diff --git a/micromamba/src/completer.cpp b/micromamba/src/completer.cpp
index 7895ce84a8..978b480f41 100644
--- a/micromamba/src/completer.cpp
+++ b/micromamba/src/completer.cpp
@@ -134,7 +134,6 @@ add_activate_completion(
 
     // Mock functions just for completion
     CLI::App* activate_subcom = app->add_subcommand("activate");
-    app->add_subcommand("deactivate");
     activate_subcom->callback(
         [app, &completer_args, &completed, &config]()
         {
diff --git a/micromamba/tests/test_completer.py b/micromamba/tests/test_completer.py
new file mode 100644
index 0000000000..2bcd6c3056
--- /dev/null
+++ b/micromamba/tests/test_completer.py
@@ -0,0 +1,59 @@
+import pytest
+
+from . import helpers
+
+
+def test_completer_lists_top_level_commands(tmp_home, tmp_root_prefix):
+    """Regression: duplicate deactivate must not abort completer (CLI::OptionAlreadyAdded)."""
+    umamba = helpers.get_umamba()
+    out = helpers.subprocess_run(umamba, "completer", "").decode()
+
+    for command in (
+        "activate",
+        "deactivate",
+        "create",
+        "install",
+        "env",
+        "shell",
+        "update",
+        "remove",
+        "list",
+        "info",
+    ):
+        assert command in out
+
+
+@pytest.mark.parametrize(
+    "prefix, expected",
+    [
+        ("a", ("activate", "auth")),
+        ("de", ("deactivate",)),
+        ("in", ("info", "install")),
+        ("rem", ("remove",)),
+        ("sh", ("shell",)),
+    ],
+)
+def test_completer_prefix_matches_commands(tmp_home, tmp_root_prefix, prefix, expected):
+    umamba = helpers.get_umamba()
+    out = helpers.subprocess_run(umamba, "completer", prefix).decode()
+    for command in expected:
+        assert command in out
+
+
+def test_completer_activate_lists_env_names(tmp_home, tmp_root_prefix, tmp_empty_env, tmp_env_name):
+    umamba = helpers.get_umamba()
+    out = helpers.subprocess_run(umamba, "completer", "-n", "").decode()
+    assert tmp_env_name in out
+
+
+def test_completer_shell_lists_subcommands(tmp_home, tmp_root_prefix):
+    umamba = helpers.get_umamba()
+    out = helpers.subprocess_run(umamba, "completer", "shell", "").decode()
+    for command in ("activate", "deactivate", "hook", "init"):
+        assert command in out
+
+
+def test_completer_install_option_prefix(tmp_home, tmp_root_prefix):
+    umamba = helpers.get_umamba()
+    out = helpers.subprocess_run(umamba, "completer", "install", "--c").decode()
+    assert "--channel" in out