diff --git a/litellm/router_strategy/adaptive_router/hooks.py b/litellm/router_strategy/adaptive_router/hooks.py
index b59ce6e3621..779eae48533 100644
--- a/litellm/router_strategy/adaptive_router/hooks.py
+++ b/litellm/router_strategy/adaptive_router/hooks.py
@@ -55,9 +55,13 @@ def _resolve_session_key(kwargs: dict[str, Any]) -> str | None:
     sid = litellm_params.get("litellm_session_id")
     if sid:
         return str(sid)
-    metadata: Final = litellm_params.get("metadata") or {}
+    metadata: Final = (
+        litellm_params.get("metadata")
+        if isinstance(litellm_params.get("metadata"), dict)
+        else (kwargs.get("metadata") if isinstance(kwargs.get("metadata"), dict) else kwargs.get("litellm_metadata"))
+    ) or {}
     if isinstance(metadata, dict):
-        sid = metadata.get("session_id") or metadata.get("litellm_session_id")
+        sid = metadata.get("session_id") or metadata.get("litellm_session_id") or metadata.get("prompt_cache_key")
         if sid:
             return str(sid)
 
diff --git a/litellm/router_strategy/complexity_router/complexity_router.py b/litellm/router_strategy/complexity_router/complexity_router.py
index f1f791ba72e..851513d4c98 100644
--- a/litellm/router_strategy/complexity_router/complexity_router.py
+++ b/litellm/router_strategy/complexity_router/complexity_router.py
@@ -16,6 +16,7 @@ Inspired by ClawRouter: https://github.com/BlockRunAI/ClawRouter
 from __future__ import annotations
 
 import asyncio
+import hashlib
 import random
 import re
 from collections.abc import Iterator, Mapping, Sequence
@@ -2151,6 +2152,162 @@ class ComplexityRouter(CustomLogger):
                 return str(session_id)
         return None
 
+    @staticmethod
+    def _get_prompt_cache_key_from_request_kwargs(request_kwargs: dict) -> str | None:
+        """Resolve a client-supplied prompt_cache_key from request_kwargs, metadata, litellm_params, extra_body, or headers."""
+        val = request_kwargs.get("prompt_cache_key")
+        if val is not None and str(val).strip():
+            return str(val).strip()
+
+        for metadata in ComplexityRouter._iter_metadata_dicts(request_kwargs):
+            val = metadata.get("prompt_cache_key")
+            if val is not None and str(val).strip():
+                return str(val).strip()
+
+        litellm_params = request_kwargs.get("litellm_params")
+        if isinstance(litellm_params, dict):
+            val = litellm_params.get("prompt_cache_key")
+            if val is not None and str(val).strip():
+                return str(val).strip()
+            lp_meta = litellm_params.get("metadata")
+            if isinstance(lp_meta, dict):
+                val = lp_meta.get("prompt_cache_key")
+                if val is not None and str(val).strip():
+                    return str(val).strip()
+
+        extra_body = request_kwargs.get("extra_body")
+        if isinstance(extra_body, dict):
+            val = extra_body.get("prompt_cache_key")
+            if val is not None and str(val).strip():
+                return str(val).strip()
+
+        headers = request_kwargs.get("headers")
+        if isinstance(headers, dict):
+            for hkey in ("prompt_cache_key", "prompt-cache-key", "x-prompt-cache-key", "X-Prompt-Cache-Key"):
+                val = headers.get(hkey)
+                if val is not None and str(val).strip():
+                    return str(val).strip()
+
+        return None
+
+    @staticmethod
+    def _get_user_identifier_from_request_kwargs(request_kwargs: dict) -> str:
+        """Extract user identifier (user_api_key_hash, user_api_key, user_id, or user)."""
+        for metadata in ComplexityRouter._iter_metadata_dicts(request_kwargs):
+            for key in ("user_api_key_hash", "user_api_key", "user_api_key_user_id", "user_id", "user"):
+                val = metadata.get(key)
+                if val is not None and str(val).strip():
+                    return str(val).strip()
+        user = request_kwargs.get("user")
+        if user is not None and str(user).strip():
+            return str(user).strip()
+        return ""
+
+    @classmethod
+    def _extract_message_text(cls, content: Any) -> str:
+        """Extract text from message content (str or structured parts)."""
+        if isinstance(content, str):
+            return content
+        if isinstance(content, list):
+            parts: list[str] = []
+            for part in content:
+                if isinstance(part, str):
+                    parts.append(part)
+                elif isinstance(part, dict):
+                    if part.get("type") == "text" and "text" in part:
+                        text_val = part["text"]
+                        if isinstance(text_val, str):
+                            parts.append(text_val)
+                    elif "content" in part and isinstance(part["content"], str):
+                        parts.append(part["content"])
+            return "".join(parts)
+        if isinstance(content, dict):
+            if content.get("type") == "text" and "text" in content:
+                text_val = content["text"]
+                if isinstance(text_val, str):
+                    return text_val
+            elif "content" in content and isinstance(content["content"], str):
+                return content["content"]
+            return ""
+        if content is None:
+            return ""
+        return str(content)
+
+    def _derive_prefix_hash(
+        self,
+        request_kwargs: dict,
+        resolved_messages: list[dict[str, Any]] | None = None,
+        model: str | None = None,
+    ) -> str | None:
+        """
+        Derive a session key hash via SHA256 of:
+        (user_api_key / user_id) + (model / model_group) + (normalized first system message) + (normalized first user message)
+        """
+        messages = resolved_messages or request_kwargs.get("messages") or []
+        first_system_msg = ""
+        first_user_msg = ""
+
+        if isinstance(messages, list):
+            for msg in messages:
+                if not isinstance(msg, dict):
+                    continue
+                role = msg.get("role")
+                if not first_system_msg and role in ("system", "developer"):
+                    first_system_msg = self._extract_message_text(msg.get("content"))[:1024].strip()
+                elif not first_user_msg and role == "user":
+                    first_user_msg = self._extract_message_text(msg.get("content"))[:1024].strip()
+                if first_system_msg and first_user_msg:
+                    break
+
+        if not first_system_msg and not first_user_msg:
+            return None
+
+        user_identifier = self._get_user_identifier_from_request_kwargs(request_kwargs)
+        model_group = model or self.model_name or request_kwargs.get("model") or ""
+
+        payload = f"litellm-session-key:{user_identifier}:{model_group}:{first_system_msg}:{first_user_msg}"
+        return hashlib.sha256(payload.encode("utf-8", errors="replace")).hexdigest()
+
+    def _resolve_session_id(
+        self,
+        request_kwargs: dict,
+        resolved_messages: list[dict[str, Any]] | None = None,
+        model: str | None = None,
+    ) -> str | None:
+        """Resolve a session_id: client-supplied first, then session_key_fallback if configured."""
+        session_id = self._get_session_id_from_request_kwargs(request_kwargs)
+        if session_id is not None:
+            return session_id
+
+        fallback = self.config.session_key_fallback
+        if fallback == "none" or not fallback:
+            return None
+
+        derived_key: str | None = None
+        if fallback == "prompt_cache_key":
+            derived_key = self._get_prompt_cache_key_from_request_kwargs(request_kwargs)
+        elif fallback == "prefix_hash":
+            derived_key = self._derive_prefix_hash(
+                request_kwargs=request_kwargs,
+                resolved_messages=resolved_messages,
+                model=model,
+            )
+
+        if derived_key is not None:
+            sanitized_key = derived_key.replace("\r", "").replace("\n", "")[:32]
+            verbose_router_logger.info(
+                "ComplexityRouter: resolved fallback session key '%s' using strategy '%s'",
+                sanitized_key,
+                fallback,
+            )
+            metadata_key = "litellm_metadata" if "litellm_metadata" in request_kwargs else "metadata"
+            metadata = request_kwargs.setdefault(metadata_key, {})
+            if isinstance(metadata, dict) and "session_id" not in metadata:
+                metadata["session_id"] = derived_key
+            return derived_key
+
+        return None
+
     @staticmethod
     def _get_user_api_key_hash_from_request_kwargs(request_kwargs: dict) -> str | None:
         """Resolve the proxy-derived API key hash, the same trust boundary
@@ -2227,8 +2384,21 @@ class ComplexityRouter(CustomLogger):
         conversation_continuing: Final = _conversation_is_continuing(resolved_messages)
 
         use_session_affinity: Final = self._uses_tier_pin
-        session_id: Final = self._get_session_id_from_request_kwargs(request_kwargs) if use_session_affinity else None
-        cache_key = self._get_session_affinity_cache_key(session_id, request_kwargs) if session_id is not None else None
+        use_deployment_affinity: Final = self._uses_deployment_pin
+        session_id: Final = (
+            self._resolve_session_id(
+                request_kwargs=request_kwargs,
+                resolved_messages=resolved_messages,
+                model=model,
+            )
+            if (use_session_affinity or use_deployment_affinity or self.config.adaptive)
+            else None
+        )
+        cache_key = (
+            self._get_session_affinity_cache_key(session_id, request_kwargs)
+            if (use_session_affinity and session_id is not None)
+            else None
+        )
 
         if cache_key is not None:
             pinned_value: Final = await self.litellm_router_instance.cache.async_get_cache(key=cache_key)
diff --git a/litellm/router_strategy/complexity_router/config.py b/litellm/router_strategy/complexity_router/config.py
index 9b2a25f5d28..adea84999a5 100644
--- a/litellm/router_strategy/complexity_router/config.py
+++ b/litellm/router_strategy/complexity_router/config.py
@@ -829,6 +829,14 @@ class ComplexityRouterConfig(BaseModel):
             "idle time for the session's routing decisions rather than total session length"
         ),
     )
+    session_key_fallback: Literal["none", "prompt_cache_key", "prefix_hash"] = Field(
+        default="none",
+        description=(
+            "Fallback method to derive a session key when session_id is absent from request metadata. "
+            "Supported values: 'none' (default, no fallback), 'prompt_cache_key' (use prompt_cache_key from request), "
+            "or 'prefix_hash' (SHA256 hash of user_api_key/user_id + model/model_group + normalized first system message + normalized first user message)."
+        ),
+    )
 
     plugins: list[RoutingPlugin] | None = Field(
         default=None,
diff --git a/litellm/types/router.py b/litellm/types/router.py
index 97bd93f3f47..405e64e9fb5 100644
--- a/litellm/types/router.py
+++ b/litellm/types/router.py
@@ -1029,6 +1029,7 @@ class AdaptiveRouterWeights(BaseModel):
 class AdaptiveRouterConfig(BaseModel):
     available_models: list[str]
     weights: AdaptiveRouterWeights = Field(default_factory=AdaptiveRouterWeights)
+    session_key_fallback: Literal["none", "prompt_cache_key", "prefix_hash"] = "none"
 
 
 class AdaptiveRouterPreferences(BaseModel):
diff --git a/tests/test_litellm/router_strategy/test_session_key_fallback.py b/tests/test_litellm/router_strategy/test_session_key_fallback.py
new file mode 100644
index 00000000000..5e336e86a2a
--- /dev/null
+++ b/tests/test_litellm/router_strategy/test_session_key_fallback.py
@@ -0,0 +1,388 @@
+"""
+Unit tests for Auto Router v2 session_key_fallback derivation (Issue #34766).
+
+Tests cover:
+- Session resolution with explicit session_id.
+- Fallback to prompt_cache_key when configured.
+- Fallback to prefix_hash when configured.
+- Normal operation with "none" (default behavior unchanged).
+- Deployment affinity with session_key_fallback.
+- Adaptive router session key resolution with fallback metadata.
+"""
+
+import hashlib
+from unittest.mock import AsyncMock
+
+import pytest
+
+import litellm
+from litellm.caching.dual_cache import DualCache
+from litellm.router_strategy.adaptive_router.hooks import _resolve_session_key
+from litellm.router_strategy.complexity_router.complexity_router import ComplexityRouter
+from litellm.router_strategy.complexity_router.config import ComplexityRouterConfig
+
+
+@pytest.fixture
+def mock_router_instance():
+    class _MockRouter:
+        def __init__(self):
+            self.cache = DualCache()
+            self.model_list = [
+                {
+                    "model_name": "gpt-4o-mini",
+                    "litellm_params": {"model": "openai/gpt-4o-mini", "input_cost_per_token": 0.0},
+                    "model_info": {},
+                },
+                {
+                    "model_name": "gpt-4o",
+                    "litellm_params": {"model": "openai/gpt-4o", "input_cost_per_token": 0.0},
+                    "model_info": {},
+                },
+                {
+                    "model_name": "claude-sonnet-4-20250514",
+                    "litellm_params": {"model": "anthropic/claude-sonnet-4-20250514", "input_cost_per_token": 0.0},
+                    "model_info": {},
+                },
+                {
+                    "model_name": "o1-preview",
+                    "litellm_params": {"model": "openai/o1-preview", "input_cost_per_token": 0.0},
+                    "model_info": {},
+                },
+            ]
+            self.model_name_to_deployment_indices = {
+                "gpt-4o-mini": [0],
+                "gpt-4o": [1],
+                "claude-sonnet-4-20250514": [2],
+                "o1-preview": [3],
+            }
+
+    return _MockRouter()
+
+
+@pytest.fixture
+def base_config():
+    return {
+        "tiers": {
+            "SIMPLE": "gpt-4o-mini",
+            "MEDIUM": "gpt-4o",
+            "COMPLEX": "claude-sonnet-4-20250514",
+            "REASONING": "o1-preview",
+        },
+        "tier_boundaries": {
+            "simple_medium": 0.25,
+            "medium_complex": 0.50,
+            "complex_reasoning": 0.75,
+        },
+        "session_affinity": True,
+        "session_affinity_ttl_seconds": 3600,
+    }
+
+
+class TestSessionKeyFallback:
+    SIMPLE_MESSAGE = [{"role": "user", "content": "Hello!"}]
+    REASONING_MESSAGE = [
+        {"role": "system", "content": "You are a mathematics tutor."},
+        {
+            "role": "user",
+            "content": "Let's think step by step and reason through this problem carefully.",
+        },
+    ]
+
+    def test_config_default_fallback_is_none(self):
+        cfg = ComplexityRouterConfig(tiers={"SIMPLE": "gpt-4o-mini"})
+        assert cfg.session_key_fallback == "none"
+
+    def test_config_supports_valid_fallback_modes(self):
+        cfg_cache = ComplexityRouterConfig(tiers={"SIMPLE": "gpt-4o-mini"}, session_key_fallback="prompt_cache_key")
+        assert cfg_cache.session_key_fallback == "prompt_cache_key"
+
+        cfg_prefix = ComplexityRouterConfig(tiers={"SIMPLE": "gpt-4o-mini"}, session_key_fallback="prefix_hash")
+        assert cfg_prefix.session_key_fallback == "prefix_hash"
+
+        cfg_none = ComplexityRouterConfig(tiers={"SIMPLE": "gpt-4o-mini"}, session_key_fallback="none")
+        assert cfg_none.session_key_fallback == "none"
+
+    @pytest.mark.asyncio
+    async def test_explicit_session_id_takes_precedence_over_fallback(self, mock_router_instance, base_config):
+        """When an explicit session_id is provided, it must be used directly, ignoring session_key_fallback."""
+        config = {**base_config, "session_key_fallback": "prefix_hash"}
+        router = ComplexityRouter(
+            model_name="test-router",
+            litellm_router_instance=mock_router_instance,
+            complexity_router_config=config,
+        )
+
+        request_kwargs = {
+            "metadata": {"session_id": "my-explicit-session", "user_api_key_hash": "key123"},
+            "prompt_cache_key": "cache-key-should-be-ignored",
+        }
+
+        resolved_id = router._resolve_session_id(
+            request_kwargs=request_kwargs,
+            resolved_messages=self.REASONING_MESSAGE,
+            model="test-router",
+        )
+        assert resolved_id == "my-explicit-session"
+
+        # Pre routing hook pins model under explicit session id
+        res1 = await router.async_pre_routing_hook(
+            model="test-router",
+            request_kwargs=request_kwargs,
+            messages=self.REASONING_MESSAGE,
+        )
+        assert res1 is not None
+        assert res1.model == "o1-preview"
+
+        # Turn 2: simple message under same explicit session_id should hit cache pin
+        req2_kwargs = {"metadata": {"session_id": "my-explicit-session", "user_api_key_hash": "key123"}}
+        res2 = await router.async_pre_routing_hook(
+            model="test-router",
+            request_kwargs=req2_kwargs,
+            messages=self.SIMPLE_MESSAGE,
+        )
+        assert res2 is not None
+        assert res2.model == "o1-preview"
+        assert res2.routing_decision["cause"] == "session_affinity_pin"
+
+    @pytest.mark.asyncio
+    async def test_fallback_to_prompt_cache_key(self, mock_router_instance, base_config):
+        """When session_id is absent and session_key_fallback='prompt_cache_key', use prompt_cache_key."""
+        config = {**base_config, "session_key_fallback": "prompt_cache_key"}
+        router = ComplexityRouter(
+            model_name="test-router",
+            litellm_router_instance=mock_router_instance,
+            complexity_router_config=config,
+        )
+
+        request_kwargs = {
+            "prompt_cache_key": "custom-prompt-cache-key-999",
+            "metadata": {"user_api_key_hash": "key123"},
+        }
+
+        resolved_id = router._resolve_session_id(
+            request_kwargs=request_kwargs,
+            resolved_messages=self.REASONING_MESSAGE,
+            model="test-router",
+        )
+        assert resolved_id == "custom-prompt-cache-key-999"
+        assert request_kwargs["metadata"]["session_id"] == "custom-prompt-cache-key-999"
+
+        # Pre routing hook turn 1
+        res1 = await router.async_pre_routing_hook(
+            model="test-router",
+            request_kwargs=request_kwargs,
+            messages=self.REASONING_MESSAGE,
+        )
+        assert res1 is not None
+        assert res1.model == "o1-preview"
+
+        # Pre routing hook turn 2 with same prompt_cache_key
+        req2_kwargs = {
+            "prompt_cache_key": "custom-prompt-cache-key-999",
+            "metadata": {"user_api_key_hash": "key123"},
+        }
+        res2 = await router.async_pre_routing_hook(
+            model="test-router",
+            request_kwargs=req2_kwargs,
+            messages=self.SIMPLE_MESSAGE,
+        )
+        assert res2 is not None
+        assert res2.model == "o1-preview"
+        assert res2.routing_decision["cause"] == "session_affinity_pin"
+
+    @pytest.mark.asyncio
+    async def test_fallback_prompt_cache_key_from_extra_body_or_headers(self, mock_router_instance, base_config):
+        """prompt_cache_key in extra_body or headers is also resolved."""
+        config = {**base_config, "session_key_fallback": "prompt_cache_key"}
+        router = ComplexityRouter(
+            model_name="test-router",
+            litellm_router_instance=mock_router_instance,
+            complexity_router_config=config,
+        )
+
+        req_extra_body = {"extra_body": {"prompt_cache_key": "extra-body-key"}}
+        resolved = router._resolve_session_id(req_extra_body)
+        assert resolved == "extra-body-key"
+
+        req_headers = {"headers": {"x-prompt-cache-key": "header-key"}}
+        resolved_hdr = router._resolve_session_id(req_headers)
+        assert resolved_hdr == "header-key"
+
+    @pytest.mark.asyncio
+    async def test_fallback_prompt_cache_key_missing_returns_none(self, mock_router_instance, base_config):
+        """When prompt_cache_key is absent and fallback='prompt_cache_key', returns None (reclassifies)."""
+        config = {**base_config, "session_key_fallback": "prompt_cache_key"}
+        router = ComplexityRouter(
+            model_name="test-router",
+            litellm_router_instance=mock_router_instance,
+            complexity_router_config=config,
+        )
+
+        request_kwargs = {"metadata": {}}
+        resolved = router._resolve_session_id(request_kwargs, resolved_messages=self.SIMPLE_MESSAGE)
+        assert resolved is None
+
+    @pytest.mark.asyncio
+    async def test_fallback_to_prefix_hash(self, mock_router_instance, base_config):
+        """When session_id is absent and session_key_fallback='prefix_hash', derive SHA256 prefix hash."""
+        config = {**base_config, "session_key_fallback": "prefix_hash"}
+        router = ComplexityRouter(
+            model_name="test-router",
+            litellm_router_instance=mock_router_instance,
+            complexity_router_config=config,
+        )
+
+        request_kwargs = {
+            "metadata": {"user_api_key_hash": "user_hash_abc"},
+        }
+
+        # Expected SHA256 computation:
+        # user_api_key_hash : model_group : first_system_msg : first_user_msg
+        system_text = "You are a mathematics tutor."
+        user_text = "Let's think step by step and reason through this problem carefully."
+        expected_payload = f"litellm-session-key:user_hash_abc:test-router:{system_text}:{user_text}"
+        expected_hash = hashlib.sha256(expected_payload.encode("utf-8")).hexdigest()
+
+        derived_id = router._resolve_session_id(
+            request_kwargs=request_kwargs,
+            resolved_messages=self.REASONING_MESSAGE,
+            model="test-router",
+        )
+        assert derived_id == expected_hash
+        assert request_kwargs["metadata"]["session_id"] == expected_hash
+
+        # Turn 1: Classifies as REASONING (o1-preview) and pins it under derived prefix hash
+        res1 = await router.async_pre_routing_hook(
+            model="test-router",
+            request_kwargs=request_kwargs,
+            messages=self.REASONING_MESSAGE,
+        )
+        assert res1 is not None
+        assert res1.model == "o1-preview"
+
+        # Turn 2: Follow-up message in the same multi-turn conversation
+        turn2_messages = [
+            {"role": "system", "content": "You are a mathematics tutor."},
+            {
+                "role": "user",
+                "content": "Let's think step by step and reason through this problem carefully.",
+            },
+            {"role": "assistant", "content": "Here is step 1..."},
+            {"role": "user", "content": "Thanks! Can you summarize step 1 in one line?"},
+        ]
+        turn2_kwargs = {
+            "metadata": {"user_api_key_hash": "user_hash_abc"},
+        }
+
+        turn2_derived_id = router._resolve_session_id(
+            request_kwargs=turn2_kwargs,
+            resolved_messages=turn2_messages,
+            model="test-router",
+        )
+        # Prefix hash MUST match Turn 1 because initial system & user message are identical
+        assert turn2_derived_id == expected_hash
+
+        res2 = await router.async_pre_routing_hook(
+            model="test-router",
+            request_kwargs=turn2_kwargs,
+            messages=turn2_messages,
+        )
+        assert res2 is not None
+        assert res2.model == "o1-preview"
+        assert res2.routing_decision["cause"] == "session_affinity_pin"
+
+    @pytest.mark.asyncio
+    async def test_fallback_prefix_hash_different_conversations_segregate(self, mock_router_instance, base_config):
+        """Different initial prompts or different users produce different prefix hashes and route independently."""
+        config = {**base_config, "session_key_fallback": "prefix_hash"}
+        router = ComplexityRouter(
+            model_name="test-router",
+            litellm_router_instance=mock_router_instance,
+            complexity_router_config=config,
+        )
+
+        conv_a_kwargs = {"metadata": {"user_api_key_hash": "user_a"}}
+        conv_b_kwargs = {"metadata": {"user_api_key_hash": "user_b"}}
+
+        res_a = await router.async_pre_routing_hook(
+            model="test-router",
+            request_kwargs=conv_a_kwargs,
+            messages=self.REASONING_MESSAGE,
+        )
+        res_b = await router.async_pre_routing_hook(
+            model="test-router",
+            request_kwargs=conv_b_kwargs,
+            messages=self.SIMPLE_MESSAGE,
+        )
+
+        assert res_a.model == "o1-preview"
+        assert res_b.model == "gpt-4o-mini"
+        assert conv_a_kwargs["metadata"]["session_id"] != conv_b_kwargs["metadata"]["session_id"]
+
+    @pytest.mark.asyncio
+    async def test_fallback_prefix_hash_empty_messages_returns_none(self, mock_router_instance, base_config):
+        """When no message content is present, prefix_hash cannot be derived and returns None."""
+        config = {**base_config, "session_key_fallback": "prefix_hash"}
+        router = ComplexityRouter(
+            model_name="test-router",
+            litellm_router_instance=mock_router_instance,
+            complexity_router_config=config,
+        )
+        resolved = router._resolve_session_id(request_kwargs={}, resolved_messages=[])
+        assert resolved is None
+
+    @pytest.mark.asyncio
+    async def test_default_none_behavior_unchanged(self, mock_router_instance, base_config):
+        """When session_key_fallback is 'none' (default), no fallback key is derived."""
+        config = {**base_config, "session_key_fallback": "none"}
+        router = ComplexityRouter(
+            model_name="test-router",
+            litellm_router_instance=mock_router_instance,
+            complexity_router_config=config,
+        )
+
+        cache = AsyncMock()
+        mock_router_instance.cache = cache
+
+        # Without session_id, no cache lookup or cache set happens
+        res = await router.async_pre_routing_hook(
+            model="test-router",
+            request_kwargs={},
+            messages=self.SIMPLE_MESSAGE,
+        )
+        assert res.model == "gpt-4o-mini"
+        cache.async_get_cache.assert_not_called()
+        cache.async_set_cache.assert_not_called()
+
+    @pytest.mark.asyncio
+    async def test_deployment_affinity_uses_fallback_session_id(self, mock_router_instance, base_config):
+        """When deployment_affinity is active and fallback derives session_id, deployment pin TTL is added."""
+        config = {
+            **base_config,
+            "session_affinity": False,
+            "deployment_affinity": True,
+            "session_key_fallback": "prompt_cache_key",
+        }
+        router = ComplexityRouter(
+            model_name="test-router",
+            litellm_router_instance=mock_router_instance,
+            complexity_router_config=config,
+        )
+
+        request_kwargs = {"prompt_cache_key": "cache-dep-123"}
+        res = await router.async_pre_routing_hook(
+            model="test-router",
+            request_kwargs=request_kwargs,
+            messages=self.SIMPLE_MESSAGE,
+        )
+        assert res is not None
+        assert res.session_affinity_ttl_seconds == 3600
+        assert request_kwargs["metadata"]["session_id"] == "cache-dep-123"
+
+    def test_adaptive_router_hooks_resolve_session_key_with_fallback_metadata(self):
+        """Adaptive router post call hook picks up session_id populated in metadata by fallback."""
+        kwargs = {
+            "metadata": {"session_id": "fallback-derived-session-id-123"},
+        }
+        key = _resolve_session_key(kwargs)
+        assert key == "fallback-derived-session-id-123"