diff --git a/src/builtin_mcp.py b/src/builtin_mcp.py
index 2a4b748ee..b777e99a4 100644
--- a/src/builtin_mcp.py
+++ b/src/builtin_mcp.py
@@ -104,6 +104,21 @@ def _spawn_bg(coro) -> asyncio.Task:
     return task
 
 
+def builtin_python_env(base_dir: str) -> dict[str, str]:
+    """Environment for built-in Python MCP subprocesses.
+
+    The app root must be importable so mcp_servers can import local modules, but
+    replacing PYTHONPATH entirely hides site-packages in container/dev launches
+    that rely on PYTHONPATH for their active environment.
+    """
+    existing = os.environ.get("PYTHONPATH", "")
+    parts = [base_dir]
+    for item in existing.split(os.pathsep):
+        if item and item not in parts:
+            parts.append(item)
+    return {"PYTHONPATH": os.pathsep.join(parts)}
+
+
 async def register_builtin_servers(mcp_manager):
     """Connect all built-in MCP servers to the manager."""
     if MCP_DISABLED:
@@ -121,7 +136,7 @@ async def register_builtin_servers(mcp_manager):
                 transport="stdio",
                 command=python,
                 args=[script_path],
-                env={"PYTHONPATH": base_dir},
+                env=builtin_python_env(base_dir),
             )
             if ok:
                 logger.info(f"Built-in MCP server registered: {name}")
diff --git a/src/mcp_manager.py b/src/mcp_manager.py
index 8f4322375..90430c78f 100644
--- a/src/mcp_manager.py
+++ b/src/mcp_manager.py
@@ -504,7 +504,7 @@ class McpManager:
     async def _reconnect_builtin(self, server_id: str) -> bool:
         """Tear down and reconnect a crashed builtin MCP server."""
         import sys
-        from src.builtin_mcp import _BUILTIN_SERVERS
+        from src.builtin_mcp import _BUILTIN_SERVERS, builtin_python_env
 
         if server_id not in _BUILTIN_SERVERS:
             return False
@@ -523,7 +523,7 @@ class McpManager:
                 transport="stdio",
                 command=sys.executable,
                 args=[script_path],
-                env={"PYTHONPATH": base_dir},
+                env=builtin_python_env(base_dir),
             )
             if ok:
                 logger.info(f"Reconnected builtin MCP server: {name}")
diff --git a/tests/test_builtin_mcp_pythonpath.py b/tests/test_builtin_mcp_pythonpath.py
new file mode 100644
index 000000000..9400fd35c
--- /dev/null
+++ b/tests/test_builtin_mcp_pythonpath.py
@@ -0,0 +1,22 @@
+import os
+
+from src.builtin_mcp import builtin_python_env
+
+
+def test_builtin_python_env_preserves_existing_pythonpath(monkeypatch):
+    monkeypatch.setenv(
+        "PYTHONPATH",
+        os.pathsep.join(["/app/venv/lib/python3.13/site-packages", "/app", "/extra"]),
+    )
+
+    env = builtin_python_env("/app")
+
+    assert env == {
+        "PYTHONPATH": os.pathsep.join(["/app", "/app/venv/lib/python3.13/site-packages", "/extra"])
+    }
+
+
+def test_builtin_python_env_uses_app_root_without_existing_pythonpath(monkeypatch):
+    monkeypatch.delenv("PYTHONPATH", raising=False)
+
+    assert builtin_python_env("/srv/odysseus") == {"PYTHONPATH": "/srv/odysseus"}