diff --git a/backend/src/agents/lead_agent/agent.py b/backend/src/agents/lead_agent/agent.py
index 29ca905..f407733 100644
--- a/backend/src/agents/lead_agent/agent.py
+++ b/backend/src/agents/lead_agent/agent.py
@@ -6,20 +6,17 @@ from langchain_core.runnables import RunnableConfig
 
 from src.agents.lead_agent.prompt import apply_prompt_template
 from src.agents.middlewares.clarification_middleware import ClarificationMiddleware
-from src.agents.middlewares.dangling_tool_call_middleware import DanglingToolCallMiddleware
 from src.agents.middlewares.memory_middleware import MemoryMiddleware
 from src.agents.middlewares.subagent_limit_middleware import SubagentLimitMiddleware
-from src.agents.middlewares.thread_data_middleware import ThreadDataMiddleware
 from src.agents.middlewares.title_middleware import TitleMiddleware
 from src.agents.middlewares.todo_middleware import TodoMiddleware
-from src.agents.middlewares.uploads_middleware import UploadsMiddleware
+from src.agents.middlewares.tool_error_handling_middleware import build_lead_runtime_middlewares
 from src.agents.middlewares.view_image_middleware import ViewImageMiddleware
 from src.agents.thread_state import ThreadState
 from src.config.agents_config import load_agent_config
 from src.config.app_config import get_app_config
 from src.config.summarization_config import get_summarization_config
 from src.models import create_chat_model
-from src.sandbox.middleware import SandboxMiddleware
 
 logger = logging.getLogger(__name__)
 
@@ -204,6 +201,7 @@ Being proactive with task management demonstrates thoroughness and ensures all r
 # TitleMiddleware generates title after first exchange
 # MemoryMiddleware queues conversation for memory update (after TitleMiddleware)
 # ViewImageMiddleware should be before ClarificationMiddleware to inject image details before LLM
+# ToolErrorHandlingMiddleware should be before ClarificationMiddleware to convert tool exceptions to ToolMessages
 # ClarificationMiddleware should be last to intercept clarification requests after model calls
 def _build_middlewares(config: RunnableConfig, model_name: str | None, agent_name: str | None = None):
     """Build middleware chain based on runtime configuration.
@@ -215,7 +213,7 @@ def _build_middlewares(config: RunnableConfig, model_name: str | None, agent_nam
     Returns:
         List of middleware instances.
     """
-    middlewares = [ThreadDataMiddleware(), UploadsMiddleware(), SandboxMiddleware(), DanglingToolCallMiddleware()]
+    middlewares = build_lead_runtime_middlewares(lazy_init=True)
 
     # Add summarization middleware if enabled
     summarization_middleware = _create_summarization_middleware()
diff --git a/backend/src/agents/middlewares/tool_error_handling_middleware.py b/backend/src/agents/middlewares/tool_error_handling_middleware.py
new file mode 100644
index 0000000..7a22fbc
--- /dev/null
+++ b/backend/src/agents/middlewares/tool_error_handling_middleware.py
@@ -0,0 +1,115 @@
+"""Tool error handling middleware and shared runtime middleware builders."""
+
+import logging
+from collections.abc import Awaitable, Callable
+from typing import override
+
+from langchain.agents import AgentState
+from langchain.agents.middleware import AgentMiddleware
+from langchain_core.messages import ToolMessage
+from langgraph.errors import GraphBubbleUp
+from langgraph.prebuilt.tool_node import ToolCallRequest
+from langgraph.types import Command
+
+logger = logging.getLogger(__name__)
+
+_MISSING_TOOL_CALL_ID = "missing_tool_call_id"
+
+
+class ToolErrorHandlingMiddleware(AgentMiddleware[AgentState]):
+    """Convert tool exceptions into error ToolMessages so the run can continue."""
+
+    def _build_error_message(self, request: ToolCallRequest, exc: Exception) -> ToolMessage:
+        tool_name = str(request.tool_call.get("name") or "unknown_tool")
+        tool_call_id = str(request.tool_call.get("id") or _MISSING_TOOL_CALL_ID)
+        detail = str(exc).strip() or exc.__class__.__name__
+        if len(detail) > 500:
+            detail = detail[:497] + "..."
+
+        content = (
+            f"Error: Tool '{tool_name}' failed with {exc.__class__.__name__}: {detail}. "
+            "Continue with available context, or choose an alternative tool."
+        )
+        return ToolMessage(
+            content=content,
+            tool_call_id=tool_call_id,
+            name=tool_name,
+            status="error",
+        )
+
+    @override
+    def wrap_tool_call(
+        self,
+        request: ToolCallRequest,
+        handler: Callable[[ToolCallRequest], ToolMessage | Command],
+    ) -> ToolMessage | Command:
+        try:
+            return handler(request)
+        except GraphBubbleUp:
+            # Preserve LangGraph control-flow signals (interrupt/pause/resume).
+            raise
+        except Exception as exc:
+            logger.exception("Tool execution failed (sync): name=%s id=%s", request.tool_call.get("name"), request.tool_call.get("id"))
+            return self._build_error_message(request, exc)
+
+    @override
+    async def awrap_tool_call(
+        self,
+        request: ToolCallRequest,
+        handler: Callable[[ToolCallRequest], Awaitable[ToolMessage | Command]],
+    ) -> ToolMessage | Command:
+        try:
+            return await handler(request)
+        except GraphBubbleUp:
+            # Preserve LangGraph control-flow signals (interrupt/pause/resume).
+            raise
+        except Exception as exc:
+            logger.exception("Tool execution failed (async): name=%s id=%s", request.tool_call.get("name"), request.tool_call.get("id"))
+            return self._build_error_message(request, exc)
+
+
+def _build_runtime_middlewares(
+    *,
+    include_uploads: bool,
+    include_dangling_tool_call_patch: bool,
+    lazy_init: bool = True,
+) -> list[AgentMiddleware]:
+    """Build shared base middlewares for agent execution."""
+    from src.agents.middlewares.thread_data_middleware import ThreadDataMiddleware
+    from src.sandbox.middleware import SandboxMiddleware
+
+    middlewares: list[AgentMiddleware] = [
+        ThreadDataMiddleware(lazy_init=lazy_init),
+        SandboxMiddleware(lazy_init=lazy_init),
+    ]
+
+    if include_uploads:
+        from src.agents.middlewares.uploads_middleware import UploadsMiddleware
+
+        middlewares.insert(1, UploadsMiddleware())
+
+    if include_dangling_tool_call_patch:
+        from src.agents.middlewares.dangling_tool_call_middleware import DanglingToolCallMiddleware
+
+        middlewares.append(DanglingToolCallMiddleware())
+
+    middlewares.append(ToolErrorHandlingMiddleware())
+    return middlewares
+
+
+def build_lead_runtime_middlewares(*, lazy_init: bool = True) -> list[AgentMiddleware]:
+    """Middlewares shared by lead agent runtime before lead-only middlewares."""
+    return _build_runtime_middlewares(
+        include_uploads=True,
+        include_dangling_tool_call_patch=True,
+        lazy_init=lazy_init,
+    )
+
+
+def build_subagent_runtime_middlewares(*, lazy_init: bool = True) -> list[AgentMiddleware]:
+    """Middlewares shared by subagent runtime before subagent-only middlewares."""
+    return _build_runtime_middlewares(
+        include_uploads=False,
+        include_dangling_tool_call_patch=False,
+        lazy_init=lazy_init,
+    )
diff --git a/backend/src/subagents/executor.py b/backend/src/subagents/executor.py
index 1493133..b269dab 100644
--- a/backend/src/subagents/executor.py
+++ b/backend/src/subagents/executor.py
@@ -166,15 +166,10 @@ class SubagentExecutor:
         model_name = _get_model_name(self.config, self.parent_model)
         model = create_chat_model(name=model_name, thinking_enabled=False)
 
-        # Subagents need minimal middlewares to ensure tools can access sandbox and thread_data
-        # These middlewares will reuse the sandbox/thread_data from parent agent
-        from src.agents.middlewares.thread_data_middleware import ThreadDataMiddleware
-        from src.sandbox.middleware import SandboxMiddleware
+        from src.agents.middlewares.tool_error_handling_middleware import build_subagent_runtime_middlewares
 
-        middlewares = [
-            ThreadDataMiddleware(lazy_init=True),  # Compute thread paths
-            SandboxMiddleware(lazy_init=True),  # Reuse parent's sandbox (no re-acquisition)
-        ]
+        # Reuse shared middleware composition with lead agent.
+        middlewares = build_subagent_runtime_middlewares(lazy_init=True)
 
         return create_agent(
             model=model,
diff --git a/backend/tests/test_tool_error_handling_middleware.py b/backend/tests/test_tool_error_handling_middleware.py
new file mode 100644
index 0000000..60e8981
--- /dev/null
+++ b/backend/tests/test_tool_error_handling_middleware.py
@@ -0,0 +1,96 @@
+from types import SimpleNamespace
+
+import pytest
+from langchain_core.messages import ToolMessage
+from langgraph.errors import GraphInterrupt
+
+from src.agents.middlewares.tool_error_handling_middleware import ToolErrorHandlingMiddleware
+
+
+def _request(name: str = "web_search", tool_call_id: str | None = "tc-1"):
+    tool_call = {"name": name}
+    if tool_call_id is not None:
+        tool_call["id"] = tool_call_id
+    return SimpleNamespace(tool_call=tool_call)
+
+
+def test_wrap_tool_call_passthrough_on_success():
+    middleware = ToolErrorHandlingMiddleware()
+    req = _request()
+    expected = ToolMessage(content="ok", tool_call_id="tc-1", name="web_search")
+
+    result = middleware.wrap_tool_call(req, lambda _req: expected)
+
+    assert result is expected
+
+
+def test_wrap_tool_call_returns_error_tool_message_on_exception():
+    middleware = ToolErrorHandlingMiddleware()
+    req = _request(name="web_search", tool_call_id="tc-42")
+
+    def _boom(_req):
+        raise RuntimeError("network down")
+
+    result = middleware.wrap_tool_call(req, _boom)
+
+    assert isinstance(result, ToolMessage)
+    assert result.tool_call_id == "tc-42"
+    assert result.name == "web_search"
+    assert result.status == "error"
+    assert "Tool 'web_search' failed" in result.text
+    assert "network down" in result.text
+
+
+def test_wrap_tool_call_uses_fallback_tool_call_id_when_missing():
+    middleware = ToolErrorHandlingMiddleware()
+    req = _request(name="mcp_tool", tool_call_id=None)
+
+    def _boom(_req):
+        raise ValueError("bad request")
+
+    result = middleware.wrap_tool_call(req, _boom)
+
+    assert isinstance(result, ToolMessage)
+    assert result.tool_call_id == "missing_tool_call_id"
+    assert result.name == "mcp_tool"
+    assert result.status == "error"
+
+
+def test_wrap_tool_call_reraises_graph_interrupt():
+    middleware = ToolErrorHandlingMiddleware()
+    req = _request(name="ask_clarification", tool_call_id="tc-int")
+
+    def _interrupt(_req):
+        raise GraphInterrupt(())
+
+    with pytest.raises(GraphInterrupt):
+        middleware.wrap_tool_call(req, _interrupt)
+
+
+@pytest.mark.anyio
+async def test_awrap_tool_call_returns_error_tool_message_on_exception():
+    middleware = ToolErrorHandlingMiddleware()
+    req = _request(name="mcp_tool", tool_call_id="tc-async")
+
+    async def _boom(_req):
+        raise TimeoutError("request timed out")
+
+    result = await middleware.awrap_tool_call(req, _boom)
+
+    assert isinstance(result, ToolMessage)
+    assert result.tool_call_id == "tc-async"
+    assert result.name == "mcp_tool"
+    assert result.status == "error"
+    assert "request timed out" in result.text
+
+
+@pytest.mark.anyio
+async def test_awrap_tool_call_reraises_graph_interrupt():
+    middleware = ToolErrorHandlingMiddleware()
+    req = _request(name="ask_clarification", tool_call_id="tc-int-async")
+
+    async def _interrupt(_req):
+        raise GraphInterrupt(())
+
+    with pytest.raises(GraphInterrupt):
+        await middleware.awrap_tool_call(req, _interrupt)
diff --git a/scripts/tool-error-degradation-detection.sh b/scripts/tool-error-degradation-detection.sh
new file mode 100755
index 0000000..3bc8c9a
--- /dev/null
+++ b/scripts/tool-error-degradation-detection.sh
@@ -0,0 +1,218 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Detect whether the current branch has working tool-failure downgrade:
+# - Lead agent middleware chain includes error-handling
+# - Subagent middleware chain includes error-handling
+# - Failing tool call does not abort the whole call sequence
+# - Subsequent successful tool call result is still preserved
+
+ROOT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+BACKEND_DIR="${ROOT_DIR}/backend"
+
+if ! command -v uv >/dev/null 2>&1; then
+  echo "[FAIL] uv is required but not found in PATH."
+  exit 1
+fi
+
+export UV_CACHE_DIR="${UV_CACHE_DIR:-/tmp/uv-cache}"
+
+echo "[INFO] Root:    ${ROOT_DIR}"
+echo "[INFO] Backend: ${BACKEND_DIR}"
+echo "[INFO] UV cache: ${UV_CACHE_DIR}"
+echo "[INFO] Running tool-failure downgrade detector..."
+
+cd "${BACKEND_DIR}"
+
+uv run python -u - <<'PY'
+import asyncio
+import logging
+import ssl
+from types import SimpleNamespace
+
+from requests.exceptions import SSLError
+
+from langchain.agents.middleware import AgentMiddleware
+from langchain_core.messages import ToolMessage
+
+from src.agents.lead_agent.agent import _build_middlewares
+from src.config import get_app_config
+from src.sandbox.middleware import SandboxMiddleware
+
+from src.agents.middlewares.thread_data_middleware import ThreadDataMiddleware
+
+HANDSHAKE_ERROR = "[SSL: UNEXPECTED_EOF_WHILE_READING] EOF occurred in violation of protocol (_ssl.c:1000)"
+logging.getLogger("src.agents.middlewares.tool_error_handling_middleware").setLevel(logging.CRITICAL)
+
+
+def _make_ssl_error():
+    return SSLError(ssl.SSLEOFError(8, HANDSHAKE_ERROR))
+
+print("[STEP 1] Prepare simulated Tavily SSL handshake failure.")
+print(f"[INFO] Handshake error payload: {HANDSHAKE_ERROR}")
+
+TOOL_CALLS = [
+    {"name": "web_search", "id": "tc-fail", "args": {"query": "latest agent news"}},
+    {"name": "web_fetch", "id": "tc-ok", "args": {"url": "https://example.com"}},
+]
+
+
+def _sync_handler(req):
+    tool_name = req.tool_call.get("name", "unknown_tool")
+    if tool_name == "web_search":
+        raise _make_ssl_error()
+    return ToolMessage(
+        content=f"{tool_name} success",
+        tool_call_id=req.tool_call.get("id", "missing-id"),
+        name=tool_name,
+        status="success",
+    )
+
+
+async def _async_handler(req):
+    tool_name = req.tool_call.get("name", "unknown_tool")
+    if tool_name == "web_search":
+        raise _make_ssl_error()
+    return ToolMessage(
+        content=f"{tool_name} success",
+        tool_call_id=req.tool_call.get("id", "missing-id"),
+        name=tool_name,
+        status="success",
+    )
+
+
+def _collect_sync_wrappers(middlewares):
+    return [
+        m.wrap_tool_call
+        for m in middlewares
+        if m.__class__.wrap_tool_call is not AgentMiddleware.wrap_tool_call
+        or m.__class__.awrap_tool_call is not AgentMiddleware.awrap_tool_call
+    ]
+
+
+def _collect_async_wrappers(middlewares):
+    return [
+        m.awrap_tool_call
+        for m in middlewares
+        if m.__class__.awrap_tool_call is not AgentMiddleware.awrap_tool_call
+        or m.__class__.wrap_tool_call is not AgentMiddleware.wrap_tool_call
+    ]
+
+
+def _compose_sync(wrappers):
+    def execute(req):
+        return _sync_handler(req)
+
+    for wrapper in reversed(wrappers):
+        previous = execute
+
+        def execute(req, wrapper=wrapper, previous=previous):
+            return wrapper(req, previous)
+
+    return execute
+
+
+def _compose_async(wrappers):
+    async def execute(req):
+        return await _async_handler(req)
+
+    for wrapper in reversed(wrappers):
+        previous = execute
+
+        async def execute(req, wrapper=wrapper, previous=previous):
+            return await wrapper(req, previous)
+
+    return execute
+
+
+def _validate_outputs(label, outputs):
+    if len(outputs) != 2:
+        print(f"[FAIL] {label}: expected 2 tool outputs, got {len(outputs)}")
+        raise SystemExit(2)
+    first, second = outputs
+    if not isinstance(first, ToolMessage) or not isinstance(second, ToolMessage):
+        print(f"[FAIL] {label}: outputs are not ToolMessage instances")
+        raise SystemExit(3)
+    if first.status != "error":
+        print(f"[FAIL] {label}: first tool should be status=error, got {first.status}")
+        raise SystemExit(4)
+    if second.status != "success":
+        print(f"[FAIL] {label}: second tool should be status=success, got {second.status}")
+        raise SystemExit(5)
+    if "Error: Tool 'web_search' failed" not in first.text:
+        print(f"[FAIL] {label}: first tool error text missing")
+        raise SystemExit(6)
+    if "web_fetch success" not in second.text:
+        print(f"[FAIL] {label}: second tool success text missing")
+        raise SystemExit(7)
+    print(f"[INFO] {label}: no crash, outputs preserved (error + success).")
+
+
+def _build_sub_middlewares():
+    try:
+        from src.agents.middlewares.tool_error_handling_middleware import build_subagent_runtime_middlewares
+    except Exception:
+        return [
+            ThreadDataMiddleware(lazy_init=True),
+            SandboxMiddleware(lazy_init=True),
+        ]
+    return build_subagent_runtime_middlewares()
+
+
+def _run_sync_sequence(executor):
+    outputs = []
+    try:
+        for call in TOOL_CALLS:
+            req = SimpleNamespace(tool_call=call)
+            outputs.append(executor(req))
+    except Exception as exc:
+        return outputs, exc
+    return outputs, None
+
+
+async def _run_async_sequence(executor):
+    outputs = []
+    try:
+        for call in TOOL_CALLS:
+            req = SimpleNamespace(tool_call=call)
+            outputs.append(await executor(req))
+    except Exception as exc:
+        return outputs, exc
+    return outputs, None
+
+
+print("[STEP 2] Load current branch middleware chains.")
+app_cfg = get_app_config()
+model_name = app_cfg.models[0].name if app_cfg.models else None
+if not model_name:
+    print("[FAIL] No model configured; cannot evaluate lead middleware chain.")
+    raise SystemExit(8)
+
+lead_middlewares = _build_middlewares({"configurable": {}}, model_name=model_name)
+sub_middlewares = _build_sub_middlewares()
+
+print("[STEP 3] Simulate two sequential tool calls and check whether conversation flow aborts.")
+any_crash = False
+for label, middlewares in [("lead", lead_middlewares), ("subagent", sub_middlewares)]:
+    sync_exec = _compose_sync(_collect_sync_wrappers(middlewares))
+    sync_outputs, sync_exc = _run_sync_sequence(sync_exec)
+    if sync_exc is not None:
+        any_crash = True
+        print(f"[INFO] {label}/sync: conversation aborted after tool error ({sync_exc.__class__.__name__}: {sync_exc}).")
+    else:
+        _validate_outputs(f"{label}/sync", sync_outputs)
+
+    async_exec = _compose_async(_collect_async_wrappers(middlewares))
+    async_outputs, async_exc = asyncio.run(_run_async_sequence(async_exec))
+    if async_exc is not None:
+        any_crash = True
+        print(f"[INFO] {label}/async: conversation aborted after tool error ({async_exc.__class__.__name__}: {async_exc}).")
+    else:
+        _validate_outputs(f"{label}/async", async_outputs)
+
+if any_crash:
+    print("[FAIL] Tool exception caused conversation flow to abort (no effective downgrade).")
+    raise SystemExit(9)
+
+print("[PASS] Tool exceptions were downgraded; conversation flow continued with remaining tool results.")
+PY