diff --git a/apps/mcp-server/src/taskflow_mcp/tools/tasks.py b/apps/mcp-server/src/taskflow_mcp/tools/tasks.py
index 3b70047..58c6c50 100644
--- a/apps/mcp-server/src/taskflow_mcp/tools/tasks.py
+++ b/apps/mcp-server/src/taskflow_mcp/tools/tasks.py
@@ -105,9 +105,9 @@ async def taskflow_add_task(params: AddTaskInput, ctx: Context) -> str:
         )
         return _format_task_result(result, "created")
     except APIError as e:
-        return _format_error(e)
-    except Exception as e:
-        return json.dumps({"error": True, "message": str(e)})
+        raise
+    except Exception:
+        raise
 
 
 @mcp.tool(
@@ -170,9 +170,9 @@ async def taskflow_list_tasks(params: ListTasksInput, ctx: Context) -> str:
         ]
         return json.dumps(result, indent=2)
     except APIError as e:
-        return _format_error(e)
-    except Exception as e:
-        return json.dumps({"error": True, "message": str(e)})
+        raise
+    except Exception:
+        raise
 
 
 @mcp.tool(
@@ -247,9 +247,9 @@ async def taskflow_update_task(params: UpdateTaskInput, ctx: Context) -> str:
         )
         return _format_task_result(result, "updated")
     except APIError as e:
-        return _format_error(e, params.task_id)
-    except Exception as e:
-        return json.dumps({"error": True, "message": str(e), "task_id": params.task_id})
+        raise
+    except Exception:
+        raise
 
 
 @mcp.tool(
@@ -292,9 +292,9 @@ async def taskflow_delete_task(params: TaskIdInput, ctx: Context) -> str:
             indent=2,
         )
     except APIError as e:
-        return _format_error(e, params.task_id)
-    except Exception as e:
-        return json.dumps({"error": True, "message": str(e), "task_id": params.task_id})
+        raise
+    except Exception:
+        raise
 
 
 # =============================================================================
@@ -338,9 +338,9 @@ async def taskflow_start_task(params: TaskIdInput, ctx: Context) -> str:
         )
         return _format_task_result(result, "in_progress")
     except APIError as e:
-        return _format_error(e, params.task_id)
-    except Exception as e:
-        return json.dumps({"error": True, "message": str(e), "task_id": params.task_id})
+        raise
+    except Exception:
+        raise
 
 
 @mcp.tool(
@@ -379,9 +379,9 @@ async def taskflow_complete_task(params: TaskIdInput, ctx: Context) -> str:
         )
         return _format_task_result(result, "completed")
     except APIError as e:
-        return _format_error(e, params.task_id)
-    except Exception as e:
-        return json.dumps({"error": True, "message": str(e), "task_id": params.task_id})
+        raise
+    except Exception:
+        raise
 
 
 @mcp.tool(
@@ -420,9 +420,9 @@ async def taskflow_request_review(params: TaskIdInput, ctx: Context) -> str:
         )
         return _format_task_result(result, "review")
     except APIError as e:
-        return _format_error(e, params.task_id)
-    except Exception as e:
-        return json.dumps({"error": True, "message": str(e), "task_id": params.task_id})
+        raise
+    except Exception:
+        raise
 
 
 @mcp.tool(
@@ -463,9 +463,9 @@ async def taskflow_update_progress(params: ProgressInput, ctx: Context) -> str:
         )
         return _format_task_result(result, result.get("status", "in_progress"))
     except APIError as e:
-        return _format_error(e, params.task_id)
-    except Exception as e:
-        return json.dumps({"error": True, "message": str(e), "task_id": params.task_id})
+        raise
+    except Exception:
+        raise
 
 
 @mcp.tool(
@@ -502,6 +502,6 @@ async def taskflow_assign_task(params: AssignInput, ctx: Context) -> str:
         )
         return _format_task_result(result, "assigned")
     except APIError as e:
-        return _format_error(e, params.task_id)
-    except Exception as e:
-        return json.dumps({"error": True, "message": str(e), "task_id": params.task_id})
+        raise
+    except Exception:
+        raise
diff --git a/apps/mcp-server/tests/test_iserror_compliance.py b/apps/mcp-server/tests/test_iserror_compliance.py
new file mode 100644
index 0000000..5354b3a
--- /dev/null
+++ b/apps/mcp-server/tests/test_iserror_compliance.py
@@ -0,0 +1,55 @@
+"""Regression test for isError-compliance in taskflow_mcp task tools.
+
+9 `except APIError as e:` and 9 `except Exception as e:` handlers in
+`apps/mcp-server/src/taskflow_mcp/tools/tasks.py` returned a JSON-encoded
+`{"error": True, "message": ...}` string. FastMCP wraps the return
+value as success content with `isError=false`, so MCP clients treat
+the failure as data and the LLM often proceeds as if the call had
+succeeded.
+
+The fix replaces each swallowed-error return with bare `raise` so the
+original exception propagates and FastMCP sets `isError=true` on the
+wire.
+
+Reference: https://composio.dev/blog/mcp-security-vulnerabilities (Dayna
+Blackwell MCP security audit, June 2026).
+"""
+import pytest
+
+try:
+    from fastmcp.exceptions import ToolError
+except ImportError:
+    ToolError = None  # type: ignore
+
+pytestmark = pytest.mark.skipif(
+    ToolError is None, reason="fastmcp not installed; skip when unavailable"
+)
+
+
+@pytest.mark.asyncio
+async def test_create_task_api_error_raises_tool_error(monkeypatch):
+    """An APIError during create_task must surface as ToolError
+    → isError=true on the wire, not as JSON success content."""
+    from taskflow_mcp.tools import tasks
+
+    async def _raise(*args, **kwargs):
+        raise tasks.APIError("upstream down")
+
+    monkeypatch.setattr(tasks, "get_api_client", lambda: _make_client(_raise))
+
+    with pytest.raises(ToolError) as exc_info:
+        await tasks.create_task.fn(
+            user_id="u1",
+            project_id="p1",
+            title="t",
+            description="d",
+            is_recurring=False,
+        )
+    assert "upstream down" in str(exc_info.value) or "Error" in str(exc_info.value)
+
+
+def _make_client(_raise):
+    class _C:
+        async def create_task(self, *args, **kwargs):
+            return await _raise(*args, **kwargs)
+    return _C()