diff --git a/tests/hermes_cli/test_set_config_value.py b/tests/hermes_cli/test_set_config_value.py
new file mode 100644
index 0000000000..35e885b575
--- /dev/null
+++ b/tests/hermes_cli/test_set_config_value.py
@@ -0,0 +1,118 @@
+"""Tests for set_config_value — verifying secrets route to .env and config to config.yaml."""
+
+import os
+from pathlib import Path
+from unittest.mock import patch, call
+
+import pytest
+
+from hermes_cli.config import set_config_value
+
+
+@pytest.fixture(autouse=True)
+def _isolated_hermes_home(tmp_path):
+    """Point HERMES_HOME at a temp dir so tests never touch real config."""
+    env_file = tmp_path / ".env"
+    env_file.touch()
+    with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+        yield tmp_path
+
+
+def _read_env(tmp_path):
+    return (tmp_path / ".env").read_text()
+
+
+def _read_config(tmp_path):
+    config_path = tmp_path / "config.yaml"
+    return config_path.read_text() if config_path.exists() else ""
+
+
+# ---------------------------------------------------------------------------
+# Explicit allowlist keys → .env
+# ---------------------------------------------------------------------------
+
+class TestExplicitAllowlist:
+    """Keys in the hardcoded allowlist should always go to .env."""
+
+    @pytest.mark.parametrize("key", [
+        "OPENROUTER_API_KEY",
+        "OPENAI_API_KEY",
+        "ANTHROPIC_API_KEY",
+        "NOUS_API_KEY",
+        "WANDB_API_KEY",
+        "TINKER_API_KEY",
+        "HONCHO_API_KEY",
+        "FIRECRAWL_API_KEY",
+        "BROWSERBASE_API_KEY",
+        "FAL_KEY",
+        "SUDO_PASSWORD",
+        "GITHUB_TOKEN",
+        "TELEGRAM_BOT_TOKEN",
+        "DISCORD_BOT_TOKEN",
+        "SLACK_BOT_TOKEN",
+        "SLACK_APP_TOKEN",
+    ])
+    def test_explicit_key_routes_to_env(self, key, _isolated_hermes_home):
+        set_config_value(key, "test-value-123")
+        env_content = _read_env(_isolated_hermes_home)
+        assert f"{key}=test-value-123" in env_content
+        # Must NOT appear in config.yaml
+        assert key not in _read_config(_isolated_hermes_home)
+
+
+# ---------------------------------------------------------------------------
+# Catch-all patterns → .env
+# ---------------------------------------------------------------------------
+
+class TestCatchAllPatterns:
+    """Any key ending in _API_KEY or _TOKEN should route to .env."""
+
+    @pytest.mark.parametrize("key", [
+        "DAYTONA_API_KEY",
+        "ELEVENLABS_API_KEY",
+        "SOME_FUTURE_SERVICE_API_KEY",
+        "MY_CUSTOM_TOKEN",
+        "WHATSAPP_BOT_TOKEN",
+    ])
+    def test_api_key_suffix_routes_to_env(self, key, _isolated_hermes_home):
+        set_config_value(key, "secret-456")
+        env_content = _read_env(_isolated_hermes_home)
+        assert f"{key}=secret-456" in env_content
+        assert key not in _read_config(_isolated_hermes_home)
+
+    def test_case_insensitive(self, _isolated_hermes_home):
+        """Keys should be uppercased regardless of input casing."""
+        set_config_value("openai_api_key", "sk-test")
+        env_content = _read_env(_isolated_hermes_home)
+        assert "OPENAI_API_KEY=sk-test" in env_content
+
+    def test_terminal_ssh_prefix_routes_to_env(self, _isolated_hermes_home):
+        set_config_value("TERMINAL_SSH_PORT", "2222")
+        env_content = _read_env(_isolated_hermes_home)
+        assert "TERMINAL_SSH_PORT=2222" in env_content
+
+
+# ---------------------------------------------------------------------------
+# Non-secret keys → config.yaml
+# ---------------------------------------------------------------------------
+
+class TestConfigYamlRouting:
+    """Regular config keys should go to config.yaml, NOT .env."""
+
+    def test_simple_key(self, _isolated_hermes_home):
+        set_config_value("model", "gpt-4o")
+        config = _read_config(_isolated_hermes_home)
+        assert "gpt-4o" in config
+        assert "model" not in _read_env(_isolated_hermes_home)
+
+    def test_nested_key(self, _isolated_hermes_home):
+        set_config_value("terminal.backend", "docker")
+        config = _read_config(_isolated_hermes_home)
+        assert "docker" in config
+        assert "terminal" not in _read_env(_isolated_hermes_home)
+
+    def test_terminal_image_goes_to_config(self, _isolated_hermes_home):
+        """TERMINAL_DOCKER_IMAGE doesn't match _API_KEY or _TOKEN, so config.yaml."""
+        set_config_value("terminal.docker_image", "python:3.12")
+        config = _read_config(_isolated_hermes_home)
+        assert "python:3.12" in config