diff --git a/README.md b/README.md
index de8f20f..62c972e 100644
--- a/README.md
+++ b/README.md
@@ -38,6 +38,31 @@ Agent 配置路径优先级
 - 如果要引用环境变量，请在 YAML 显式写 `${ENV_VAR}`。
 - `siliconflow` 独立 section 已移除；请在 `agent.llm / agent.tts / agent.asr` 内通过 `provider`、`api_key`、`api_url`、`model` 配置。
 
+## Backend Integration
+
+Engine runtime now supports adapter-based backend integration:
+
+- `BACKEND_MODE=auto|http|disabled`
+- `BACKEND_URL` + `BACKEND_TIMEOUT_SEC`
+- `HISTORY_ENABLED=true|false`
+
+Behavior:
+
+- `auto`: use HTTP backend only when `BACKEND_URL` is set, otherwise engine-only mode.
+- `http`: force HTTP backend; falls back to engine-only mode when URL is missing.
+- `disabled`: force engine-only mode (no backend calls).
+
+History write path is now asynchronous and buffered per session:
+
+- `HISTORY_QUEUE_MAX_SIZE`
+- `HISTORY_RETRY_MAX_ATTEMPTS`
+- `HISTORY_RETRY_BACKOFF_SEC`
+- `HISTORY_FINALIZE_DRAIN_TIMEOUT_SEC`
+
+This keeps turn processing responsive even when backend history APIs are slow/failing.
+
+Detailed notes: `docs/backend_integration.md`.
+
 测试
 
 ```
diff --git a/app/backend_adapters.py b/app/backend_adapters.py
new file mode 100644
index 0000000..a05bd8f
--- /dev/null
+++ b/app/backend_adapters.py
@@ -0,0 +1,357 @@
+"""Backend adapter implementations for engine integration ports."""
+
+from __future__ import annotations
+
+from typing import Any, Dict, List, Optional
+
+import aiohttp
+from loguru import logger
+
+from app.config import settings
+
+
+class NullBackendAdapter:
+    """No-op adapter for engine-only runtime without backend dependencies."""
+
+    async def fetch_assistant_config(self, assistant_id: str) -> Optional[Dict[str, Any]]:
+        _ = assistant_id
+        return None
+
+    async def create_call_record(
+        self,
+        *,
+        user_id: int,
+        assistant_id: Optional[str],
+        source: str = "debug",
+    ) -> Optional[str]:
+        _ = (user_id, assistant_id, source)
+        return None
+
+    async def add_transcript(
+        self,
+        *,
+        call_id: str,
+        turn_index: int,
+        speaker: str,
+        content: str,
+        start_ms: int,
+        end_ms: int,
+        confidence: Optional[float] = None,
+        duration_ms: Optional[int] = None,
+    ) -> bool:
+        _ = (call_id, turn_index, speaker, content, start_ms, end_ms, confidence, duration_ms)
+        return False
+
+    async def finalize_call_record(
+        self,
+        *,
+        call_id: str,
+        status: str,
+        duration_seconds: int,
+    ) -> bool:
+        _ = (call_id, status, duration_seconds)
+        return False
+
+    async def search_knowledge_context(
+        self,
+        *,
+        kb_id: str,
+        query: str,
+        n_results: int = 5,
+    ) -> List[Dict[str, Any]]:
+        _ = (kb_id, query, n_results)
+        return []
+
+    async def fetch_tool_resource(self, tool_id: str) -> Optional[Dict[str, Any]]:
+        _ = tool_id
+        return None
+
+
+class HistoryDisabledBackendAdapter:
+    """Adapter wrapper that disables history writes while keeping reads available."""
+
+    def __init__(self, delegate: HttpBackendAdapter | NullBackendAdapter):
+        self._delegate = delegate
+
+    async def fetch_assistant_config(self, assistant_id: str) -> Optional[Dict[str, Any]]:
+        return await self._delegate.fetch_assistant_config(assistant_id)
+
+    async def create_call_record(
+        self,
+        *,
+        user_id: int,
+        assistant_id: Optional[str],
+        source: str = "debug",
+    ) -> Optional[str]:
+        _ = (user_id, assistant_id, source)
+        return None
+
+    async def add_transcript(
+        self,
+        *,
+        call_id: str,
+        turn_index: int,
+        speaker: str,
+        content: str,
+        start_ms: int,
+        end_ms: int,
+        confidence: Optional[float] = None,
+        duration_ms: Optional[int] = None,
+    ) -> bool:
+        _ = (call_id, turn_index, speaker, content, start_ms, end_ms, confidence, duration_ms)
+        return False
+
+    async def finalize_call_record(
+        self,
+        *,
+        call_id: str,
+        status: str,
+        duration_seconds: int,
+    ) -> bool:
+        _ = (call_id, status, duration_seconds)
+        return False
+
+    async def search_knowledge_context(
+        self,
+        *,
+        kb_id: str,
+        query: str,
+        n_results: int = 5,
+    ) -> List[Dict[str, Any]]:
+        return await self._delegate.search_knowledge_context(
+            kb_id=kb_id,
+            query=query,
+            n_results=n_results,
+        )
+
+    async def fetch_tool_resource(self, tool_id: str) -> Optional[Dict[str, Any]]:
+        return await self._delegate.fetch_tool_resource(tool_id)
+
+
+class HttpBackendAdapter:
+    """HTTP implementation of backend integration ports."""
+
+    def __init__(self, backend_url: str, timeout_sec: int = 10):
+        base_url = str(backend_url or "").strip().rstrip("/")
+        if not base_url:
+            raise ValueError("backend_url is required for HttpBackendAdapter")
+        self._base_url = base_url
+        self._timeout_sec = timeout_sec
+
+    def _timeout(self) -> aiohttp.ClientTimeout:
+        return aiohttp.ClientTimeout(total=self._timeout_sec)
+
+    async def fetch_assistant_config(self, assistant_id: str) -> Optional[Dict[str, Any]]:
+        """Fetch assistant config payload from backend API.
+
+        Expected response shape:
+        {
+          "assistant": {...},
+          "voice": {...} | null
+        }
+        """
+        url = f"{self._base_url}/api/assistants/{assistant_id}/config"
+
+        try:
+            async with aiohttp.ClientSession(timeout=self._timeout()) as session:
+                async with session.get(url) as resp:
+                    if resp.status == 404:
+                        logger.warning(f"Assistant config not found: {assistant_id}")
+                        return None
+                    resp.raise_for_status()
+                    payload = await resp.json()
+                    if not isinstance(payload, dict):
+                        logger.warning("Assistant config payload is not a dict; ignoring")
+                        return None
+                    return payload
+        except Exception as exc:
+            logger.warning(f"Failed to fetch assistant config ({assistant_id}): {exc}")
+            return None
+
+    async def create_call_record(
+        self,
+        *,
+        user_id: int,
+        assistant_id: Optional[str],
+        source: str = "debug",
+    ) -> Optional[str]:
+        """Create a call record via backend history API and return call_id."""
+        url = f"{self._base_url}/api/history"
+        payload: Dict[str, Any] = {
+            "user_id": user_id,
+            "assistant_id": assistant_id,
+            "source": source,
+            "status": "connected",
+        }
+
+        try:
+            async with aiohttp.ClientSession(timeout=self._timeout()) as session:
+                async with session.post(url, json=payload) as resp:
+                    resp.raise_for_status()
+                    data = await resp.json()
+                    call_id = str((data or {}).get("id") or "")
+                    return call_id or None
+        except Exception as exc:
+            logger.warning(f"Failed to create history call record: {exc}")
+            return None
+
+    async def add_transcript(
+        self,
+        *,
+        call_id: str,
+        turn_index: int,
+        speaker: str,
+        content: str,
+        start_ms: int,
+        end_ms: int,
+        confidence: Optional[float] = None,
+        duration_ms: Optional[int] = None,
+    ) -> bool:
+        """Append a transcript segment to backend history."""
+        if not call_id:
+            return False
+
+        url = f"{self._base_url}/api/history/{call_id}/transcripts"
+        payload: Dict[str, Any] = {
+            "turn_index": turn_index,
+            "speaker": speaker,
+            "content": content,
+            "confidence": confidence,
+            "start_ms": start_ms,
+            "end_ms": end_ms,
+            "duration_ms": duration_ms,
+        }
+
+        try:
+            async with aiohttp.ClientSession(timeout=self._timeout()) as session:
+                async with session.post(url, json=payload) as resp:
+                    resp.raise_for_status()
+                    return True
+        except Exception as exc:
+            logger.warning(f"Failed to append history transcript (call_id={call_id}, turn={turn_index}): {exc}")
+            return False
+
+    async def finalize_call_record(
+        self,
+        *,
+        call_id: str,
+        status: str,
+        duration_seconds: int,
+    ) -> bool:
+        """Finalize a call record with status and duration."""
+        if not call_id:
+            return False
+
+        url = f"{self._base_url}/api/history/{call_id}"
+        payload: Dict[str, Any] = {
+            "status": status,
+            "duration_seconds": duration_seconds,
+        }
+
+        try:
+            async with aiohttp.ClientSession(timeout=self._timeout()) as session:
+                async with session.put(url, json=payload) as resp:
+                    resp.raise_for_status()
+                    return True
+        except Exception as exc:
+            logger.warning(f"Failed to finalize history call record ({call_id}): {exc}")
+            return False
+
+    async def search_knowledge_context(
+        self,
+        *,
+        kb_id: str,
+        query: str,
+        n_results: int = 5,
+    ) -> List[Dict[str, Any]]:
+        """Search backend knowledge base and return retrieval results."""
+        if not kb_id or not query.strip():
+            return []
+        try:
+            safe_n_results = max(1, int(n_results))
+        except (TypeError, ValueError):
+            safe_n_results = 5
+
+        url = f"{self._base_url}/api/knowledge/search"
+        payload: Dict[str, Any] = {
+            "kb_id": kb_id,
+            "query": query,
+            "nResults": safe_n_results,
+        }
+
+        try:
+            async with aiohttp.ClientSession(timeout=self._timeout()) as session:
+                async with session.post(url, json=payload) as resp:
+                    if resp.status == 404:
+                        logger.warning(f"Knowledge base not found for retrieval: {kb_id}")
+                        return []
+                    resp.raise_for_status()
+                    data = await resp.json()
+                    if not isinstance(data, dict):
+                        return []
+                    results = data.get("results", [])
+                    if not isinstance(results, list):
+                        return []
+                    return [r for r in results if isinstance(r, dict)]
+        except Exception as exc:
+            logger.warning(f"Knowledge search failed (kb_id={kb_id}): {exc}")
+            return []
+
+    async def fetch_tool_resource(self, tool_id: str) -> Optional[Dict[str, Any]]:
+        """Fetch tool resource configuration from backend API."""
+        if not tool_id:
+            return None
+
+        url = f"{self._base_url}/api/tools/resources/{tool_id}"
+        try:
+            async with aiohttp.ClientSession(timeout=self._timeout()) as session:
+                async with session.get(url) as resp:
+                    if resp.status == 404:
+                        return None
+                    resp.raise_for_status()
+                    data = await resp.json()
+                    return data if isinstance(data, dict) else None
+        except Exception as exc:
+            logger.warning(f"Failed to fetch tool resource ({tool_id}): {exc}")
+            return None
+
+
+def build_backend_adapter(
+    *,
+    backend_url: Optional[str],
+    backend_mode: str = "auto",
+    history_enabled: bool = True,
+    timeout_sec: int = 10,
+) -> HttpBackendAdapter | NullBackendAdapter | HistoryDisabledBackendAdapter:
+    """Create backend adapter implementation based on runtime settings."""
+    mode = str(backend_mode or "auto").strip().lower()
+    has_url = bool(str(backend_url or "").strip())
+
+    base_adapter: HttpBackendAdapter | NullBackendAdapter
+    if mode in {"disabled", "off", "none", "null", "engine_only", "engine-only"}:
+        base_adapter = NullBackendAdapter()
+    elif mode == "http":
+        if has_url:
+            base_adapter = HttpBackendAdapter(backend_url=str(backend_url), timeout_sec=timeout_sec)
+        else:
+            logger.warning("BACKEND_MODE=http but BACKEND_URL is empty; falling back to NullBackendAdapter")
+            base_adapter = NullBackendAdapter()
+    else:
+        if has_url:
+            base_adapter = HttpBackendAdapter(backend_url=str(backend_url), timeout_sec=timeout_sec)
+        else:
+            base_adapter = NullBackendAdapter()
+
+    if not history_enabled:
+        return HistoryDisabledBackendAdapter(base_adapter)
+    return base_adapter
+
+
+def build_backend_adapter_from_settings() -> HttpBackendAdapter | NullBackendAdapter | HistoryDisabledBackendAdapter:
+    """Create backend adapter using current app settings."""
+    return build_backend_adapter(
+        backend_url=settings.backend_url,
+        backend_mode=settings.backend_mode,
+        history_enabled=settings.history_enabled,
+        timeout_sec=settings.backend_timeout_sec,
+    )
diff --git a/app/backend_client.py b/app/backend_client.py
index b750564..93ea183 100644
--- a/app/backend_client.py
+++ b/app/backend_client.py
@@ -1,56 +1,19 @@
-"""Backend API client for assistant config and history persistence."""
+"""Compatibility wrappers around backend adapter implementations."""
 
 from __future__ import annotations
 
 from typing import Any, Dict, List, Optional
 
-import aiohttp
-from loguru import logger
+from app.backend_adapters import build_backend_adapter_from_settings
 
-from app.config import settings
+
+def _adapter():
+    return build_backend_adapter_from_settings()
 
 
 async def fetch_assistant_config(assistant_id: str) -> Optional[Dict[str, Any]]:
-    """Fetch assistant config payload from backend API.
-
-    Expected response shape:
-    {
-      "assistant": {...},
-      "voice": {...} | null
-    }
-    """
-    if not settings.backend_url:
-        logger.warning("BACKEND_URL not set; skipping assistant config fetch")
-        return None
-
-    url = f"{settings.backend_url.rstrip('/')}/api/assistants/{assistant_id}/config"
-    timeout = aiohttp.ClientTimeout(total=settings.backend_timeout_sec)
-
-    try:
-        async with aiohttp.ClientSession(timeout=timeout) as session:
-            async with session.get(url) as resp:
-                if resp.status == 404:
-                    logger.warning(f"Assistant config not found: {assistant_id}")
-                    return None
-                resp.raise_for_status()
-                payload = await resp.json()
-                if not isinstance(payload, dict):
-                    logger.warning("Assistant config payload is not a dict; ignoring")
-                    return None
-                return payload
-    except Exception as exc:
-        logger.warning(f"Failed to fetch assistant config ({assistant_id}): {exc}")
-        return None
-
-
-def _backend_base_url() -> Optional[str]:
-    if not settings.backend_url:
-        return None
-    return settings.backend_url.rstrip("/")
-
-
-def _timeout() -> aiohttp.ClientTimeout:
-    return aiohttp.ClientTimeout(total=settings.backend_timeout_sec)
+    """Fetch assistant config payload from backend adapter."""
+    return await _adapter().fetch_assistant_config(assistant_id)
 
 
 async def create_history_call_record(
@@ -60,28 +23,11 @@ async def create_history_call_record(
     source: str = "debug",
 ) -> Optional[str]:
     """Create a call record via backend history API and return call_id."""
-    base_url = _backend_base_url()
-    if not base_url:
-        return None
-
-    url = f"{base_url}/api/history"
-    payload: Dict[str, Any] = {
-        "user_id": user_id,
-        "assistant_id": assistant_id,
-        "source": source,
-        "status": "connected",
-    }
-
-    try:
-        async with aiohttp.ClientSession(timeout=_timeout()) as session:
-            async with session.post(url, json=payload) as resp:
-                resp.raise_for_status()
-                data = await resp.json()
-                call_id = str((data or {}).get("id") or "")
-                return call_id or None
-    except Exception as exc:
-        logger.warning(f"Failed to create history call record: {exc}")
-        return None
+    return await _adapter().create_call_record(
+        user_id=user_id,
+        assistant_id=assistant_id,
+        source=source,
+    )
 
 
 async def add_history_transcript(
@@ -96,29 +42,16 @@ async def add_history_transcript(
     duration_ms: Optional[int] = None,
 ) -> bool:
     """Append a transcript segment to backend history."""
-    base_url = _backend_base_url()
-    if not base_url or not call_id:
-        return False
-
-    url = f"{base_url}/api/history/{call_id}/transcripts"
-    payload: Dict[str, Any] = {
-        "turn_index": turn_index,
-        "speaker": speaker,
-        "content": content,
-        "confidence": confidence,
-        "start_ms": start_ms,
-        "end_ms": end_ms,
-        "duration_ms": duration_ms,
-    }
-
-    try:
-        async with aiohttp.ClientSession(timeout=_timeout()) as session:
-            async with session.post(url, json=payload) as resp:
-                resp.raise_for_status()
-                return True
-    except Exception as exc:
-        logger.warning(f"Failed to append history transcript (call_id={call_id}, turn={turn_index}): {exc}")
-        return False
+    return await _adapter().add_transcript(
+        call_id=call_id,
+        turn_index=turn_index,
+        speaker=speaker,
+        content=content,
+        start_ms=start_ms,
+        end_ms=end_ms,
+        confidence=confidence,
+        duration_ms=duration_ms,
+    )
 
 
 async def finalize_history_call_record(
@@ -128,24 +61,11 @@ async def finalize_history_call_record(
     duration_seconds: int,
 ) -> bool:
     """Finalize a call record with status and duration."""
-    base_url = _backend_base_url()
-    if not base_url or not call_id:
-        return False
-
-    url = f"{base_url}/api/history/{call_id}"
-    payload: Dict[str, Any] = {
-        "status": status,
-        "duration_seconds": duration_seconds,
-    }
-
-    try:
-        async with aiohttp.ClientSession(timeout=_timeout()) as session:
-            async with session.put(url, json=payload) as resp:
-                resp.raise_for_status()
-                return True
-    except Exception as exc:
-        logger.warning(f"Failed to finalize history call record ({call_id}): {exc}")
-        return False
+    return await _adapter().finalize_call_record(
+        call_id=call_id,
+        status=status,
+        duration_seconds=duration_seconds,
+    )
 
 
 async def search_knowledge_context(
@@ -155,57 +75,13 @@ async def search_knowledge_context(
     n_results: int = 5,
 ) -> List[Dict[str, Any]]:
     """Search backend knowledge base and return retrieval results."""
-    base_url = _backend_base_url()
-    if not base_url:
-        return []
-    if not kb_id or not query.strip():
-        return []
-    try:
-        safe_n_results = max(1, int(n_results))
-    except (TypeError, ValueError):
-        safe_n_results = 5
-
-    url = f"{base_url}/api/knowledge/search"
-    payload: Dict[str, Any] = {
-        "kb_id": kb_id,
-        "query": query,
-        "nResults": safe_n_results,
-    }
-
-    try:
-        async with aiohttp.ClientSession(timeout=_timeout()) as session:
-            async with session.post(url, json=payload) as resp:
-                if resp.status == 404:
-                    logger.warning(f"Knowledge base not found for retrieval: {kb_id}")
-                    return []
-                resp.raise_for_status()
-                data = await resp.json()
-                if not isinstance(data, dict):
-                    return []
-                results = data.get("results", [])
-                if not isinstance(results, list):
-                    return []
-                return [r for r in results if isinstance(r, dict)]
-    except Exception as exc:
-        logger.warning(f"Knowledge search failed (kb_id={kb_id}): {exc}")
-        return []
+    return await _adapter().search_knowledge_context(
+        kb_id=kb_id,
+        query=query,
+        n_results=n_results,
+    )
 
 
 async def fetch_tool_resource(tool_id: str) -> Optional[Dict[str, Any]]:
     """Fetch tool resource configuration from backend API."""
-    base_url = _backend_base_url()
-    if not base_url or not tool_id:
-        return None
-
-    url = f"{base_url}/api/tools/resources/{tool_id}"
-    try:
-        async with aiohttp.ClientSession(timeout=_timeout()) as session:
-            async with session.get(url) as resp:
-                if resp.status == 404:
-                    return None
-                resp.raise_for_status()
-                data = await resp.json()
-                return data if isinstance(data, dict) else None
-    except Exception as exc:
-        logger.warning(f"Failed to fetch tool resource ({tool_id}): {exc}")
-        return None
+    return await _adapter().fetch_tool_resource(tool_id)
diff --git a/app/config.py b/app/config.py
index 7eaf74d..1f6a2ea 100644
--- a/app/config.py
+++ b/app/config.py
@@ -468,9 +468,21 @@ class Settings(BaseSettings):
     ws_require_auth: bool = Field(default=False, description="Require auth in hello message even when ws_api_key is not set")
 
     # Backend bridge configuration (for call/transcript persistence)
+    backend_mode: str = Field(
+        default="auto",
+        description="Backend integration mode: auto | http | disabled"
+    )
     backend_url: Optional[str] = Field(default=None, description="Backend API base URL (e.g. http://localhost:8787)")
     backend_timeout_sec: int = Field(default=10, description="Backend API request timeout in seconds")
+    history_enabled: bool = Field(default=True, description="Enable history write bridge")
     history_default_user_id: int = Field(default=1, description="Fallback user_id for history records")
+    history_queue_max_size: int = Field(default=256, description="Max buffered transcript writes per session")
+    history_retry_max_attempts: int = Field(default=2, description="Retry attempts for each transcript write")
+    history_retry_backoff_sec: float = Field(default=0.2, description="Base retry backoff for transcript writes")
+    history_finalize_drain_timeout_sec: float = Field(
+        default=1.5,
+        description="Max wait before finalizing history when queue is still draining"
+    )
 
     # Agent YAML metadata
     agent_config_path: Optional[str] = Field(default=None, description="Resolved agent YAML path")
diff --git a/app/main.py b/app/main.py
index bf05cc9..c13daba 100644
--- a/app/main.py
+++ b/app/main.py
@@ -20,6 +20,7 @@ except ImportError:
     logger.warning("aiortc not available - WebRTC endpoint will be disabled")
 
 from app.config import settings
+from app.backend_adapters import build_backend_adapter_from_settings
 from core.transports import SocketTransport, WebRtcTransport, BaseTransport
 from core.session import Session
 from processors.tracks import Resampled16kTrack
@@ -75,6 +76,7 @@ app.add_middleware(
 
 # Active sessions storage
 active_sessions: Dict[str, Session] = {}
+backend_gateway = build_backend_adapter_from_settings()
 
 # Configure logging
 logger.remove()
@@ -164,7 +166,7 @@ async def websocket_endpoint(websocket: WebSocket):
 
     # Create transport and session
     transport = SocketTransport(websocket)
-    session = Session(session_id, transport)
+    session = Session(session_id, transport, backend_gateway=backend_gateway)
     active_sessions[session_id] = session
 
     logger.info(f"WebSocket connection established: {session_id}")
@@ -243,7 +245,7 @@ async def webrtc_endpoint(websocket: WebSocket):
 
     # Create transport and session
     transport = WebRtcTransport(websocket, pc)
-    session = Session(session_id, transport)
+    session = Session(session_id, transport, backend_gateway=backend_gateway)
     active_sessions[session_id] = session
 
     logger.info(f"WebRTC connection established: {session_id}")
diff --git a/core/duplex_pipeline.py b/core/duplex_pipeline.py
index a148fc6..551c63c 100644
--- a/core/duplex_pipeline.py
+++ b/core/duplex_pipeline.py
@@ -15,7 +15,7 @@ import asyncio
 import json
 import time
 import uuid
-from typing import Any, Callable, Dict, List, Optional, Tuple
+from typing import Any, Awaitable, Callable, Dict, List, Optional, Tuple
 
 import numpy as np
 from loguru import logger
@@ -86,7 +86,16 @@ class DuplexPipeline:
         tts_service: Optional[BaseTTSService] = None,
         asr_service: Optional[BaseASRService] = None,
         system_prompt: Optional[str] = None,
-        greeting: Optional[str] = None
+        greeting: Optional[str] = None,
+        knowledge_searcher: Optional[
+            Callable[..., Awaitable[List[Dict[str, Any]]]]
+        ] = None,
+        tool_resource_resolver: Optional[
+            Callable[[str], Awaitable[Optional[Dict[str, Any]]]]
+        ] = None,
+        server_tool_executor: Optional[
+            Callable[[Dict[str, Any]], Awaitable[Dict[str, Any]]]
+        ] = None,
     ):
         """
         Initialize duplex pipeline.
@@ -127,6 +136,9 @@ class DuplexPipeline:
         self.llm_service = llm_service
         self.tts_service = tts_service
         self.asr_service = asr_service  # Will be initialized in start()
+        self._knowledge_searcher = knowledge_searcher
+        self._tool_resource_resolver = tool_resource_resolver
+        self._server_tool_executor = server_tool_executor
 
         # Track last sent transcript to avoid duplicates
         self._last_sent_transcript = ""
@@ -215,6 +227,18 @@ class DuplexPipeline:
         self._pending_llm_delta: str = ""
         self._last_llm_delta_emit_ms: float = 0.0
 
+        if self._server_tool_executor is None:
+            if self._tool_resource_resolver:
+                async def _executor(call: Dict[str, Any]) -> Dict[str, Any]:
+                    return await execute_server_tool(
+                        call,
+                        tool_resource_fetcher=self._tool_resource_resolver,
+                    )
+
+                self._server_tool_executor = _executor
+            else:
+                self._server_tool_executor = execute_server_tool
+
         logger.info(f"DuplexPipeline initialized for session {session_id}")
 
     def set_event_sequence_provider(self, provider: Callable[[], int]) -> None:
@@ -559,6 +583,7 @@ class DuplexPipeline:
                         base_url=llm_base_url,
                         model=llm_model,
                         knowledge_config=self._resolved_knowledge_config(),
+                        knowledge_searcher=self._knowledge_searcher,
                     )
                 else:
                     logger.warning("LLM provider unsupported or API key missing - using mock LLM")
@@ -1491,7 +1516,7 @@ class DuplexPipeline:
 
                     try:
                         result = await asyncio.wait_for(
-                            execute_server_tool(call),
+                            self._server_tool_executor(call),
                             timeout=self._SERVER_TOOL_TIMEOUT_SECONDS,
                         )
                     except asyncio.TimeoutError:
diff --git a/core/history_bridge.py b/core/history_bridge.py
new file mode 100644
index 0000000..ead9a3b
--- /dev/null
+++ b/core/history_bridge.py
@@ -0,0 +1,244 @@
+"""Async history bridge for non-blocking transcript persistence."""
+
+from __future__ import annotations
+
+import asyncio
+import time
+from dataclasses import dataclass
+from typing import Any, Optional
+
+from loguru import logger
+
+
+@dataclass
+class _HistoryTranscriptJob:
+    call_id: str
+    turn_index: int
+    speaker: str
+    content: str
+    start_ms: int
+    end_ms: int
+    duration_ms: int
+
+
+class SessionHistoryBridge:
+    """Session-scoped buffered history writer with background retries."""
+
+    _STOP_SENTINEL = object()
+
+    def __init__(
+        self,
+        *,
+        history_writer: Any,
+        enabled: bool,
+        queue_max_size: int,
+        retry_max_attempts: int,
+        retry_backoff_sec: float,
+        finalize_drain_timeout_sec: float,
+    ):
+        self._history_writer = history_writer
+        self._enabled = bool(enabled and history_writer is not None)
+        self._queue_max_size = max(1, int(queue_max_size))
+        self._retry_max_attempts = max(0, int(retry_max_attempts))
+        self._retry_backoff_sec = max(0.0, float(retry_backoff_sec))
+        self._finalize_drain_timeout_sec = max(0.0, float(finalize_drain_timeout_sec))
+
+        self._call_id: Optional[str] = None
+        self._turn_index: int = 0
+        self._started_mono: Optional[float] = None
+        self._finalized: bool = False
+        self._worker_task: Optional[asyncio.Task] = None
+        self._finalize_lock = asyncio.Lock()
+        self._queue: asyncio.Queue[_HistoryTranscriptJob | object] = asyncio.Queue(maxsize=self._queue_max_size)
+
+    @property
+    def enabled(self) -> bool:
+        return self._enabled
+
+    @property
+    def call_id(self) -> Optional[str]:
+        return self._call_id
+
+    async def start_call(
+        self,
+        *,
+        user_id: int,
+        assistant_id: Optional[str],
+        source: str,
+    ) -> Optional[str]:
+        """Create remote call record and start background worker."""
+        if not self._enabled or self._call_id:
+            return self._call_id
+
+        call_id = await self._history_writer.create_call_record(
+            user_id=user_id,
+            assistant_id=assistant_id,
+            source=source,
+        )
+        if not call_id:
+            return None
+
+        self._call_id = str(call_id)
+        self._turn_index = 0
+        self._finalized = False
+        self._started_mono = time.monotonic()
+        self._ensure_worker()
+        return self._call_id
+
+    def elapsed_ms(self) -> int:
+        if self._started_mono is None:
+            return 0
+        return max(0, int((time.monotonic() - self._started_mono) * 1000))
+
+    def enqueue_turn(self, *, role: str, text: str) -> bool:
+        """Queue one transcript write without blocking the caller."""
+        if not self._enabled or not self._call_id or self._finalized:
+            return False
+
+        content = str(text or "").strip()
+        if not content:
+            return False
+
+        speaker = "human" if str(role or "").strip().lower() == "user" else "ai"
+        end_ms = self.elapsed_ms()
+        estimated_duration_ms = max(300, min(12000, len(content) * 80))
+        start_ms = max(0, end_ms - estimated_duration_ms)
+
+        job = _HistoryTranscriptJob(
+            call_id=self._call_id,
+            turn_index=self._turn_index,
+            speaker=speaker,
+            content=content,
+            start_ms=start_ms,
+            end_ms=end_ms,
+            duration_ms=max(1, end_ms - start_ms),
+        )
+        self._turn_index += 1
+        self._ensure_worker()
+
+        try:
+            self._queue.put_nowait(job)
+            return True
+        except asyncio.QueueFull:
+            logger.warning(
+                "History queue full; dropping transcript call_id={} turn={}",
+                self._call_id,
+                job.turn_index,
+            )
+            return False
+
+    async def finalize(self, *, status: str) -> bool:
+        """Finalize history record once; waits briefly for queue drain."""
+        if not self._enabled or not self._call_id:
+            return False
+
+        async with self._finalize_lock:
+            if self._finalized:
+                return True
+
+            await self._drain_queue()
+            ok = await self._history_writer.finalize_call_record(
+                call_id=self._call_id,
+                status=status,
+                duration_seconds=self.duration_seconds(),
+            )
+            if ok:
+                self._finalized = True
+            await self._stop_worker()
+            return ok
+
+    async def shutdown(self) -> None:
+        """Stop worker task and release queue resources."""
+        await self._stop_worker()
+
+    def duration_seconds(self) -> int:
+        if self._started_mono is None:
+            return 0
+        return max(0, int(time.monotonic() - self._started_mono))
+
+    def _ensure_worker(self) -> None:
+        if self._worker_task and not self._worker_task.done():
+            return
+        self._worker_task = asyncio.create_task(self._worker_loop())
+
+    async def _drain_queue(self) -> None:
+        if self._finalize_drain_timeout_sec <= 0:
+            return
+        try:
+            await asyncio.wait_for(self._queue.join(), timeout=self._finalize_drain_timeout_sec)
+        except asyncio.TimeoutError:
+            logger.warning("History queue drain timed out after {}s", self._finalize_drain_timeout_sec)
+
+    async def _stop_worker(self) -> None:
+        task = self._worker_task
+        if not task:
+            return
+        if task.done():
+            self._worker_task = None
+            return
+
+        sent = False
+        try:
+            self._queue.put_nowait(self._STOP_SENTINEL)
+            sent = True
+        except asyncio.QueueFull:
+            pass
+
+        if not sent:
+            try:
+                await asyncio.wait_for(self._queue.put(self._STOP_SENTINEL), timeout=0.5)
+            except asyncio.TimeoutError:
+                task.cancel()
+
+        try:
+            await asyncio.wait_for(task, timeout=1.5)
+        except asyncio.TimeoutError:
+            task.cancel()
+            try:
+                await task
+            except Exception:
+                pass
+        except asyncio.CancelledError:
+            pass
+        finally:
+            self._worker_task = None
+
+    async def _worker_loop(self) -> None:
+        while True:
+            item = await self._queue.get()
+            try:
+                if item is self._STOP_SENTINEL:
+                    return
+
+                assert isinstance(item, _HistoryTranscriptJob)
+                await self._write_with_retry(item)
+            except Exception as exc:
+                logger.warning("History worker write failed unexpectedly: {}", exc)
+            finally:
+                self._queue.task_done()
+
+    async def _write_with_retry(self, job: _HistoryTranscriptJob) -> bool:
+        for attempt in range(self._retry_max_attempts + 1):
+            ok = await self._history_writer.add_transcript(
+                call_id=job.call_id,
+                turn_index=job.turn_index,
+                speaker=job.speaker,
+                content=job.content,
+                start_ms=job.start_ms,
+                end_ms=job.end_ms,
+                duration_ms=job.duration_ms,
+            )
+            if ok:
+                return True
+
+            if attempt >= self._retry_max_attempts:
+                logger.warning(
+                    "History write dropped after retries call_id={} turn={}",
+                    job.call_id,
+                    job.turn_index,
+                )
+                return False
+
+            if self._retry_backoff_sec > 0:
+                await asyncio.sleep(self._retry_backoff_sec * (2**attempt))
+        return False
diff --git a/core/ports/__init__.py b/core/ports/__init__.py
new file mode 100644
index 0000000..7d7c9dd
--- /dev/null
+++ b/core/ports/__init__.py
@@ -0,0 +1,17 @@
+"""Port interfaces for engine-side integration boundaries."""
+
+from core.ports.backend import (
+    AssistantConfigProvider,
+    BackendGateway,
+    HistoryWriter,
+    KnowledgeSearcher,
+    ToolResourceResolver,
+)
+
+__all__ = [
+    "AssistantConfigProvider",
+    "BackendGateway",
+    "HistoryWriter",
+    "KnowledgeSearcher",
+    "ToolResourceResolver",
+]
diff --git a/core/ports/backend.py b/core/ports/backend.py
new file mode 100644
index 0000000..227c743
--- /dev/null
+++ b/core/ports/backend.py
@@ -0,0 +1,84 @@
+"""Backend integration ports.
+
+These interfaces define the boundary between engine runtime logic and
+backend-side capabilities (config lookup, history persistence, retrieval,
+and tool resource discovery).
+"""
+
+from __future__ import annotations
+
+from typing import Any, Dict, List, Optional, Protocol
+
+
+class AssistantConfigProvider(Protocol):
+    """Port for loading trusted assistant runtime configuration."""
+
+    async def fetch_assistant_config(self, assistant_id: str) -> Optional[Dict[str, Any]]:
+        """Fetch assistant configuration payload."""
+
+
+class HistoryWriter(Protocol):
+    """Port for persisting call and transcript history."""
+
+    async def create_call_record(
+        self,
+        *,
+        user_id: int,
+        assistant_id: Optional[str],
+        source: str = "debug",
+    ) -> Optional[str]:
+        """Create a call record and return backend call ID."""
+
+    async def add_transcript(
+        self,
+        *,
+        call_id: str,
+        turn_index: int,
+        speaker: str,
+        content: str,
+        start_ms: int,
+        end_ms: int,
+        confidence: Optional[float] = None,
+        duration_ms: Optional[int] = None,
+    ) -> bool:
+        """Append one transcript turn segment."""
+
+    async def finalize_call_record(
+        self,
+        *,
+        call_id: str,
+        status: str,
+        duration_seconds: int,
+    ) -> bool:
+        """Finalize a call record."""
+
+
+class KnowledgeSearcher(Protocol):
+    """Port for RAG / knowledge retrieval operations."""
+
+    async def search_knowledge_context(
+        self,
+        *,
+        kb_id: str,
+        query: str,
+        n_results: int = 5,
+    ) -> List[Dict[str, Any]]:
+        """Search a knowledge source and return ranked snippets."""
+
+
+class ToolResourceResolver(Protocol):
+    """Port for resolving tool metadata/configuration."""
+
+    async def fetch_tool_resource(self, tool_id: str) -> Optional[Dict[str, Any]]:
+        """Fetch tool resource configuration."""
+
+
+class BackendGateway(
+    AssistantConfigProvider,
+    HistoryWriter,
+    KnowledgeSearcher,
+    ToolResourceResolver,
+    Protocol,
+):
+    """Composite backend gateway interface used by engine services."""
+
diff --git a/core/session.py b/core/session.py
index b741425..2be41e2 100644
--- a/core/session.py
+++ b/core/session.py
@@ -9,15 +9,11 @@ from enum import Enum
 from typing import Optional, Dict, Any, List
 from loguru import logger
 
-from app.backend_client import (
-    fetch_assistant_config,
-    create_history_call_record,
-    add_history_transcript,
-    finalize_history_call_record,
-)
+from app.backend_adapters import build_backend_adapter_from_settings
 from core.transports import BaseTransport
 from core.duplex_pipeline import DuplexPipeline
 from core.conversation import ConversationTurn
+from core.history_bridge import SessionHistoryBridge
 from core.workflow_runner import WorkflowRunner, WorkflowTransition, WorkflowNodeDef, WorkflowEdgeDef
 from app.config import settings
 from services.base import LLMMessage
@@ -76,7 +72,13 @@ class Session:
         "config_version_id",
     }
 
-    def __init__(self, session_id: str, transport: BaseTransport, use_duplex: bool = None):
+    def __init__(
+        self,
+        session_id: str,
+        transport: BaseTransport,
+        use_duplex: bool = None,
+        backend_gateway: Optional[Any] = None,
+    ):
         """
         Initialize session.
 
@@ -88,12 +90,23 @@ class Session:
         self.id = session_id
         self.transport = transport
         self.use_duplex = use_duplex if use_duplex is not None else settings.duplex_enabled
+        self._backend_gateway = backend_gateway or build_backend_adapter_from_settings()
+        self._history_bridge = SessionHistoryBridge(
+            history_writer=self._backend_gateway,
+            enabled=settings.history_enabled,
+            queue_max_size=settings.history_queue_max_size,
+            retry_max_attempts=settings.history_retry_max_attempts,
+            retry_backoff_sec=settings.history_retry_backoff_sec,
+            finalize_drain_timeout_sec=settings.history_finalize_drain_timeout_sec,
+        )
 
         self.pipeline = DuplexPipeline(
             transport=transport,
             session_id=session_id,
             system_prompt=settings.duplex_system_prompt,
-            greeting=settings.duplex_greeting
+            greeting=settings.duplex_greeting,
+            knowledge_searcher=getattr(self._backend_gateway, "search_knowledge_context", None),
+            tool_resource_resolver=getattr(self._backend_gateway, "fetch_tool_resource", None),
         )
 
         # Session state
@@ -107,10 +120,6 @@ class Session:
         # Track IDs
         self.current_track_id: str = self.TRACK_CONTROL
         self._event_seq: int = 0
-        self._history_call_id: Optional[str] = None
-        self._history_turn_index: int = 0
-        self._history_call_started_mono: Optional[float] = None
-        self._history_finalized: bool = False
         self._cleanup_lock = asyncio.Lock()
         self._cleaned_up = False
         self.workflow_runner: Optional[WorkflowRunner] = None
@@ -424,11 +433,12 @@ class Session:
             logger.info(f"Session {self.id} cleaning up")
             await self._finalize_history(status="connected")
             await self.pipeline.cleanup()
+            await self._history_bridge.shutdown()
             await self.transport.close()
 
     async def _start_history_bridge(self, metadata: Dict[str, Any]) -> None:
         """Initialize backend history call record for this session."""
-        if self._history_call_id:
+        if self._history_bridge.call_id:
             return
 
         history_meta: Dict[str, Any] = {}
@@ -444,7 +454,7 @@ class Session:
         assistant_id = history_meta.get("assistantId", metadata.get("assistantId"))
         source = str(history_meta.get("source", metadata.get("source", "debug")))
 
-        call_id = await create_history_call_record(
+        call_id = await self._history_bridge.start_call(
             user_id=user_id,
             assistant_id=str(assistant_id) if assistant_id else None,
             source=source,
@@ -452,10 +462,6 @@ class Session:
         if not call_id:
             return
 
-        self._history_call_id = call_id
-        self._history_call_started_mono = time.monotonic()
-        self._history_turn_index = 0
-        self._history_finalized = False
         logger.info(f"Session {self.id} history bridge enabled (call_id={call_id}, source={source})")
 
     async def _on_turn_complete(self, turn: ConversationTurn) -> None:
@@ -467,48 +473,11 @@ class Session:
             elif role == "assistant":
                 await self._maybe_advance_workflow(turn.text.strip())
 
-        if not self._history_call_id:
-            return
-        if not turn.text or not turn.text.strip():
-            return
-
-        role = (turn.role or "").lower()
-        speaker = "human" if role == "user" else "ai"
-
-        end_ms = 0
-        if self._history_call_started_mono is not None:
-            end_ms = max(0, int((time.monotonic() - self._history_call_started_mono) * 1000))
-        estimated_duration_ms = max(300, min(12000, len(turn.text.strip()) * 80))
-        start_ms = max(0, end_ms - estimated_duration_ms)
-
-        turn_index = self._history_turn_index
-        await add_history_transcript(
-            call_id=self._history_call_id,
-            turn_index=turn_index,
-            speaker=speaker,
-            content=turn.text.strip(),
-            start_ms=start_ms,
-            end_ms=end_ms,
-            duration_ms=max(1, end_ms - start_ms),
-        )
-        self._history_turn_index += 1
+        self._history_bridge.enqueue_turn(role=turn.role or "", text=turn.text or "")
 
     async def _finalize_history(self, status: str) -> None:
         """Finalize history call record once."""
-        if not self._history_call_id or self._history_finalized:
-            return
-
-        duration_seconds = 0
-        if self._history_call_started_mono is not None:
-            duration_seconds = max(0, int(time.monotonic() - self._history_call_started_mono))
-
-        ok = await finalize_history_call_record(
-            call_id=self._history_call_id,
-            status=status,
-            duration_seconds=duration_seconds,
-        )
-        if ok:
-            self._history_finalized = True
+        await self._history_bridge.finalize(status=status)
 
     def _bootstrap_workflow(self, metadata: Dict[str, Any]) -> Dict[str, Any]:
         """Parse workflow payload and return initial runtime overrides."""
@@ -795,10 +764,12 @@ class Session:
         )
         if assistant_id is None:
             return {}
-        if not settings.backend_url:
+
+        provider = getattr(self._backend_gateway, "fetch_assistant_config", None)
+        if not callable(provider):
             return {}
 
-        payload = await fetch_assistant_config(str(assistant_id).strip())
+        payload = await provider(str(assistant_id).strip())
         if not isinstance(payload, dict):
             return {}
 
diff --git a/core/tool_executor.py b/core/tool_executor.py
index 407e199..4505436 100644
--- a/core/tool_executor.py
+++ b/core/tool_executor.py
@@ -4,11 +4,13 @@ import asyncio
 import ast
 import operator
 from datetime import datetime
-from typing import Any, Dict
+from typing import Any, Awaitable, Callable, Dict, Optional
 
 import aiohttp
 
-from app.backend_client import fetch_tool_resource
+from app.backend_adapters import build_backend_adapter_from_settings
+
+ToolResourceFetcher = Callable[[str], Awaitable[Optional[Dict[str, Any]]]]
 
 _BIN_OPS = {
     ast.Add: operator.add,
@@ -170,11 +172,21 @@ def _extract_tool_args(tool_call: Dict[str, Any]) -> Dict[str, Any]:
         return {}
 
 
-async def execute_server_tool(tool_call: Dict[str, Any]) -> Dict[str, Any]:
+async def fetch_tool_resource(tool_id: str) -> Optional[Dict[str, Any]]:
+    """Default tool resource resolver via backend adapter."""
+    adapter = build_backend_adapter_from_settings()
+    return await adapter.fetch_tool_resource(tool_id)
+
+
+async def execute_server_tool(
+    tool_call: Dict[str, Any],
+    tool_resource_fetcher: Optional[ToolResourceFetcher] = None,
+) -> Dict[str, Any]:
     """Execute a server-side tool and return normalized result payload."""
     call_id = str(tool_call.get("id") or "").strip()
     tool_name = _extract_tool_name(tool_call)
     args = _extract_tool_args(tool_call)
+    resource_fetcher = tool_resource_fetcher or fetch_tool_resource
 
     if tool_name == "calculator":
         expression = str(args.get("expression") or "").strip()
@@ -257,7 +269,7 @@ async def execute_server_tool(tool_call: Dict[str, Any]) -> Dict[str, Any]:
         }
 
     if tool_name and tool_name not in {"calculator", "code_interpreter", "current_time"}:
-        resource = await fetch_tool_resource(tool_name)
+        resource = await resource_fetcher(tool_name)
         if resource and str(resource.get("category") or "") == "query":
             method = str(resource.get("http_method") or "GET").strip().upper()
             if method not in {"GET", "POST", "PUT", "PATCH", "DELETE"}:
diff --git a/docs/backend_integration.md b/docs/backend_integration.md
new file mode 100644
index 0000000..1f5d14d
--- /dev/null
+++ b/docs/backend_integration.md
@@ -0,0 +1,47 @@
+# Backend Integration and History Bridge
+
+This engine uses adapter-based backend integration so core runtime logic can run
+with or without an external backend service.
+
+## Runtime Modes
+
+Configure with environment variables:
+
+- `BACKEND_MODE=auto|http|disabled`
+- `BACKEND_URL`
+- `BACKEND_TIMEOUT_SEC`
+- `HISTORY_ENABLED=true|false`
+
+Mode behavior:
+
+- `auto`: use HTTP backend adapter only when `BACKEND_URL` is set.
+- `http`: force HTTP backend adapter (falls back to null adapter when URL is missing).
+- `disabled`: force null adapter and run engine-only.
+
+## Architecture
+
+- Ports: `core/ports/backend.py`
+- Adapters: `app/backend_adapters.py`
+- Compatibility wrappers: `app/backend_client.py`
+
+`Session` and `DuplexPipeline` receive backend capabilities via injected adapter
+methods instead of hard-coding backend client imports.
+
+## Async History Writes
+
+Session history persistence is handled by `core/history_bridge.py`.
+
+Design:
+
+- transcript writes are queued with `put_nowait` (non-blocking turn path)
+- background worker drains queue
+- failed writes retry with exponential backoff
+- finalize waits briefly for queue drain before sending call finalize
+- finalize is idempotent
+
+Related settings:
+
+- `HISTORY_QUEUE_MAX_SIZE`
+- `HISTORY_RETRY_MAX_ATTEMPTS`
+- `HISTORY_RETRY_BACKOFF_SEC`
+- `HISTORY_FINALIZE_DRAIN_TIMEOUT_SEC`
diff --git a/services/llm.py b/services/llm.py
index 51bfbe4..eb7f89c 100644
--- a/services/llm.py
+++ b/services/llm.py
@@ -7,10 +7,10 @@ for real-time voice conversation.
 import os
 import asyncio
 import uuid
-from typing import AsyncIterator, Optional, List, Dict, Any
+from typing import AsyncIterator, Optional, List, Dict, Any, Callable, Awaitable
 from loguru import logger
 
-from app.backend_client import search_knowledge_context
+from app.backend_adapters import build_backend_adapter_from_settings
 from services.base import BaseLLMService, LLMMessage, LLMStreamEvent, ServiceState
 
 # Try to import openai
@@ -37,6 +37,7 @@ class OpenAILLMService(BaseLLMService):
         base_url: Optional[str] = None,
         system_prompt: Optional[str] = None,
         knowledge_config: Optional[Dict[str, Any]] = None,
+        knowledge_searcher: Optional[Callable[..., Awaitable[List[Dict[str, Any]]]]] = None,
     ):
         """
         Initialize OpenAI LLM service.
@@ -60,6 +61,11 @@ class OpenAILLMService(BaseLLMService):
         self.client: Optional[AsyncOpenAI] = None
         self._cancel_event = asyncio.Event()
         self._knowledge_config: Dict[str, Any] = knowledge_config or {}
+        if knowledge_searcher is None:
+            adapter = build_backend_adapter_from_settings()
+            self._knowledge_searcher = adapter.search_knowledge_context
+        else:
+            self._knowledge_searcher = knowledge_searcher
         self._tool_schemas: List[Dict[str, Any]] = []
 
     _RAG_DEFAULT_RESULTS = 5
@@ -224,7 +230,7 @@ class OpenAILLMService(BaseLLMService):
         n_results = self._coerce_int(cfg.get("nResults"), self._RAG_DEFAULT_RESULTS)
         n_results = max(1, min(n_results, self._RAG_MAX_RESULTS))
 
-        results = await search_knowledge_context(
+        results = await self._knowledge_searcher(
             kb_id=kb_id,
             query=latest_user,
             n_results=n_results,
diff --git a/tests/test_backend_adapters.py b/tests/test_backend_adapters.py
new file mode 100644
index 0000000..d55f5e2
--- /dev/null
+++ b/tests/test_backend_adapters.py
@@ -0,0 +1,150 @@
+import aiohttp
+import pytest
+
+from app.backend_adapters import (
+    HistoryDisabledBackendAdapter,
+    HttpBackendAdapter,
+    NullBackendAdapter,
+    build_backend_adapter,
+)
+
+
+@pytest.mark.asyncio
+async def test_build_backend_adapter_without_url_returns_null_adapter():
+    adapter = build_backend_adapter(
+        backend_url=None,
+        backend_mode="auto",
+        history_enabled=True,
+        timeout_sec=3,
+    )
+    assert isinstance(adapter, NullBackendAdapter)
+
+    assert await adapter.fetch_assistant_config("assistant_1") is None
+    assert (
+        await adapter.create_call_record(
+            user_id=1,
+            assistant_id="assistant_1",
+            source="debug",
+        )
+        is None
+    )
+    assert (
+        await adapter.add_transcript(
+            call_id="call_1",
+            turn_index=0,
+            speaker="human",
+            content="hi",
+            start_ms=0,
+            end_ms=100,
+            confidence=0.9,
+            duration_ms=100,
+        )
+        is False
+    )
+    assert (
+        await adapter.finalize_call_record(
+            call_id="call_1",
+            status="connected",
+            duration_seconds=2,
+        )
+        is False
+    )
+    assert await adapter.search_knowledge_context(kb_id="kb_1", query="hello", n_results=3) == []
+    assert await adapter.fetch_tool_resource("tool_1") is None
+
+
+@pytest.mark.asyncio
+async def test_http_backend_adapter_create_call_record_posts_expected_payload(monkeypatch):
+    captured = {}
+
+    class _FakeResponse:
+        def __init__(self, status=200, payload=None):
+            self.status = status
+            self._payload = payload if payload is not None else {}
+
+        async def __aenter__(self):
+            return self
+
+        async def __aexit__(self, exc_type, exc, tb):
+            return None
+
+        async def json(self):
+            return self._payload
+
+        def raise_for_status(self):
+            if self.status >= 400:
+                raise RuntimeError("http_error")
+
+    class _FakeClientSession:
+        def __init__(self, timeout=None):
+            captured["timeout"] = timeout
+
+        async def __aenter__(self):
+            return self
+
+        async def __aexit__(self, exc_type, exc, tb):
+            return None
+
+        def post(self, url, json=None):
+            captured["url"] = url
+            captured["json"] = json
+            return _FakeResponse(status=200, payload={"id": "call_123"})
+
+    monkeypatch.setattr("app.backend_adapters.aiohttp.ClientSession", _FakeClientSession)
+
+    adapter = build_backend_adapter(
+        backend_url="http://localhost:8100",
+        backend_mode="auto",
+        history_enabled=True,
+        timeout_sec=7,
+    )
+    assert isinstance(adapter, HttpBackendAdapter)
+
+    call_id = await adapter.create_call_record(
+        user_id=99,
+        assistant_id="assistant_9",
+        source="debug",
+    )
+
+    assert call_id == "call_123"
+    assert captured["url"] == "http://localhost:8100/api/history"
+    assert captured["json"] == {
+        "user_id": 99,
+        "assistant_id": "assistant_9",
+        "source": "debug",
+        "status": "connected",
+    }
+    assert isinstance(captured["timeout"], aiohttp.ClientTimeout)
+    assert captured["timeout"].total == 7
+
+
+@pytest.mark.asyncio
+async def test_backend_mode_disabled_forces_null_even_with_url():
+    adapter = build_backend_adapter(
+        backend_url="http://localhost:8100",
+        backend_mode="disabled",
+        history_enabled=True,
+        timeout_sec=7,
+    )
+    assert isinstance(adapter, NullBackendAdapter)
+
+
+@pytest.mark.asyncio
+async def test_history_disabled_wraps_backend_adapter():
+    adapter = build_backend_adapter(
+        backend_url="http://localhost:8100",
+        backend_mode="auto",
+        history_enabled=False,
+        timeout_sec=7,
+    )
+    assert isinstance(adapter, HistoryDisabledBackendAdapter)
+    assert await adapter.create_call_record(user_id=1, assistant_id="a1", source="debug") is None
+    assert await adapter.add_transcript(
+        call_id="c1",
+        turn_index=0,
+        speaker="human",
+        content="hi",
+        start_ms=0,
+        end_ms=10,
+        duration_ms=10,
+    ) is False
diff --git a/tests/test_history_bridge.py b/tests/test_history_bridge.py
new file mode 100644
index 0000000..2f9dd80
--- /dev/null
+++ b/tests/test_history_bridge.py
@@ -0,0 +1,147 @@
+import asyncio
+import time
+
+import pytest
+
+from core.history_bridge import SessionHistoryBridge
+
+
+class _FakeHistoryWriter:
+    def __init__(self, *, add_delay_s: float = 0.0, add_result: bool = True):
+        self.add_delay_s = add_delay_s
+        self.add_result = add_result
+        self.created_call_ids = []
+        self.transcripts = []
+        self.finalize_calls = 0
+        self.finalize_statuses = []
+        self.finalize_at = None
+        self.last_transcript_at = None
+
+    async def create_call_record(self, *, user_id: int, assistant_id: str | None, source: str = "debug"):
+        _ = (user_id, assistant_id, source)
+        call_id = "call_test_1"
+        self.created_call_ids.append(call_id)
+        return call_id
+
+    async def add_transcript(
+        self,
+        *,
+        call_id: str,
+        turn_index: int,
+        speaker: str,
+        content: str,
+        start_ms: int,
+        end_ms: int,
+        confidence: float | None = None,
+        duration_ms: int | None = None,
+    ) -> bool:
+        _ = confidence
+        if self.add_delay_s > 0:
+            await asyncio.sleep(self.add_delay_s)
+        self.transcripts.append(
+            {
+                "call_id": call_id,
+                "turn_index": turn_index,
+                "speaker": speaker,
+                "content": content,
+                "start_ms": start_ms,
+                "end_ms": end_ms,
+                "duration_ms": duration_ms,
+            }
+        )
+        self.last_transcript_at = time.monotonic()
+        return self.add_result
+
+    async def finalize_call_record(self, *, call_id: str, status: str, duration_seconds: int) -> bool:
+        _ = (call_id, duration_seconds)
+        self.finalize_calls += 1
+        self.finalize_statuses.append(status)
+        self.finalize_at = time.monotonic()
+        return True
+
+@pytest.mark.asyncio
+async def test_slow_backend_does_not_block_enqueue():
+    writer = _FakeHistoryWriter(add_delay_s=0.15, add_result=True)
+    bridge = SessionHistoryBridge(
+        history_writer=writer,
+        enabled=True,
+        queue_max_size=32,
+        retry_max_attempts=0,
+        retry_backoff_sec=0.01,
+        finalize_drain_timeout_sec=1.0,
+    )
+
+    try:
+        call_id = await bridge.start_call(user_id=1, assistant_id="assistant_1", source="debug")
+        assert call_id == "call_test_1"
+
+        t0 = time.perf_counter()
+        queued = bridge.enqueue_turn(role="user", text="hello world")
+        elapsed_s = time.perf_counter() - t0
+
+        assert queued is True
+        assert elapsed_s < 0.02
+
+        await bridge.finalize(status="connected")
+        assert len(writer.transcripts) == 1
+        assert writer.finalize_calls == 1
+    finally:
+        await bridge.shutdown()
+
+
+@pytest.mark.asyncio
+async def test_failing_backend_retries_but_enqueue_remains_non_blocking():
+    writer = _FakeHistoryWriter(add_delay_s=0.01, add_result=False)
+    bridge = SessionHistoryBridge(
+        history_writer=writer,
+        enabled=True,
+        queue_max_size=32,
+        retry_max_attempts=2,
+        retry_backoff_sec=0.01,
+        finalize_drain_timeout_sec=0.5,
+    )
+
+    try:
+        await bridge.start_call(user_id=1, assistant_id="assistant_1", source="debug")
+        t0 = time.perf_counter()
+        assert bridge.enqueue_turn(role="assistant", text="retry me")
+        elapsed_s = time.perf_counter() - t0
+        assert elapsed_s < 0.02
+
+        await bridge.finalize(status="connected")
+
+        # Initial try + 2 retries
+        assert len(writer.transcripts) == 3
+        assert writer.finalize_calls == 1
+    finally:
+        await bridge.shutdown()
+
+
+@pytest.mark.asyncio
+async def test_finalize_is_idempotent_and_waits_for_queue_drain():
+    writer = _FakeHistoryWriter(add_delay_s=0.05, add_result=True)
+    bridge = SessionHistoryBridge(
+        history_writer=writer,
+        enabled=True,
+        queue_max_size=32,
+        retry_max_attempts=0,
+        retry_backoff_sec=0.01,
+        finalize_drain_timeout_sec=1.0,
+    )
+
+    try:
+        await bridge.start_call(user_id=1, assistant_id="assistant_1", source="debug")
+        assert bridge.enqueue_turn(role="user", text="first")
+
+        ok_1 = await bridge.finalize(status="connected")
+        ok_2 = await bridge.finalize(status="connected")
+
+        assert ok_1 is True
+        assert ok_2 is True
+        assert len(writer.transcripts) == 1
+        assert writer.finalize_calls == 1
+        assert writer.last_transcript_at is not None
+        assert writer.finalize_at is not None
+        assert writer.finalize_at >= writer.last_transcript_at
+    finally:
+        await bridge.shutdown()