diff --git a/agentix/runtime/PROTOCOL.md b/agentix/runtime/PROTOCOL.md
index 784d4b7..9b6a37a 100644
--- a/agentix/runtime/PROTOCOL.md
+++ b/agentix/runtime/PROTOCOL.md
@@ -22,7 +22,8 @@ instances are intentionally outside the callable boundary. Put remote
 code behind an importable top-level function instead.
 
 Args and kwargs travel separately as `arguments = pickle.dumps((args, kwargs))`.
-Return values travel as `value = pickle.dumps(result)`.
+Return values travel as `value = pack(result)` using
+`agentix.runtime.shared.codec`.
 
 ```python
 from my_project.tasks import run
@@ -51,7 +52,7 @@ runtime.
 
 ```text
 call          {call_id, callable, arguments}
-call:result   {call_id, value}                  # value is pickle.dumps(result)
+call:result   {call_id, value}                  # value is pack(result)
 call:error    {call_id, error}
 cancel        {call_id}
 ```
@@ -127,4 +128,4 @@ Client mapping:
 - Per-call timeouts; callers use `asyncio.wait_for(...)`.
 - Retries; calls are at-most-once.
 - Auth/TLS; providers own that layer.
-- Annotation-driven validation on the wire (args/return are pickle today).
+- Annotation-driven validation on the wire (args are pickle today).
diff --git a/agentix/runtime/client/client.py b/agentix/runtime/client/client.py
index 51178e8..d17cdfd 100644
--- a/agentix/runtime/client/client.py
+++ b/agentix/runtime/client/client.py
@@ -108,8 +108,23 @@ def _decode_payload(raw: Any) -> dict[str, Any]:
     return raw
 
 
-def _unpickle_value(raw: Any) -> Any:
-    return pickle.loads(raw) if raw is not None else None
+def _decode_result_value(raw: Any) -> Any:
+    if raw is None:
+        return None
+    if isinstance(raw, memoryview):
+        raw = raw.tobytes()
+    elif isinstance(raw, bytearray):
+        raw = bytes(raw)
+    if not isinstance(raw, bytes):
+        raise RuntimeError(
+            "sandbox return value must be msgpack bytes; "
+            f"got {type(raw).__name__}. "
+            "This security boundary prevents host-side pickle deserialization."
+        )
+    try:
+        return unpack(raw)
+    except Exception as exc:
+        raise RuntimeError(f"failed to decode sandbox return payload: {type(exc).__name__}: {exc}") from exc
 
 
 class RuntimeClient:
@@ -271,7 +286,7 @@ async def _try_http_fast_path(
         if not isinstance(reply, dict):
             raise RuntimeError("invalid /call reply payload")
         if reply.get("ok") is True:
-            return "result", _unpickle_value(reply.get("value"))
+            return "result", _decode_result_value(reply.get("value"))
         if reply.get("ok") is False:
             err = RemoteError.model_validate(reply.get("error") or {})
             return "error", err
@@ -325,7 +340,7 @@ async def remote(
                     kind, data = await q.get()
                     if kind == "result":
                         terminated = True
-                        return cast(R, _unpickle_value(data.get("value")))
+                        return cast(R, _decode_result_value(data.get("value")))
                     if kind == "error":
                         err = RemoteError.model_validate(data["error"])
                         terminated = True
diff --git a/agentix/runtime/server/worker/invoker.py b/agentix/runtime/server/worker/invoker.py
index c08a461..5cbe9e2 100644
--- a/agentix/runtime/server/worker/invoker.py
+++ b/agentix/runtime/server/worker/invoker.py
@@ -1,8 +1,7 @@
 """Worker-side callable execution.
 
-The worker unpickles the callable + args/kwargs, calls it, and pickles
-the result back. No shape detection, no TypeAdapter validation — pickle
-preserves Python object identity end to end.
+The worker unpickles the callable + args/kwargs, calls it, and
+msgpack-encodes the result back.
 
 A coroutine function is awaited directly on the worker's event loop; a
 plain (sync) function runs in a thread via ``asyncio.to_thread`` so a
@@ -31,6 +30,7 @@
 from typing import Any
 
 from agentix.runtime.shared.callables import display_name_for
+from agentix.runtime.shared.codec import pack
 from agentix.runtime.shared.models import RemoteError, RemoteRequest, RemoteResponse
 from agentix.utils import context as _context
 from agentix.utils.trace._bridge import DISPATCH_CALL_ID
@@ -86,13 +86,13 @@ async def call(self, fn: Any, request: RemoteRequest) -> RemoteResponse:
                 ),
             )
         try:
-            payload = pickle.dumps(result)
+            payload = pack(result)
         except Exception as exc:
             return RemoteResponse(
                 ok=False,
                 error=RemoteError(
                     type="ResultEncodeError",
-                    message=f"failed to pickle return value: {exc}",
+                    message=f"failed to encode return value: {exc}",
                 ),
             )
         return RemoteResponse(ok=True, value=payload)
diff --git a/agentix/runtime/shared/framing.py b/agentix/runtime/shared/framing.py
index c9820c2..80d3ab4 100644
--- a/agentix/runtime/shared/framing.py
+++ b/agentix/runtime/shared/framing.py
@@ -19,7 +19,7 @@
   ─── worker → runtime ─────────────────────────────────────
     ready        {}                               — sent once after worker startup
     boot_error   {error}                          — sent once if startup fails
-    result       {call_id, value}                 — call succeeded (value is pickle bytes)
+    result       {call_id, value}                 — call succeeded (value is msgpack bytes)
     error        {call_id, error}                 — call failed
     sio_open     {namespace}                      — open a side-channel namespace
     sio_emit     {namespace, event, data}         — emit side-channel data
@@ -32,7 +32,7 @@
 
 `callable` is an import-path `RemoteCallable` string
 (`module::qualname`); `arguments` is pickle.dumps((args, kwargs)); the
-worker pickles the return value back into `value`.
+worker msgpack-encodes the return value back into `value`.
 """
 
 from __future__ import annotations
diff --git a/agentix/runtime/shared/models.py b/agentix/runtime/shared/models.py
index 454ab2e..eac7c62 100644
--- a/agentix/runtime/shared/models.py
+++ b/agentix/runtime/shared/models.py
@@ -5,8 +5,8 @@
 and the worker subprocess. Both client and server import from here.
 
 Wire encoding: callable identity travels as an import path
-(`module::qualname`). Args, kwargs, and return values travel as stdlib
-pickle blobs so arbitrary Python values can cross the boundary.
+(`module::qualname`). Args/kwargs travel as stdlib pickle blobs, while
+return values travel as msgpack blobs via `agentix.runtime.shared.codec`.
 """
 
 from __future__ import annotations
@@ -57,7 +57,7 @@ class RemoteError(BaseModel):
 
 
 class RemoteResponse(BaseModel):
-    """Internal worker response. `value` is a pickle blob on success."""
+    """Internal worker response. `value` is a msgpack blob on success."""
 
     ok: bool
     value: bytes | None = None
diff --git a/docs/reference/architecture.mdx b/docs/reference/architecture.mdx
index 3b562a8..24d1716 100644
--- a/docs/reference/architecture.mdx
+++ b/docs/reference/architecture.mdx
@@ -90,7 +90,7 @@ arguments = pickle.dumps((args, kwargs))
 ```
 
 The runtime worker imports the module, resolves the function, unpickles
-the arguments, and invokes it. Return values travel back as pickle
+the arguments, and invokes it. Return values travel back as msgpack
 blobs inside `call:result`.
 
 ## Call Flow
@@ -102,8 +102,8 @@ blobs inside `call:result`.
 4. Client emits `call` on Socket.IO `/rpc`.
 5. Runtime server forwards the request to the worker subprocess.
 6. Worker imports `app`, resolves `run`, unpickles args, and calls it.
-7. Worker pickles the return value.
-8. Server emits `call:result`; client unpickles the value and returns it.
+7. Worker msgpack-encodes the return value.
+8. Server emits `call:result`; client msgpack-decodes the value and returns it.
 ```
 
 ## Transports
@@ -158,7 +158,7 @@ For each call, the worker:
 1. resolves the `RemoteCallable` import path
 2. unpickles `(args, kwargs)`
 3. calls the callable (awaiting when the return value is awaitable)
-4. pickles the return value
+4. msgpack-encodes the return value
 
 The worker also hosts the sandbox-side `agentix.sio` bridge. Plugin
 namespaces register inside the worker and forward events through
diff --git a/tests/runtime/client/test_main_module.py b/tests/runtime/client/test_main_module.py
index 7594047..5a2fe4c 100644
--- a/tests/runtime/client/test_main_module.py
+++ b/tests/runtime/client/test_main_module.py
@@ -17,16 +17,16 @@ async def test_remote_call_to_importable_module():
     from tests._user_app_target import add, greet
 
     try:
-        import pickle
+        from agentix.runtime.shared.codec import unpack
 
         resp = await mp.call(request_for(greet, kwargs={"name": "world"}))
         assert resp.ok, resp.error
-        assert pickle.loads(resp.value) == "hello world"
+        assert unpack(resp.value) == "hello world"
 
         # Second function on the same module should reuse the same worker.
         resp2 = await mp.call(request_for(add, kwargs={"a": 3, "b": 4}))
         assert resp2.ok, resp2.error
-        assert pickle.loads(resp2.value) == 7
+        assert unpack(resp2.value) == 7
 
     finally:
         await mp.shutdown()
diff --git a/tests/runtime/client/test_robustness.py b/tests/runtime/client/test_robustness.py
index b505642..60d66ed 100644
--- a/tests/runtime/client/test_robustness.py
+++ b/tests/runtime/client/test_robustness.py
@@ -8,6 +8,8 @@
 
 from __future__ import annotations
 
+import os
+import pickle
 import socket
 
 import pytest
@@ -18,10 +20,17 @@
     RuntimeClient,
     RuntimeUnreachable,
     WorkerExited,
+    _decode_result_value,
 )
+from agentix.runtime.shared.codec import pack
 from tests._worker_target import count_exec_and_sleep, self_sigkill
 
 
+class _SandboxPickleExploit:
+    def __reduce__(self):
+        return (os.putenv, ("AGENTIX_UNSAFE_PICKLE_TRIGGERED", "1"))
+
+
 @pytest.mark.asyncio
 async def test_call_timeout_when_call_exceeds_deadline(live_server):
     base_url = await live_server()
@@ -80,3 +89,15 @@ async def test_fail_pending_drains_queues_with_fatal_error():
         assert data is err
     finally:
         await client._client.aclose()
+
+
+def test_decode_result_value_accepts_msgpack_value() -> None:
+    assert _decode_result_value(pack({"ok": True})) == {"ok": True}
+
+
+def test_decode_result_value_rejects_pickle_exploit(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.delenv("AGENTIX_UNSAFE_PICKLE_TRIGGERED", raising=False)
+    payload = pickle.dumps(_SandboxPickleExploit())
+    with pytest.raises(RuntimeError, match="failed to decode sandbox return payload"):
+        _decode_result_value(payload)
+    assert os.environ.get("AGENTIX_UNSAFE_PICKLE_TRIGGERED") is None
diff --git a/tests/runtime/server/worker/test_subprocess.py b/tests/runtime/server/worker/test_subprocess.py
index 643a80f..b799fdb 100644
--- a/tests/runtime/server/worker/test_subprocess.py
+++ b/tests/runtime/server/worker/test_subprocess.py
@@ -17,6 +17,7 @@
 import pytest
 
 from agentix.runtime.server.worker import RuntimeWorkerClient
+from agentix.runtime.shared.codec import unpack
 from agentix.runtime.shared.models import RemoteRequest
 from tests import _worker_target as target
 from tests._rpc_helpers import request_for
@@ -34,8 +35,8 @@ async def test_subprocess_worker_round_trip():
     try:
         resp = await mp.call(request_for(target.echo, kwargs={"msg": "hi"}))
         assert resp.ok, resp.error
-        result = pickle.loads(resp.value)
-        assert result.msg == "echo:hi"
+        result = unpack(resp.value)
+        assert result["msg"] == "echo:hi"
     finally:
         await mp.shutdown()
 
@@ -53,7 +54,7 @@ async def test_subprocess_worker_handles_many_concurrent_calls():
         responses = await asyncio.gather(
             *(mp.call(request_for(target.echo, kwargs={"msg": str(i)})) for i in range(25))
         )
-        msgs = sorted(pickle.loads(r.value).msg for r in responses)
+        msgs = sorted(unpack(r.value)["msg"] for r in responses)
         assert msgs == sorted(f"echo:{i}" for i in range(25))
     finally:
         await mp.shutdown()
@@ -93,8 +94,8 @@ async def test_subprocess_worker_ignores_cwd_agentix_package(
     try:
         resp = await mp.call(request_for(target.echo, kwargs={"msg": "shadow"}))
         assert resp.ok, resp.error
-        result = pickle.loads(resp.value)
-        assert result.msg == "echo:shadow"
+        result = unpack(resp.value)
+        assert result["msg"] == "echo:shadow"
     finally:
         await mp.shutdown()
 
@@ -110,13 +111,13 @@ async def test_subprocess_worker_child_reading_stdin_does_not_steal_frames():
     try:
         r1 = await mp.call(request_for(target.spawn_stdin_reading_child))
         assert r1.ok, r1.error
-        assert pickle.loads(r1.value) == 0
+        assert unpack(r1.value) == 0
 
         # If the child had stolen frame bytes, the pipe would be desynced
         # and this call would hang or return garbage.
         r2 = await mp.call(request_for(target.echo, kwargs={"msg": "after"}))
         assert r2.ok, r2.error
-        assert pickle.loads(r2.value).msg == "echo:after"
+        assert unpack(r2.value)["msg"] == "echo:after"
     finally:
         await mp.shutdown()
 
@@ -179,7 +180,7 @@ async def test_subprocess_worker_respawns_after_death():
 
         r2 = await mp.call(request_for(target.echo, kwargs={"msg": "two"}))
         assert r2.ok, r2.error
-        assert pickle.loads(r2.value).msg == "echo:two"
+        assert unpack(r2.value)["msg"] == "echo:two"
         assert mp._worker is not worker1  # a fresh worker was spawned
 
         # The dead worker must be torn down on respawn, not leaked: its drain
diff --git a/tests/runtime/shared/test_models.py b/tests/runtime/shared/test_models.py
index ce196bb..31a2275 100644
--- a/tests/runtime/shared/test_models.py
+++ b/tests/runtime/shared/test_models.py
@@ -8,6 +8,7 @@
 
 from agentix.provider.base import SandboxConfig, SandboxResource
 from agentix.runtime.shared.callables import RemoteCallable
+from agentix.runtime.shared.codec import pack, unpack
 from agentix.runtime.shared.models import RemoteError, RemoteRequest, RemoteResponse
 
 BUNDLE_REF = "/cache/agentix/bundles/sha256-deadbeef"
@@ -33,9 +34,9 @@ def test_remote_callable_rejects_non_callable():
 
 
 def test_remote_response_ok_shape():
-    resp = RemoteResponse(ok=True, value=pickle.dumps({"x": 1}))
+    resp = RemoteResponse(ok=True, value=pack({"x": 1}))
     assert resp.error is None
-    assert pickle.loads(resp.value) == {"x": 1}
+    assert unpack(resp.value) == {"x": 1}
 
 
 def test_remote_response_error_shape():
diff --git a/tests/runtime/test_protocol.py b/tests/runtime/test_protocol.py
index c099f9e..301d060 100644
--- a/tests/runtime/test_protocol.py
+++ b/tests/runtime/test_protocol.py
@@ -55,10 +55,8 @@ async def _on_result(data):
         await sio.disconnect()
 
     assert payload["call_id"] == "call-ok"
-    import pickle
-
-    result = pickle.loads(payload["value"])
-    assert result.msg == "echo:hi"
+    result = unpack(payload["value"])
+    assert result["msg"] == "echo:hi"
 
 
 async def test_socketio_bad_callable_returns_error(use_inprocess_worker, live_server):
@@ -97,7 +95,7 @@ async def test_client_remote_round_trip(use_inprocess_worker, live_server):
     base_url = await live_server()
     async with RuntimeClient(base_url) as c:
         result = await c.remote(target.echo, msg="hello")
-    assert result.msg == "echo:hello"
+    assert result["msg"] == "echo:hello"
 
 
 async def test_client_remote_http_fast_path_falls_back_to_sio(use_inprocess_worker, live_server):
@@ -157,8 +155,7 @@ async def _on_result(data):
         await sio.disconnect()
 
     assert payload["call_id"] == call_id
-    import pickle as _pickle
-    assert _pickle.loads(payload["value"]) == 1, "fn must have run exactly once"
+    assert unpack(payload["value"]) == 1, "fn must have run exactly once"
 
 
 async def test_runtime_replays_unacked_result_after_reconnect(use_inprocess_worker, live_server):
@@ -210,8 +207,7 @@ async def _on_result(data):
         await sio_b.disconnect()
 
     assert payload["call_id"] == call_id
-    import pickle as _pickle
-    assert _pickle.loads(payload["value"]) == 1, "fn must run exactly once"
+    assert unpack(payload["value"]) == 1, "fn must run exactly once"
 
 
 async def test_client_remote_http_fallback_does_not_double_execute(use_inprocess_worker, live_server):