diff --git a/litellm/llms/chatgpt/common_utils.py b/litellm/llms/chatgpt/common_utils.py
index 3946ce53975..661de929552 100644
--- a/litellm/llms/chatgpt/common_utils.py
+++ b/litellm/llms/chatgpt/common_utils.py
@@ -5,13 +5,14 @@ Constants and helpers for ChatGPT subscription OAuth.
 import hashlib
 import os
 import platform
-from typing import Any, Final
+from typing import Final
 from uuid import uuid4
 
 import httpx
 
 from litellm.constants import SESSION_ID_GENERATED_METADATA_KEY
 from litellm.llms.base_llm.chat.transformation import BaseLLMException
+from litellm.types.router import GenericLiteLLMParams
 
 # OAuth + API constants (derived from openai/codex)
 CHATGPT_AUTH_BASE: Final = "https://auth.openai.com"
@@ -252,25 +253,18 @@ def get_chatgpt_default_instructions() -> str:
     return os.getenv("CHATGPT_DEFAULT_INSTRUCTIONS") or CHATGPT_DEFAULT_INSTRUCTIONS
 
 
-def _normalize_litellm_params(litellm_params: Any | None) -> dict[str, object]:
+def _normalize_litellm_params(litellm_params: dict[str, object] | GenericLiteLLMParams | None) -> dict[str, object]:
     if litellm_params is None:
         return {}
     if isinstance(litellm_params, dict):
         return litellm_params
-    if hasattr(litellm_params, "model_dump"):
-        try:
-            return litellm_params.model_dump()
-        except Exception:
-            return {}
-    if hasattr(litellm_params, "dict"):
-        try:
-            return litellm_params.dict()
-        except Exception:
-            return {}
-    return {}
+    try:
+        return litellm_params.model_dump()
+    except Exception:
+        return {}
 
 
-def get_chatgpt_session_id(litellm_params: object) -> str | None:
+def get_chatgpt_session_id(litellm_params: dict[str, object] | GenericLiteLLMParams | None) -> str | None:
     params: Final = _normalize_litellm_params(litellm_params)
     metadata: Final = params.get("metadata")
     generated: Final = any(
@@ -292,7 +286,7 @@ def get_chatgpt_session_id(litellm_params: object) -> str | None:
         key = str(prompt_cache_key)
         safe = _safe_header_value(key)
         # hashing avoids collisions from _safe_header_value's replacement char
-        return safe if safe == key else hashlib.sha256(key.encode()).hexdigest()
+        return safe if safe == key else hashlib.sha256(key.encode("utf-8", "surrogatepass")).hexdigest()
     if generated:
         return None
     for key in ("litellm_trace_id", "litellm_call_id"):
@@ -302,5 +296,5 @@ def get_chatgpt_session_id(litellm_params: object) -> str | None:
     return None
 
 
-def ensure_chatgpt_session_id(litellm_params: object) -> str:
+def ensure_chatgpt_session_id(litellm_params: dict[str, object] | GenericLiteLLMParams | None) -> str:
     return get_chatgpt_session_id(litellm_params) or str(uuid4())
diff --git a/tests/unit/llms/chatgpt/responses/test_chatgpt_responses_transformation.py b/tests/unit/llms/chatgpt/responses/test_chatgpt_responses_transformation.py
index ed64e4b73c0..be0678e3571 100644
--- a/tests/unit/llms/chatgpt/responses/test_chatgpt_responses_transformation.py
+++ b/tests/unit/llms/chatgpt/responses/test_chatgpt_responses_transformation.py
@@ -371,36 +371,48 @@ class TestChatGPTResponsesAPITransformation:
         assert exc_info.value.status_code == 502
 
 
-class TestChatGPTSessionId:
-    def test_explicit_session_ids_win(self):
-        assert get_chatgpt_session_id({"session_id": "s", "prompt_cache_key": "k"}) == "s"
-        assert get_chatgpt_session_id({"litellm_session_id": "ls", "prompt_cache_key": "k"}) == "ls"
-        assert get_chatgpt_session_id({"metadata": {"session_id": "ms"}, "prompt_cache_key": "k"}) == "ms"
+def test_explicit_session_ids_win():
+    assert get_chatgpt_session_id({"session_id": "s", "prompt_cache_key": "k"}) == "s"
+    assert get_chatgpt_session_id({"litellm_session_id": "ls", "prompt_cache_key": "k"}) == "ls"
+    assert get_chatgpt_session_id({"metadata": {"session_id": "ms"}, "prompt_cache_key": "k"}) == "ms"
 
-    def test_prompt_cache_key_becomes_session_id(self):
-        assert get_chatgpt_session_id({"prompt_cache_key": "conv-abc"}) == "conv-abc"
-        assert ensure_chatgpt_session_id({"prompt_cache_key": "conv-abc"}) == "conv-abc"
 
-    def test_prompt_cache_key_beats_internal_request_ids(self):
-        assert get_chatgpt_session_id({"litellm_call_id": "c", "prompt_cache_key": "k"}) == "k"
-        assert get_chatgpt_session_id({"litellm_trace_id": "t", "prompt_cache_key": "k"}) == "k"
+def test_prompt_cache_key_becomes_session_id():
+    assert get_chatgpt_session_id({"prompt_cache_key": "conv-abc"}) == "conv-abc"
+    assert ensure_chatgpt_session_id({"prompt_cache_key": "conv-abc"}) == "conv-abc"
 
-    def test_unsafe_cache_keys_are_hashed_not_collapsed(self):
-        mangled_a = get_chatgpt_session_id({"prompt_cache_key": "a\nb"})
-        mangled_b = get_chatgpt_session_id({"prompt_cache_key": "a\tb"})
-        assert mangled_a == hashlib.sha256(b"a\nb").hexdigest()
-        assert mangled_b == hashlib.sha256(b"a\tb").hexdigest()
-        assert mangled_a != mangled_b
 
-    def test_generated_session_ids_are_skipped(self):
-        for metadata_key in ("metadata", "litellm_metadata"):
-            generated = {
-                "litellm_session_id": "gen",
-                metadata_key: {"session_id": "gen", "litellm_session_id_generated": True},
-            }
-            assert get_chatgpt_session_id({**generated, "prompt_cache_key": "k"}) == "k"
-            assert get_chatgpt_session_id(generated) is None
-            assert ensure_chatgpt_session_id(generated) != ensure_chatgpt_session_id(generated) != "gen"
+def test_prompt_cache_key_beats_internal_request_ids():
+    assert get_chatgpt_session_id({"litellm_call_id": "c", "prompt_cache_key": "k"}) == "k"
+    assert get_chatgpt_session_id({"litellm_trace_id": "t", "prompt_cache_key": "k"}) == "k"
 
-    def test_uuid4_fallback_without_any_key(self):
-        assert ensure_chatgpt_session_id({}) != ensure_chatgpt_session_id({})
+
+def test_unsafe_cache_keys_are_hashed_not_collapsed():
+    mangled_a = get_chatgpt_session_id({"prompt_cache_key": "a\nb"})
+    mangled_b = get_chatgpt_session_id({"prompt_cache_key": "a\tb"})
+    assert mangled_a == hashlib.sha256(b"a\nb").hexdigest()
+    assert mangled_b == hashlib.sha256(b"a\tb").hexdigest()
+    assert mangled_a != mangled_b
+
+
+def test_lone_surrogate_cache_keys_hash_to_distinct_session_ids():
+    high = get_chatgpt_session_id({"prompt_cache_key": json.loads('"conv-\\ud800"')})
+    low = get_chatgpt_session_id({"prompt_cache_key": json.loads('"conv-\\udc00"')})
+    assert high == hashlib.sha256(b"conv-\xed\xa0\x80").hexdigest()
+    assert low == hashlib.sha256(b"conv-\xed\xb0\x80").hexdigest()
+    assert high != low
+
+
+def test_generated_session_ids_are_skipped():
+    for metadata_key in ("metadata", "litellm_metadata"):
+        generated = {
+            "litellm_session_id": "gen",
+            metadata_key: {"session_id": "gen", "litellm_session_id_generated": True},
+        }
+        assert get_chatgpt_session_id({**generated, "prompt_cache_key": "k"}) == "k"
+        assert get_chatgpt_session_id(generated) is None
+        assert ensure_chatgpt_session_id(generated) != ensure_chatgpt_session_id(generated) != "gen"
+
+
+def test_uuid4_fallback_without_any_key():
+    assert ensure_chatgpt_session_id({}) != ensure_chatgpt_session_id({})