diff --git a/config_overlay.py b/config_overlay.py
new file mode 100644
index 00000000..cb3510e2
--- /dev/null
+++ b/config_overlay.py
@@ -0,0 +1,130 @@
+#!/usr/bin/env python3
+"""
+config_overlay.py — Environment-specific config overlays.
+
+Merges a base config YAML with an environment-specific overlay.
+Overlay keys override base keys (deep merge for dicts, replace for lists).
+
+Usage:
+    from config_overlay import load_config
+    config = load_config('config.yaml', env='prod')
+    # Loads config.yaml + config.prod.yaml, merges, returns dict
+
+Environments: dev, prod, cron, gateway (or custom)
+"""
+import os
+import copy
+import yaml
+from pathlib import Path
+
+
+def deep_merge(base: dict, overlay: dict) -> dict:
+    """Deep merge overlay into base. Overlay wins on conflicts.
+
+    - Dicts are merged recursively
+    - Lists are replaced (not extended)
+    - Scalars are replaced
+    - None in overlay removes the key from base
+    """
+    result = copy.deepcopy(base)
+    for key, value in overlay.items():
+        if value is None:
+            result.pop(key, None)
+        elif key in result and isinstance(result[key], dict) and isinstance(value, dict):
+            result[key] = deep_merge(result[key], value)
+        else:
+            result[key] = copy.deepcopy(value)
+    return result
+
+
+def find_config(base_path: str, env: str = None) -> tuple:
+    """Find base and overlay config paths.
+
+    Returns: (base_path, overlay_path_or_None)
+    """
+    base = Path(base_path)
+    if not base.exists():
+        raise FileNotFoundError(f"Base config not found: {base_path}")
+
+    overlay = None
+    if env:
+        # Try: config.{env}.yaml, config/{env}.yaml, {env}.overlay.yaml
+        candidates = [
+            base.parent / f"{base.stem}.{env}{base.suffix}",
+            base.parent / env / f"{base.name}",
+            base.parent / f"{env}.overlay{base.suffix}",
+        ]
+        for c in candidates:
+            if c.exists():
+                overlay = c
+                break
+
+    return base, overlay
+
+
+def load_config(base_path: str, env: str = None) -> dict:
+    """Load base config and merge with environment overlay.
+
+    Args:
+        base_path: Path to base config YAML
+        env: Environment name (dev, prod, cron, gateway, or custom)
+
+    Returns:
+        Merged config dict
+    """
+    base_file, overlay_file = find_config(base_path, env)
+
+    with open(base_file) as f:
+        base_config = yaml.safe_load(f) or {}
+
+    if overlay_file and overlay_file.exists():
+        with open(overlay_file) as f:
+            overlay_config = yaml.safe_load(f) or {}
+        return deep_merge(base_config, overlay_config)
+
+    return base_config
+
+
+def detect_env() -> str:
+    """Auto-detect environment from env vars.
+
+    Checks: TIMMY_ENV, HERMES_ENV, ENVIRONMENT, NODE_ENV
+    """
+    for var in ('TIMMY_ENV', 'HERMES_ENV', 'ENVIRONMENT', 'NODE_ENV'):
+        val = os.environ.get(var, '').strip().lower()
+        if val:
+            return val
+    return None
+
+
+def load_config_auto(base_path: str) -> dict:
+    """Load config with auto-detected environment."""
+    env = detect_env()
+    return load_config(base_path, env=env)
+
+
+def list_overlays(base_path: str) -> list:
+    """List available overlay files for a base config."""
+    base = Path(base_path)
+    pattern = f"{base.stem}.*{base.suffix}"
+    overlays = []
+    for f in sorted(base.parent.glob(pattern)):
+        if f == base:
+            continue
+        env_name = f.stem.replace(base.stem + '.', '')
+        overlays.append({
+            'env': env_name,
+            'path': str(f),
+        })
+    return overlays
+
+
+if __name__ == '__main__':
+    import sys
+    import json
+
+    base = sys.argv[1] if len(sys.argv) > 1 else 'config.yaml'
+    env = sys.argv[2] if len(sys.argv) > 2 else None
+
+    config = load_config(base, env=env)
+    print(json.dumps(config, indent=2, default=str))