diff --git a/packages/claude-code-plugin/hooks/lib/runtime_mode.py b/packages/claude-code-plugin/hooks/lib/runtime_mode.py
new file mode 100644
index 00000000..a6af9c65
--- /dev/null
+++ b/packages/claude-code-plugin/hooks/lib/runtime_mode.py
@@ -0,0 +1,40 @@
+"""Runtime mode detection — MCP vs standalone.
+
+Checks ~/.claude/mcp.json for codingbuddy MCP server entry.
+Used by prompt_injection, user-prompt-submit, and health_check.
+"""
+import json
+import os
+from typing import Optional
+
+
+def detect_runtime_mode(home_dir: Optional[str] = None) -> str:
+    """Detect if CodingBuddy MCP server is configured.
+
+    Checks ~/.claude/mcp.json for an entry containing 'codingbuddy'.
+
+    Args:
+        home_dir: Override home directory (for testing).
+
+    Returns:
+        'mcp' if codingbuddy MCP server found, 'standalone' otherwise.
+    """
+    home = home_dir or os.path.expanduser("~")
+    mcp_json_path = os.path.join(home, ".claude", "mcp.json")
+
+    try:
+        with open(mcp_json_path, "r", encoding="utf-8") as f:
+            data = json.load(f)
+
+        mcp_servers = data.get("mcpServers", {})
+        for key in mcp_servers:
+            if "codingbuddy" in key.lower():
+                return "mcp"
+        return "standalone"
+    except (OSError, json.JSONDecodeError, TypeError):
+        return "standalone"
+
+
+def is_mcp_available(home_dir: Optional[str] = None) -> bool:
+    """Convenience wrapper: True if MCP mode detected."""
+    return detect_runtime_mode(home_dir) == "mcp"
diff --git a/packages/claude-code-plugin/tests/test_runtime_mode.py b/packages/claude-code-plugin/tests/test_runtime_mode.py
new file mode 100644
index 00000000..b4e67a97
--- /dev/null
+++ b/packages/claude-code-plugin/tests/test_runtime_mode.py
@@ -0,0 +1,74 @@
+"""Tests for runtime_mode detection utility."""
+import json
+import os
+
+import pytest
+
+from hooks.lib.runtime_mode import detect_runtime_mode, is_mcp_available
+
+
+@pytest.fixture()
+def fake_home(tmp_path):
+    """Create a fake home directory with .claude/ structure."""
+    claude_dir = tmp_path / ".claude"
+    claude_dir.mkdir()
+    return tmp_path
+
+
+def _write_mcp_json(home_path, data):
+    mcp_path = os.path.join(str(home_path), ".claude", "mcp.json")
+    with open(mcp_path, "w", encoding="utf-8") as f:
+        json.dump(data, f)
+
+
+class TestDetectRuntimeMode:
+    def test_returns_mcp_when_codingbuddy_entry_exists(self, fake_home):
+        _write_mcp_json(fake_home, {
+            "mcpServers": {
+                "codingbuddy": {"command": "npx", "args": ["codingbuddy"]}
+            }
+        })
+        assert detect_runtime_mode(str(fake_home)) == "mcp"
+
+    def test_returns_mcp_for_codingbuddy_rules_entry(self, fake_home):
+        """Case-insensitive match on key containing 'codingbuddy'."""
+        _write_mcp_json(fake_home, {
+            "mcpServers": {
+                "CodingBuddy-Rules": {"command": "npx", "args": ["codingbuddy-rules"]}
+            }
+        })
+        assert detect_runtime_mode(str(fake_home)) == "mcp"
+
+    def test_returns_standalone_without_codingbuddy(self, fake_home):
+        _write_mcp_json(fake_home, {
+            "mcpServers": {
+                "other-server": {"command": "npx", "args": ["other"]}
+            }
+        })
+        assert detect_runtime_mode(str(fake_home)) == "standalone"
+
+    def test_returns_standalone_when_mcp_json_missing(self, tmp_path):
+        assert detect_runtime_mode(str(tmp_path)) == "standalone"
+
+    def test_returns_standalone_on_parse_error(self, fake_home):
+        mcp_path = os.path.join(str(fake_home), ".claude", "mcp.json")
+        with open(mcp_path, "w", encoding="utf-8") as f:
+            f.write("{invalid json!!!")
+        assert detect_runtime_mode(str(fake_home)) == "standalone"
+
+    def test_returns_standalone_when_mcp_servers_missing(self, fake_home):
+        _write_mcp_json(fake_home, {"otherKey": "value"})
+        assert detect_runtime_mode(str(fake_home)) == "standalone"
+
+
+class TestIsMcpAvailable:
+    def test_returns_true_when_mcp(self, fake_home):
+        _write_mcp_json(fake_home, {
+            "mcpServers": {
+                "codingbuddy": {"command": "npx"}
+            }
+        })
+        assert is_mcp_available(str(fake_home)) is True
+
+    def test_returns_false_when_standalone(self, tmp_path):
+        assert is_mcp_available(str(tmp_path)) is False