diff --git a/model_tools.py b/model_tools.py
index 36cea8f304..788b9dec68 100644
--- a/model_tools.py
+++ b/model_tools.py
@@ -137,12 +137,19 @@ def _run_async(coro):
 
 discover_builtin_tools()
 
-# MCP tool discovery (external MCP servers from config)
-try:
-    from tools.mcp_tool import discover_mcp_tools
-    discover_mcp_tools()
-except Exception as e:
-    logger.debug("MCP tool discovery failed: %s", e)
+# MCP tool discovery (external MCP servers from config).
+# Skipped when HERMES_MCP_DISCOVERY=0 -- the TUI slash_worker sets this because
+# the TUI server already handles MCP; running it again would spawn duplicate
+# ``hermes mcp serve`` children per session (#15275).
+import os as _os_mod
+if _os_mod.environ.get("HERMES_MCP_DISCOVERY") != "0":
+    try:
+        from tools.mcp_tool import discover_mcp_tools
+        discover_mcp_tools()
+    except Exception as e:
+        logger.debug("MCP tool discovery failed: %s", e)
+else:
+    logger.debug("MCP tool discovery suppressed (HERMES_MCP_DISCOVERY=0)")
 
 # Plugin tool discovery (user/project/pip plugins)
 try:
diff --git a/tests/tui_gateway/test_slash_worker_mcp.py b/tests/tui_gateway/test_slash_worker_mcp.py
new file mode 100644
index 0000000000..f3cd8d4df0
--- /dev/null
+++ b/tests/tui_gateway/test_slash_worker_mcp.py
@@ -0,0 +1,84 @@
+"""Verify the TUI slash_worker suppresses MCP discovery at import time.
+
+Regression test for #15275 -- without the HERMES_MCP_DISCOVERY=0 guard in
+slash_worker.py, importing the cli module triggers discover_mcp_tools() via
+model_tools, spawning duplicate ``hermes mcp serve`` children per session.
+"""
+
+import ast
+import os
+
+
+def test_slash_worker_sets_mcp_discovery_before_cli_import():
+    """slash_worker.main() must set HERMES_MCP_DISCOVERY=0 before importing cli."""
+    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:
+        lines = fh.readlines()
+
+    # Find the line that sets the env var and the first cli import.
+    env_line_idx = None
+    cli_import_idx = None
+    for i, line in enumerate(lines):
+        stripped = line.strip()
+        if "HERMES_MCP_DISCOVERY" in stripped and "environ" in stripped:
+            env_line_idx = i
+        # Only match actual import statements, not comments or strings
+        if cli_import_idx is None and stripped.startswith(("import cli", "from cli ")):
+            cli_import_idx = i
+
+    assert env_line_idx is not None, (
+        "slash_worker.py must set HERMES_MCP_DISCOVERY env var"
+    )
+    assert cli_import_idx is not None, (
+        "slash_worker.py must import cli"
+    )
+    assert env_line_idx < cli_import_idx, (
+        "HERMES_MCP_DISCOVERY must be set before cli is imported "
+        f"(env var at line {env_line_idx + 1}, cli import at line {cli_import_idx + 1})"
+    )
+
+
+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
+    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()
+
+    tree = ast.parse(source)
+    for node in ast.iter_child_nodes(tree):
+        if isinstance(node, ast.Import):
+            for alias in node.names:
+                assert not alias.name.startswith("cli"), (
+                    f"cli must not be imported at module level (line {node.lineno})"
+                )
+        elif isinstance(node, ast.ImportFrom):
+            assert node.module is None or not node.module.startswith("cli"), (
+                f"cli must not be imported at module level (line {node.lineno})"
+            )
+
+
+def test_model_tools_skips_mcp_when_env_var_set(monkeypatch):
+    """model_tools must skip discover_mcp_tools() when HERMES_MCP_DISCOVERY=0."""
+    monkeypatch.setenv("HERMES_MCP_DISCOVERY", "0")
+
+    assert os.environ.get("HERMES_MCP_DISCOVERY") == "0"
+
+    # Simulate the guard check from model_tools.py
+    should_discover = os.environ.get("HERMES_MCP_DISCOVERY") != "0"
+    assert should_discover is False, (
+        "MCP discovery should be suppressed when HERMES_MCP_DISCOVERY=0"
+    )
+
+
+def test_model_tools_runs_mcp_when_env_var_absent(monkeypatch):
+    """model_tools must run discover_mcp_tools() when env var is not set."""
+    monkeypatch.delenv("HERMES_MCP_DISCOVERY", raising=False)
+
+    should_discover = os.environ.get("HERMES_MCP_DISCOVERY") != "0"
+    assert should_discover is True, (
+        "MCP discovery should run when HERMES_MCP_DISCOVERY is not set"
+    )
diff --git a/tui_gateway/slash_worker.py b/tui_gateway/slash_worker.py
index 631b0c7045..b70bb6f7f0 100644
--- a/tui_gateway/slash_worker.py
+++ b/tui_gateway/slash_worker.py
@@ -1,6 +1,12 @@
-"""Persistent slash-command worker — one HermesCLI per TUI session.
+"""Persistent slash-command worker -- one HermesCLI per TUI session.
 
 Protocol: reads JSON lines from stdin {id, command}, writes {id, ok, output|error} to stdout.
+
+The slash_worker only needs CLI command processing, not MCP tool bootstrapping.
+The TUI server already handles MCP discovery and spawns hermes mcp serve
+children; without the HERMES_MCP_DISCOVERY guard both code paths independently
+spawn duplicate MCP serve processes per session (#15275).  The cli module is
+imported lazily inside main() so the env var is set before model_tools runs.
 """
 
 import argparse
@@ -10,12 +16,14 @@ import json
 import os
 import sys
 
-import cli as cli_mod
-from cli import HermesCLI
 from rich.console import Console
 
+# cli module reference -- populated lazily in main() after setting the
+# HERMES_MCP_DISCOVERY env var to prevent duplicate MCP serve children.
+_cli_mod = None
 
-def _run(cli: HermesCLI, command: str) -> str:
+
+def _run(cli, command: str) -> str:
     cmd = (command or "").strip()
     if not cmd:
         return ""
@@ -29,21 +37,23 @@ def _run(cli: HermesCLI, command: str) -> str:
     # underlying file to our buffer so self.console.print() is captured.
     cli.console = Console(file=buf, force_terminal=True, width=120)
 
-    old = getattr(cli_mod, "_cprint", None)
+    old = getattr(_cli_mod, "_cprint", None)
     if old is not None:
-        cli_mod._cprint = lambda text: print(text)
+        _cli_mod._cprint = lambda text: print(text)
 
     try:
         with contextlib.redirect_stdout(buf), contextlib.redirect_stderr(buf):
             cli.process_command(cmd)
     finally:
         if old is not None:
-            cli_mod._cprint = old
+            _cli_mod._cprint = old
 
     return buf.getvalue().rstrip()
 
 
 def main():
+    global _cli_mod
+
     p = argparse.ArgumentParser(add_help=False)
     p.add_argument("--session-key", required=True)
     p.add_argument("--model", default="")
@@ -52,6 +62,16 @@ def main():
     os.environ["HERMES_SESSION_KEY"] = args.session_key
     os.environ["HERMES_INTERACTIVE"] = "1"
 
+    # 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")
+
+    import cli as cli_mod
+    from cli import HermesCLI
+
+    _cli_mod = cli_mod
+
     with contextlib.redirect_stdout(io.StringIO()), contextlib.redirect_stderr(io.StringIO()):
         cli = HermesCLI(model=args.model or None, compact=True, resume=args.session_key, verbose=False)