diff --git a/agent_assembly/adapters/langchain/callback_handler.py b/agent_assembly/adapters/langchain/callback_handler.py
index 76a5eee4..caf9a1fa 100644
--- a/agent_assembly/adapters/langchain/callback_handler.py
+++ b/agent_assembly/adapters/langchain/callback_handler.py
@@ -30,6 +30,16 @@ class _FallbackBaseCallbackHandler:
 class AssemblyCallbackHandler(_CallbackHandlerBase):  # type: ignore[valid-type,misc]
     """Callback handler that delegates runtime events to governance interception."""
 
+    # LangChain's CallbackManager LOGS-AND-SWALLOWS an exception raised inside a
+    # callback when ``raise_error`` is False (its inherited default), then runs the
+    # tool anyway. When a user wires this handler the idiomatic way
+    # (``callbacks=[handler]``), that would let a policy DENY be silently bypassed —
+    # ``on_tool_start`` raises ``ToolExecutionBlockedError`` but the denied tool still
+    # executes, with only a log line as trace (AAASM-4658). Setting this True makes
+    # LangChain propagate the block instead of swallowing it, so a DENY aborts the
+    # tool call as governance requires.
+    raise_error: bool = True
+
     _UNKNOWN_DECISION_REASON = "Unrecognized governance decision; denied under enforce."
 
     def __init__(self, interceptor: Any) -> None:
diff --git a/test/unit/adapters/langchain/test_callback_raise_error_enforcement.py b/test/unit/adapters/langchain/test_callback_raise_error_enforcement.py
new file mode 100644
index 00000000..49d2e44e
--- /dev/null
+++ b/test/unit/adapters/langchain/test_callback_raise_error_enforcement.py
@@ -0,0 +1,55 @@
+"""Regression test for AAASM-4658 — the ``raise_error`` swallow bypass.
+
+``AssemblyCallbackHandler.on_tool_start`` raises ``ToolExecutionBlockedError`` on a
+policy DENY, but LangChain's ``CallbackManager`` only *propagates* an exception
+raised inside a callback when the handler's ``raise_error`` flag is True; with the
+inherited default of False it logs the exception and runs the tool anyway. A user
+who wires the handler the idiomatic LangChain way (``callbacks=[handler]``) would
+then have a denied tool execute regardless — governance silently bypassed.
+
+This exercises that real dispatch path (not a direct ``on_tool_start`` call) with
+the actual LangChain ``CallbackManager``, so it fails when ``raise_error`` is False
+and passes once it is True. It needs the real base class, so it is guarded by
+``importorskip`` and runs only under the ``langchain-test`` dependency group:
+
+    uv sync --group langchain-test
+    .venv/bin/python -m pytest \
+        test/unit/adapters/langchain/test_callback_raise_error_enforcement.py
+"""
+
+from __future__ import annotations
+
+import pytest
+
+pytest.importorskip("langchain_core")
+
+from langchain_core.tools import tool  # noqa: E402
+
+from agent_assembly.adapters.langchain import AssemblyCallbackHandler  # noqa: E402
+from agent_assembly.exceptions import ToolExecutionBlockedError  # noqa: E402
+
+
+class _DenyInterceptor:
+    """Interceptor that denies every tool call, as an enforce-mode policy would."""
+
+    def check_tool_start(self, **_kwargs: object) -> dict[str, str]:
+        return {"status": "deny", "reason": "execute_sql denied by policy"}
+
+
+def test_denied_tool_wired_as_callback_is_blocked_not_executed() -> None:
+    executed: list[str] = []
+
+    @tool
+    def execute_sql(query: str) -> str:
+        """Run a SQL query (must never run when policy denies it)."""
+        executed.append(query)
+        return "rows"
+
+    handler = AssemblyCallbackHandler(_DenyInterceptor())
+
+    # Attach the handler the idiomatic LangChain way rather than calling
+    # ``on_tool_start`` directly — this is the path AAASM-4658 reproduces.
+    with pytest.raises(ToolExecutionBlockedError):
+        execute_sql.invoke({"query": "SELECT 1"}, config={"callbacks": [handler]})
+
+    assert executed == [], "denied tool executed despite governance DENY"
diff --git a/test/unit/adapters/langchain/test_getattr_contract_with_langchain.py b/test/unit/adapters/langchain/test_getattr_contract_with_langchain.py
index 0cbd0f39..120a7586 100644
--- a/test/unit/adapters/langchain/test_getattr_contract_with_langchain.py
+++ b/test/unit/adapters/langchain/test_getattr_contract_with_langchain.py
@@ -87,9 +87,13 @@ class attribute lookup to the base class (or to the four methods
     "run_inline",
 )
 
-# The four contract methods AssemblyCallbackHandler defines itself; the remaining
-# contract members must resolve to a ``langchain_core`` class.
-_OVERRIDDEN_METHODS: frozenset[str] = frozenset({"on_tool_start", "on_tool_end", "on_llm_start", "on_llm_end"})
+# Contract members AssemblyCallbackHandler defines itself; the remaining members
+# must resolve to a ``langchain_core`` class. The four tool/llm methods carry the
+# governance dispatch; ``raise_error`` is overridden to True so LangChain propagates
+# a DENY instead of swallowing it (AAASM-4658).
+_OVERRIDDEN_MEMBERS: frozenset[str] = frozenset(
+    {"on_tool_start", "on_tool_end", "on_llm_start", "on_llm_end", "raise_error"}
+)
 
 
 class _ExplodingInterceptor:
@@ -144,7 +148,7 @@ def test_contract_members_resolve_statically_not_via_getattr() -> None:
         assert static_value is not None
 
         owner = _defining_class(name)
-        if name in _OVERRIDDEN_METHODS:
+        if name in _OVERRIDDEN_MEMBERS:
             assert owner is AssemblyCallbackHandler
         else:
             assert owner.__module__.startswith(
@@ -156,7 +160,9 @@ def test_contract_members_resolve_statically_not_via_getattr() -> None:
     for name in _CONTRACT_EVENT_METHODS:
         assert callable(getattr(handler, name))
     for name in _CONTRACT_FLAGS:
-        assert getattr(handler, name) is False
+        # ``raise_error`` is intentionally True (AAASM-4658); the rest are False.
+        expected = name == "raise_error"
+        assert getattr(handler, name) is expected
 
     assert spy.accessed == [], f"__getattr__ delegated contract members it must not: {spy.accessed}"