_tags.py
@@ -0,0 +1,76 @@
+import json
+import uuid
+from hashlib import sha256
+from pathlib import Path
+from typing import Final
+
+import pytest
+import yaml
+from integration._support.client import Gateway, eventually
+from integration._support.database import read_rows
+from integration._support.process import owned_proxy
+from integration._support.wire import Reply, Request, wire_server
+
+MODEL: Final = "claude-sonnet-4-5-20250929"
+SENT_HEADERS: Final = {"user-agent": "claude-cli/2.0.0", "x-tenant-id": "tenant-a"}
+EXPECTED_TAGS: Final = ["User-Agent: claude-cli", "User-Agent: claude-cli/2.0.0", "x-tenant-id: tenant-a"]
+
+
+def _respond(request: Request) -> Reply:
+    assert request.method == "POST" and request.target == "/v1/messages", request.target
+    return Reply(
+        body=json.dumps(
+            {
+                "id": f"msg_{uuid.uuid4().hex}",
+                "type": "message",
+                "role": "assistant",
+                "model": MODEL,
+                "content": [{"type": "text", "text": "tagged"}],
+                "stop_reason": "end_turn",
+                "stop_sequence": None,
+                "usage": {"input_tokens": 10, "output_tokens": 2},
+            }
+        ).encode()
+    )
+
+
+def _request_tags(key: str) -> list[list[str]]:
+    rows: Final = read_rows(
+        'SELECT request_tags FROM "LiteLLM_SpendLogs" WHERE api_key=%s', (sha256(key.encode()).hexdigest(),)
+    )
+    return [
+        json.loads(row["request_tags"]) if isinstance(row["request_tags"], str) else row["request_tags"] for row in rows
+    ]
+
+
+@pytest.mark.parametrize(
+    "route", [pytest.param("/anthropic/v1/messages", id="passthrough"), pytest.param("/v1/messages", id="unified")]
+)
+def test_header_derived_spend_tags_are_recorded_on_anthropic_messages_routes(
+    gateway: Gateway, tmp_path: Path, route: str
+) -> None:
+    with wire_server(_respond) as wire:
+        config: Final = yaml.safe_load(Path("tests/integration/proxy_config.yaml").read_text())
+        config["litellm_settings"]["extra_spend_tag_headers"] = ["x-tenant-id"]
+        path: Final = tmp_path / "spend-tag-headers.yaml"
+        path.write_text(yaml.safe_dump(config))
+        environment: Final = {"ANTHROPIC_API_BASE": wire.url, "ANTHROPIC_API_KEY": "synthetic-anthropic-key"}
+        with owned_proxy(gateway, tmp_path, environment, config=path) as candidate, candidate.scenario() as scenario:
+            model: Final = scenario.model(
+                model=f"anthropic/{MODEL}", api_base=wire.url, api_key="synthetic-anthropic-key"
+            )
+            key: Final = scenario.key()
+            response: Final = candidate.request(
+                "POST",
+                route,
+                {
+                    "model": MODEL if route == "/anthropic/v1/messages" else model,
+                    "max_tokens": 16,
+                    "messages": [{"role": "user", "content": "tag me"}],
+                },
+                key=key,
+                headers={**SENT_HEADERS, "anthropic-version": "2023-06-01"},
+            )
+            assert response.status_code == 200, response.text
+            assert len(wire.drain()) == 1
+            assert eventually(lambda: _request_tags(key), lambda tags: len(tags) == 1, seconds=70) == [EXPECTED_TAGS]
diff --git a/tests/unit/litellm_core_utils/test_litellm_logging.py b/tests/unit/litellm_core_utils/test_litellm_logging.py
index e07ffe00d4c..494e82d22e7 100644
--- a/tests/unit/litellm_core_utils/test_litellm_logging.py
+++ b/tests/unit/litellm_core_utils/test_litellm_logging.py
@@ -2959,6 +2959,30 @@ def test_get_extra_header_tags():
                 delattr(litellm, "extra_spend_tag_headers")
 
 
+def test_get_request_tags_reads_header_tags_from_starlette_headers():
+    from starlette.datastructures import Headers
+
+    import litellm
+    from litellm.litellm_core_utils.litellm_logging import StandardLoggingPayloadSetup
+
+    original_extra_headers = getattr(litellm, "extra_spend_tag_headers", None)
+    original_disable_user_agent = litellm.disable_add_user_agent_to_request_tags
+    try:
+        litellm.extra_spend_tag_headers = ["x-tenant-id"]
+        litellm.disable_add_user_agent_to_request_tags = False
+        proxy_server_request = {"headers": Headers({"user-agent": "claude-cli/2.0.0", "x-tenant-id": "tenant-a"})}
+
+        assert StandardLoggingPayloadSetup._get_request_tags(
+            litellm_params={}, proxy_server_request=proxy_server_request
+        ) == ["User-Agent: claude-cli", "User-Agent: claude-cli/2.0.0", "x-tenant-id: tenant-a"]
+    finally:
+        if original_extra_headers is not None:
+            litellm.extra_spend_tag_headers = original_extra_headers
+        elif hasattr(litellm, "extra_spend_tag_headers"):
+            delattr(litellm, "extra_spend_tag_headers")
+        litellm.disable_add_user_agent_to_request_tags = original_disable_user_agent
+
+
 def test_response_cost_calculator_with_response_cost_in_hidden_params(logging_obj):
     from litellm import Router