diff --git a/tests/tui_gateway/test_slash_worker_mcp.py b/tests/tui_gateway/test_slash_worker_mcp.py
index f3cd8d4df..7588f6a17 100644
--- a/tests/tui_gateway/test_slash_worker_mcp.py
+++ b/tests/tui_gateway/test_slash_worker_mcp.py
@@ -40,6 +40,18 @@ def test_slash_worker_sets_mcp_discovery_before_cli_import():
     )
 
 
+def test_slash_worker_overrides_parent_mcp_setting():
+    """slash_worker must force MCP discovery off, not inherit a parent override."""
+    import importlib
+    src = importlib.util.find_spec("tui_gateway.slash_worker")
+    assert src is not None and src.origin is not None
+    with open(src.origin) as fh:
+        source = fh.read()
+
+    assert 'os.environ["HERMES_MCP_DISCOVERY"] = "0"' in source
+    assert "setdefault" not in source
+
+
 def test_slash_worker_does_not_import_cli_at_module_level():
     """cli must NOT be imported at module scope -- it must be lazy in main()."""
     import importlib
@@ -82,3 +94,35 @@ def test_model_tools_runs_mcp_when_env_var_absent(monkeypatch):
     assert should_discover is True, (
         "MCP discovery should run when HERMES_MCP_DISCOVERY is not set"
     )
+
+
+def test_tui_server_forces_mcp_discovery_off_for_worker(monkeypatch):
+    """The slash worker subprocess must always inherit HERMES_MCP_DISCOVERY=0."""
+    import importlib
+    server = importlib.import_module("tui_gateway.server")
+
+    popen_kwargs = {}
+
+    class DummyProc:
+        stdin = None
+        stdout = None
+        stderr = None
+
+    class DummyThread:
+        def __init__(self, *args, **kwargs):
+            pass
+
+        def start(self):
+            pass
+
+    def fake_popen(*args, **kwargs):
+        popen_kwargs.update(kwargs)
+        return DummyProc()
+
+    monkeypatch.setenv("HERMES_MCP_DISCOVERY", "1")
+    monkeypatch.setattr(server.subprocess, "Popen", fake_popen)
+    monkeypatch.setattr(server.threading, "Thread", DummyThread)
+
+    server._SlashWorker("session-1", "")
+
+    assert popen_kwargs["env"]["HERMES_MCP_DISCOVERY"] == "0"
diff --git a/tui_gateway/server.py b/tui_gateway/server.py
index 7bc0fb2e0..26a409b73 100644
--- a/tui_gateway/server.py
+++ b/tui_gateway/server.py
@@ -185,6 +185,9 @@ class _SlashWorker:
         if model:
             argv += ["--model", model]
 
+        worker_env = os.environ.copy()
+        worker_env["HERMES_MCP_DISCOVERY"] = "0"
+
         self.proc = subprocess.Popen(
             argv,
             stdin=subprocess.PIPE,
@@ -193,7 +196,7 @@ class _SlashWorker:
             text=True,
             bufsize=1,
             cwd=os.getcwd(),
-            env=os.environ.copy(),
+            env=worker_env,
         )
         threading.Thread(target=self._drain_stdout, daemon=True).start()
         threading.Thread(target=self._drain_stderr, daemon=True).start()
diff --git a/tui_gateway/slash_worker.py b/tui_gateway/slash_worker.py
index b70bb6f7f..ae9d463a4 100644
--- a/tui_gateway/slash_worker.py
+++ b/tui_gateway/slash_worker.py
@@ -65,7 +65,7 @@ def main():
     # Suppress MCP discovery -- the TUI server already handles MCP servers;
     # importing cli triggers model_tools which calls discover_mcp_tools() at
     # module scope.  This env var tells model_tools to skip that step.
-    os.environ.setdefault("HERMES_MCP_DISCOVERY", "0")
+    os.environ["HERMES_MCP_DISCOVERY"] = "0"
 
     import cli as cli_mod
     from cli import HermesCLI