diff --git a/backend/chainlit/__init__.py b/backend/chainlit/__init__.py
index 99dd9d15b2..8b83cb0a5d 100644
--- a/backend/chainlit/__init__.py
+++ b/backend/chainlit/__init__.py
@@ -84,6 +84,7 @@
     on_message,
     on_settings_edit,
     on_settings_update,
+    on_shared_thread_access_allowed,
     on_shared_thread_view,
     on_slack_reaction_added,
     on_stop,
@@ -205,6 +206,7 @@ def acall(self):
     "on_message",
     "on_settings_edit",
     "on_settings_update",
+    "on_shared_thread_access_allowed",
     "on_shared_thread_view",
     "on_slack_reaction_added",
     "on_stop",
diff --git a/backend/chainlit/callbacks.py b/backend/chainlit/callbacks.py
index 0cdd209d3b..de021f443d 100644
--- a/backend/chainlit/callbacks.py
+++ b/backend/chainlit/callbacks.py
@@ -551,3 +551,18 @@ def on_shared_thread_view(
     """
     config.code.on_shared_thread_view = wrap_user_function(func)
     return func
+
+
+def on_shared_thread_access_allowed(
+    func: Callable[[ThreadDict, Optional[User]], Awaitable[bool]],
+) -> Callable[[ThreadDict, Optional[User]], Awaitable[bool]]:
+    """Hook to add extra permission check for viewing a shared thread.
+
+    Unlike on_shared_thread_view, this callback can only deny access further.
+    If defined and returns False, the viewer is blocked regardless of other checks.
+    If undefined or returns True, normal authorization flow proceeds.
+
+    Signature: async (thread: ThreadDict, viewer: Optional[User]) -> bool
+    """
+    config.code.on_shared_thread_access_allowed = wrap_user_function(func)
+    return func
diff --git a/backend/chainlit/config.py b/backend/chainlit/config.py
index a540752b33..b33950a6fe 100644
--- a/backend/chainlit/config.py
+++ b/backend/chainlit/config.py
@@ -422,6 +422,9 @@ class CodeSettings(BaseModel):
     on_shared_thread_view: Optional[
         Callable[["ThreadDict", Optional["User"]], Awaitable[bool]]
     ] = None
+    on_shared_thread_access_allowed: Optional[
+        Callable[["ThreadDict", Optional["User"]], Awaitable[bool]]
+    ] = None
     # Auth callbacks
     password_auth_callback: Optional[
         Callable[[str, str], Awaitable[Optional["User"]]]
diff --git a/backend/chainlit/server.py b/backend/chainlit/server.py
index cab8fef9e6..cd7aa4ce55 100644
--- a/backend/chainlit/server.py
+++ b/backend/chainlit/server.py
@@ -1024,13 +1024,23 @@ async def get_shared_thread(
             )
         except Exception:
             user_can_view = False
-
     is_shared = bool(metadata.get("is_shared"))
 
-    # Proceed only raise an error if both conditions are False.
     if (not user_can_view) and (not is_shared):
         raise HTTPException(status_code=404, detail="Thread not found")
 
+    if getattr(config.code, "on_shared_thread_access_allowed", None):
+        try:
+            access_allowed = await config.code.on_shared_thread_access_allowed(
+                thread, current_user
+            )
+            if not access_allowed:
+                raise HTTPException(status_code=404, detail="Thread not found")
+        except HTTPException:
+            raise
+        except Exception:
+            raise HTTPException(status_code=404, detail="Thread not found")
+
     metadata.pop("chat_profile", None)
     metadata.pop("chat_settings", None)
     metadata.pop("env", None)
diff --git a/backend/tests/test_server.py b/backend/tests/test_server.py
index 1441a3d8b2..468550d173 100644
--- a/backend/tests/test_server.py
+++ b/backend/tests/test_server.py
@@ -1152,3 +1152,140 @@ def test_health_check(test_client: TestClient):
     response = test_client.get("/health")
     assert response.status_code == 200
     assert response.json() == {"status": "ok"}
+
+
+@pytest.mark.parametrize(
+    ("is_shared", "on_shared_thread_view_result", "allowed"),
+    [
+        (True, True, True),
+        (True, False, True),
+        (True, ValueError("error"), True),
+        (False, True, True),
+        (False, False, False),
+    ],
+)
+def test_get_shared_thread_access(
+    test_client: TestClient,
+    test_config: ChainlitConfig,
+    is_shared: bool,
+    on_shared_thread_view_result: bool | Exception,
+    allowed: bool,
+):
+    """Check if shared thread access is allowed based on is_shared and on_shared_thread_view result."""
+    import chainlit.data as data_mod
+    from chainlit.server import app as _app, get_current_user as _get_current_user
+
+    viewer = PersistedUser(
+        id="viewer1",
+        createdAt=datetime.datetime.now().isoformat(),
+        identifier="viewer",
+    )
+    _app.dependency_overrides[_get_current_user] = lambda: viewer
+
+    dl = AsyncMock()
+    dl.get_thread.return_value = {
+        "id": "shared-thread-1",
+        "name": "Shared Thread",
+        "userIdentifier": "author",
+        "metadata": {"is_shared": is_shared, "chat_profile": "pro"},
+    }
+    dl.get_thread_author.return_value = "author"
+    dl.build_debug_url.return_value = ""
+
+    data_mod._data_layer = dl
+    data_mod._data_layer_initialized = True
+
+    async def deny_cb(thread, user):
+        if isinstance(on_shared_thread_view_result, Exception):
+            raise on_shared_thread_view_result
+        return on_shared_thread_view_result
+
+    test_config.code.on_shared_thread_view = deny_cb
+
+    r = test_client.get("/project/share/shared-thread-1")
+
+    if allowed:
+        assert r.status_code == 200
+        assert r.json()["id"] == "shared-thread-1"
+    else:
+        assert r.status_code == 404
+        assert r.json() == {"detail": "Thread not found"}
+
+    # Cleanup
+    del _app.dependency_overrides[_get_current_user]
+    data_mod._data_layer = None
+    data_mod._data_layer_initialized = False
+    test_config.code.on_shared_thread_view = None
+
+
+@pytest.mark.parametrize(
+    ("is_shared", "access_allowed_result", "allowed"),
+    [
+        (True, None, True),  # No callback → falls through to is_shared (=200)
+        (True, True, True),  # Callback allows → proceeds to return thread (=200)
+        (True, False, False),  # Callback denies → 404
+        (True, ValueError("err"), False),  # Callback raises → 404
+        (False, True, False),  # is_shared=False blocks
+    ],
+)
+def test_get_shared_thread_access_allowed(
+    test_client: TestClient,
+    test_config: ChainlitConfig,
+    is_shared: bool,
+    access_allowed_result: bool | Exception | None,
+    allowed: bool,
+):
+    """Check if shared thread access respects on_shared_thread_access_allowed callback."""
+    import chainlit.data as data_mod
+    from chainlit.server import app as _app, get_current_user as _get_current_user
+
+    viewer = PersistedUser(
+        id="viewer1",
+        createdAt=datetime.datetime.now().isoformat(),
+        identifier="viewer",
+    )
+    _app.dependency_overrides[_get_current_user] = lambda: viewer
+
+    dl = AsyncMock()
+    dl.get_thread.return_value = {
+        "id": "shared-thread-1",
+        "name": "Shared Thread",
+        "userIdentifier": "author",
+        "metadata": {"is_shared": is_shared, "chat_profile": "pro"},
+    }
+    dl.get_thread_author.return_value = "author"
+    dl.build_debug_url.return_value = ""
+
+    data_mod._data_layer = dl
+    data_mod._data_layer_initialized = True
+
+    # Ensure on_shared_thread_view is not set (Tier 1+2 relies on is_shared fallback)
+    test_config.code.on_shared_thread_view = None
+
+    if access_allowed_result is not None:
+
+        async def access_cb(thread, user):
+            if isinstance(access_allowed_result, Exception):
+                raise access_allowed_result
+            return access_allowed_result
+
+        test_config.code.on_shared_thread_access_allowed = access_cb
+    else:
+        # Callback not defined — Tier 3 is skipped
+        test_config.code.on_shared_thread_access_allowed = None
+
+    r = test_client.get("/project/share/shared-thread-1")
+
+    if allowed:
+        assert r.status_code == 200
+        assert r.json()["id"] == "shared-thread-1"
+    else:
+        assert r.status_code == 404
+        assert r.json() == {"detail": "Thread not found"}
+
+    # Cleanup
+    del _app.dependency_overrides[_get_current_user]
+    data_mod._data_layer = None
+    data_mod._data_layer_initialized = False
+    test_config.code.on_shared_thread_view = None
+    test_config.code.on_shared_thread_access_allowed = None