diff --git a/core/sandbox.py b/core/sandbox.py
index 199e58e..d6620b3 100644
--- a/core/sandbox.py
+++ b/core/sandbox.py
@@ -1,23 +1,160 @@
+import importlib
+import multiprocessing
 import os
-import tempfile
-import textwrap
-import subprocess
-from typing import Dict, Any
+import re
+import traceback
+import types
+from typing import Any, Dict, Tuple
+
+# Applies to every env var name; anything matching is dropped from the child's
+# environment before user code runs, so a generated snippet reading os.environ
+# (which several tools intentionally expose) can't exfiltrate provider/DB
+# credentials. Broad on purpose: better to over-redact than miss a variable.
+_SECRET_ENV_PATTERN = re.compile(r"(KEY|SECRET|TOKEN|PASSWORD|CREDENTIAL|_AUTH)", re.IGNORECASE)
+
+# Wall-clock budget for a single run_user_code call. This is the only timeout
+# that can actually stop runaway/malicious generated code: callers invoke
+# run_user_code synchronously from inside an `async def`, so the asyncio-level
+# per-tool timeout can't preempt it (asyncio can only cancel at await points).
+DEFAULT_TIMEOUT_S = float(os.getenv("SANDBOX_TIMEOUT_S", "600"))
+
+# Resource limits are opt-in (unset by default). The tools behind run_user_code
+# span lightweight pandas queries and GPU torch/MONAI inference, so a single
+# default RLIMIT_AS/RLIMIT_CPU would either be too loose to matter or tight
+# enough to break legitimate multi-threaded/CUDA workloads (CUDA context
+# creation in particular tends to fail under a strict RLIMIT_AS regardless of
+# actual memory used). Set these only if your deployment's workload profile
+# is known and homogeneous. `tools/idc_python_worker.py` shows a working
+# example (RLIMIT_CPU + a 4GB RLIMIT_AS) for a narrower, CPU-only worker.
+_MEMORY_LIMIT_MB = os.getenv("SANDBOX_MEMORY_LIMIT_MB")
+_CPU_LIMIT_S = os.getenv("SANDBOX_CPU_LIMIT_S")
+_NPROC_LIMIT = os.getenv("SANDBOX_NPROC_LIMIT")
+
+
+def _redact_secret_env() -> None:
+    for name in list(os.environ):
+        if _SECRET_ENV_PATTERN.search(name):
+            os.environ.pop(name, None)
+
+
+def _apply_resource_limits() -> None:
+    try:
+        import resource
+    except ImportError:
+        return  # not available on Windows
+
+    if _CPU_LIMIT_S:
+        try:
+            cpu_s = int(_CPU_LIMIT_S)
+            resource.setrlimit(resource.RLIMIT_CPU, (cpu_s, cpu_s))
+        except (ValueError, OSError):
+            pass
+    if _MEMORY_LIMIT_MB:
+        try:
+            mem_bytes = int(_MEMORY_LIMIT_MB) * 1024 * 1024
+            resource.setrlimit(resource.RLIMIT_AS, (mem_bytes, mem_bytes))
+        except (ValueError, OSError):
+            pass
+    if _NPROC_LIMIT:
+        try:
+            nproc = int(_NPROC_LIMIT)
+            resource.setrlimit(resource.RLIMIT_NPROC, (nproc, nproc))
+        except (ValueError, OSError):
+            pass
+
+
+def _split_modules(local_env: Dict[str, Any]) -> Tuple[Dict[str, str], Dict[str, Any]]:
+    """Plain `pickle` can't serialize module objects, and every tool that
+    calls run_user_code stuffs modules (pd, os, plt, nib, ...) into
+    local_env. Send module names across the process boundary instead and
+    re-import them in the child, where imports are cheap (already loaded)."""
+    module_names: Dict[str, str] = {}
+    data: Dict[str, Any] = {}
+    for key, value in (local_env or {}).items():
+        if isinstance(value, types.ModuleType):
+            module_names[key] = value.__name__
+        else:
+            data[key] = value
+    return module_names, data
+
+
+def _sandboxed_worker(code: str, module_names: Dict[str, str], data_env: Dict[str, Any], conn) -> None:
+    _redact_secret_env()
+    _apply_resource_limits()
+    try:
+        env: Dict[str, Any] = {"__builtins__": __builtins__}
+        for key, name in module_names.items():
+            env[key] = importlib.import_module(name)
+        env.update(data_env)
+        exec(code, env, env)
+        result: Dict[str, Any] = {"ok": True, "res_query": env.get("res_query")}
+    except BaseException as exc:
+        result = {
+            "ok": False,
+            "error": f"{type(exc).__name__}: {exc}",
+            "traceback": traceback.format_exc(),
+        }
+    try:
+        conn.send(result)
+    except Exception as exc:
+        # res_query wasn't picklable (e.g. an object holding an open handle/lock) -
+        # report that clearly instead of hanging the parent's poll() forever.
+        conn.send({"ok": False, "error": f"Result could not be returned from the sandboxed process: {exc}"})
+    finally:
+        conn.close()
+
+
+def run_user_code_subprocess(code: str, local_env: Dict[str, Any]) -> Dict[str, Any]:
+    """Run generated code in an isolated child process.
+
+    Unlike the in-process mode, this gives the code its own address space (no
+    inherited DB/API client objects to reach via sys.modules), a redacted
+    environment, and a wall-clock timeout that's actually enforceable via
+    process termination.
+    """
+    module_names, data_env = _split_modules(local_env)
+    ctx = multiprocessing.get_context("spawn")
+    parent_conn, child_conn = ctx.Pipe(duplex=False)
+    process = ctx.Process(
+        target=_sandboxed_worker,
+        args=(code, module_names, data_env, child_conn),
+        daemon=True,
+    )
+    process.start()
+    child_conn.close()
+
+    if parent_conn.poll(DEFAULT_TIMEOUT_S):
+        result = parent_conn.recv()
+    else:
+        process.terminate()
+        process.join(5)
+        if process.is_alive():
+            process.kill()
+            process.join()
+        result = {"ok": False, "error": f"Execution timed out after {DEFAULT_TIMEOUT_S:.0f}s and was terminated."}
+
+    process.join(5)
+    parent_conn.close()
+
+    if not result.get("ok"):
+        raise RuntimeError(result.get("error", "Sandboxed code execution failed."))
+    return {"res_query": result.get("res_query")}
+
 
 def run_user_code_inproc(code: str, local_env: Dict[str, Any]) -> Dict[str, Any]:
+    """Execute generated code directly in this process. No isolation, no
+    timeout, full access to the server's own environment and memory - only
+    use this in a trusted, single-tenant, already-sandboxed deployment
+    (e.g. the whole app itself running inside a locked-down container)."""
     env = {"__builtins__": __builtins__}
     if local_env:
         env.update(local_env)
     exec(code, env, env)
-    return {
-        "res_query": env.get("res_query"),
-    }
-
-#def run_user_code_subprocess(code: str, local_env: Dict[str, Any]) -> Dict[str, Any]:
+    return {"res_query": env.get("res_query")}
 
 
-def run_user_code(code: str, local_env: Dict[str, Any]):
-    mode = os.getenv("EXECUTION_MODE", "inproc")
-    #if mode == "subprocess":
-        #return run_user_code_subprocess(code, local_env)
-    return run_user_code_inproc(code, local_env)
+def run_user_code(code: str, local_env: Dict[str, Any]) -> Dict[str, Any]:
+    mode = os.getenv("EXECUTION_MODE", "subprocess")
+    if mode == "inproc":
+        return run_user_code_inproc(code, local_env)
+    return run_user_code_subprocess(code, local_env)
diff --git a/tests/test_sandbox.py b/tests/test_sandbox.py
new file mode 100644
index 0000000..c3944c6
--- /dev/null
+++ b/tests/test_sandbox.py
@@ -0,0 +1,57 @@
+"""Regression tests for core.sandbox.run_user_code.
+
+Stdlib-only (unittest) since the project has no test runner configured yet.
+Run directly: python3 tests/test_sandbox.py
+"""
+import os
+import sys
+import time
+import unittest
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+os.environ.setdefault("SANDBOX_TIMEOUT_S", "5")
+
+from core.sandbox import run_user_code_subprocess  # noqa: E402
+
+
+class RunUserCodeSubprocessTests(unittest.TestCase):
+    def test_basic_result_round_trips(self):
+        out = run_user_code_subprocess("res_query = 1 + 41", {})
+        self.assertEqual(out["res_query"], 42)
+
+    def test_module_values_in_local_env_are_usable(self):
+        out = run_user_code_subprocess("res_query = os.path.basename('/a/b/c.txt')", {"os": os})
+        self.assertEqual(out["res_query"], "c.txt")
+
+    def test_plain_data_in_local_env_round_trips(self):
+        out = run_user_code_subprocess("res_query = X + 1", {"X": 10})
+        self.assertEqual(out["res_query"], 11)
+
+    def test_exception_surfaces_as_runtime_error(self):
+        with self.assertRaises(RuntimeError) as ctx:
+            run_user_code_subprocess("res_query = 1 / 0", {})
+        self.assertIn("ZeroDivisionError", str(ctx.exception))
+
+    def test_secret_env_vars_are_not_visible_to_generated_code(self):
+        os.environ["OPENAI_API_KEY"] = "sk-should-not-leak"
+        os.environ["POSTGRES_PASSWORD"] = "hunter2-should-not-leak"
+        try:
+            out = run_user_code_subprocess("import os\nres_query = dict(os.environ)", {"os": os})
+        finally:
+            os.environ.pop("OPENAI_API_KEY", None)
+            os.environ.pop("POSTGRES_PASSWORD", None)
+        leaked = [k for k in out["res_query"] if "API_KEY" in k or "PASSWORD" in k]
+        self.assertEqual(leaked, [])
+
+    def test_runaway_code_is_killed_after_timeout(self):
+        start = time.time()
+        with self.assertRaises(RuntimeError) as ctx:
+            run_user_code_subprocess("while True:\n    pass", {})
+        elapsed = time.time() - start
+        self.assertIn("timed out", str(ctx.exception))
+        self.assertLess(elapsed, float(os.environ["SANDBOX_TIMEOUT_S"]) + 5)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/tools/monai_infer.py b/tools/monai_infer.py
index 7783be1..a9c8413 100644
--- a/tools/monai_infer.py
+++ b/tools/monai_infer.py
@@ -83,6 +83,27 @@ def run_monai_bundle(
     return result
 
 
+def _save_pred_as_nifti(pred, out_path):
+    arr = np.asarray(pred)
+    if arr.dtype == np.int64:
+        arr = arr.astype(np.uint8)
+    aff = getattr(pred, "affine", np.eye(4))
+    nib.save(nib.Nifti1Image(arr, aff), out_path)
+
+
+def _normalize_to_HWD(x):
+    if isinstance(x, torch.Tensor):
+        x = x.detach().cpu().numpy()
+    x = np.asarray(x)
+    if x.ndim == 5 and x.shape[0] == 1:  # (B,C,H,W,D)
+        x = x.squeeze(0)
+    if x.ndim == 4:  # (C,H,W,D)
+        x = x[0] if x.shape[0] == 1 else x.argmax(axis=0).astype(np.uint8)
+    elif x.ndim != 3:
+        raise ValueError(f"Unexpected pred shape {x.shape}")
+    return x
+
+
 class MONAIAgent:
     name = "monai"
     model = "gpt-5.6-sol"
@@ -157,25 +178,6 @@ async def run(
 
         print("MONAI AGENT GENERATED CODE:\n", code)
 
-        def _save_pred_as_nifti(pred, out_path):
-            arr = np.asarray(pred)
-            if arr.dtype == np.int64:
-                arr = arr.astype(np.uint8)
-            aff = getattr(pred, "affine", np.eye(4))
-            nib.save(nib.Nifti1Image(arr, aff), out_path)
-
-        def _normalize_to_HWD(x):
-            if isinstance(x, torch.Tensor):
-                x = x.detach().cpu().numpy()
-            x = np.asarray(x)
-            if x.ndim == 5 and x.shape[0] == 1:  # (B,C,H,W,D)
-                x = x.squeeze(0)
-            if x.ndim == 4:  # (C,H,W,D)
-                x = x[0] if x.shape[0] == 1 else x.argmax(axis=0).astype(np.uint8)
-            elif x.ndim != 3:
-                raise ValueError(f"Unexpected pred shape {x.shape}")
-            return x
-
         local_env: Dict[str, Any] = {
             # MONAI execution utilities
             "run_monai_bundle": run_monai_bundle,