diff --git a/config.cron.yaml b/config.cron.yaml
new file mode 100644
index 00000000..074525ef
--- /dev/null
+++ b/config.cron.yaml
@@ -0,0 +1,19 @@
+# Cron/headless environment overlay
+# For scheduled jobs and autonomous agents
+model:
+  default: qwen3:30b
+  provider: custom
+agent:
+  max_turns: 100
+  verbose: false
+display:
+  compact: true
+  show_reasoning: false
+  streaming: false
+  resume_display: minimal
+terminal:
+  timeout: 300
+  persistent_shell: true
+memory:
+  memory_enabled: false
+  user_profile_enabled: false
diff --git a/config.dev.yaml b/config.dev.yaml
new file mode 100644
index 00000000..b6a9421a
--- /dev/null
+++ b/config.dev.yaml
@@ -0,0 +1,14 @@
+# Dev environment overlay
+# Merges with config.yaml — these keys override the base
+model:
+  default: qwen3:30b
+  provider: custom
+agent:
+  max_turns: 50
+  verbose: true
+display:
+  show_reasoning: true
+  streaming: true
+  show_cost: true
+terminal:
+  timeout: 300
diff --git a/config.gateway.yaml b/config.gateway.yaml
new file mode 100644
index 00000000..99ecaa85
--- /dev/null
+++ b/config.gateway.yaml
@@ -0,0 +1,17 @@
+# Gateway environment overlay
+# For messaging platform gateway (Telegram, Discord, etc.)
+model:
+  default: qwen3:30b
+  provider: custom
+agent:
+  max_turns: 30
+  verbose: false
+display:
+  compact: true
+  show_reasoning: false
+  streaming: false
+terminal:
+  timeout: 120
+memory:
+  memory_enabled: true
+  user_profile_enabled: true
diff --git a/config.prod.yaml b/config.prod.yaml
new file mode 100644
index 00000000..badac220
--- /dev/null
+++ b/config.prod.yaml
@@ -0,0 +1,17 @@
+# Prod environment overlay
+# Merges with config.yaml — these keys override the base
+model:
+  default: claude-opus-4-6
+  provider: anthropic
+agent:
+  max_turns: 90
+  verbose: false
+display:
+  compact: true
+  show_reasoning: false
+  streaming: false
+  show_cost: false
+privacy:
+  redact_pii: true
+security:
+  redact_secrets: true
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))
diff --git a/tests/test_config_overlay.py b/tests/test_config_overlay.py
new file mode 100644
index 00000000..dc1a92b6
--- /dev/null
+++ b/tests/test_config_overlay.py
@@ -0,0 +1,136 @@
+"""Tests for config overlay system."""
+import os
+import json
+import tempfile
+import yaml
+import pytest
+
+
+def test_deep_merge_dicts():
+    """Deep merge should recursively merge dicts."""
+    from config_overlay import deep_merge
+    base = {"a": {"b": 1, "c": 2}, "d": 3}
+    overlay = {"a": {"b": 10, "e": 5}}
+    result = deep_merge(base, overlay)
+    assert result == {"a": {"b": 10, "c": 2, "e": 5}, "d": 3}
+
+
+def test_deep_merge_lists_replaced():
+    """Lists should be replaced, not extended."""
+    from config_overlay import deep_merge
+    base = {"items": [1, 2, 3]}
+    overlay = {"items": [4, 5]}
+    result = deep_merge(base, overlay)
+    assert result == {"items": [4, 5]}
+
+
+def test_deep_merge_scalars_overridden():
+    """Scalar values should be overridden by overlay."""
+    from config_overlay import deep_merge
+    base = {"name": "base", "count": 10}
+    overlay = {"name": "override", "count": 20}
+    result = deep_merge(base, overlay)
+    assert result == {"name": "override", "count": 20}
+
+
+def test_deep_merge_none_removes_key():
+    """None in overlay should remove the key from base."""
+    from config_overlay import deep_merge
+    base = {"a": 1, "b": 2, "c": 3}
+    overlay = {"b": None}
+    result = deep_merge(base, overlay)
+    assert result == {"a": 1, "c": 3}
+
+
+def test_deep_merge_empty_overlay():
+    """Empty overlay should return base unchanged."""
+    from config_overlay import deep_merge
+    base = {"a": 1, "b": {"c": 2}}
+    result = deep_merge(base, {})
+    assert result == base
+
+
+def test_load_config_no_env(tmp_path):
+    """Load config without overlay should return base."""
+    from config_overlay import load_config
+    base = {"model": "test", "agent": {"max_turns": 10}}
+    path = tmp_path / "config.yaml"
+    path.write_text(yaml.dump(base))
+    result = load_config(str(path))
+    assert result == base
+
+
+def test_load_config_with_overlay(tmp_path):
+    """Load config with overlay should merge."""
+    from config_overlay import load_config
+    base = {"model": "base", "agent": {"max_turns": 10, "verbose": False}}
+    overlay = {"model": "override", "agent": {"verbose": True}}
+    (tmp_path / "config.yaml").write_text(yaml.dump(base))
+    (tmp_path / "config.dev.yaml").write_text(yaml.dump(overlay))
+    result = load_config(str(tmp_path / "config.yaml"), env="dev")
+    assert result["model"] == "override"
+    assert result["agent"]["max_turns"] == 10
+    assert result["agent"]["verbose"] is True
+
+
+def test_load_config_missing_overlay(tmp_path):
+    """Missing overlay should silently return base."""
+    from config_overlay import load_config
+    base = {"model": "base"}
+    (tmp_path / "config.yaml").write_text(yaml.dump(base))
+    result = load_config(str(tmp_path / "config.yaml"), env="nonexistent")
+    assert result == base
+
+
+def test_find_config(tmp_path):
+    """find_config should locate base and overlay."""
+    from config_overlay import find_config
+    base = tmp_path / "config.yaml"
+    base.write_text("a: 1")
+    overlay = tmp_path / "config.prod.yaml"
+    overlay.write_text("a: 2")
+    b, o = find_config(str(base), "prod")
+    assert b == base
+    assert o == overlay
+
+
+def test_list_overlays(tmp_path):
+    """list_overlays should find all overlay files."""
+    from config_overlay import list_overlays
+    (tmp_path / "config.yaml").write_text("a: 1")
+    (tmp_path / "config.dev.yaml").write_text("a: 2")
+    (tmp_path / "config.prod.yaml").write_text("a: 3")
+    overlays = list_overlays(str(tmp_path / "config.yaml"))
+    envs = [o['env'] for o in overlays]
+    assert 'dev' in envs
+    assert 'prod' in envs
+
+
+def test_detect_env_from_var(tmp_path, monkeypatch):
+    """detect_env should check TIMMY_ENV first."""
+    from config_overlay import detect_env
+    monkeypatch.setenv("TIMMY_ENV", "prod")
+    assert detect_env() == "prod"
+
+
+def test_detect_env_fallback(tmp_path, monkeypatch):
+    """detect_env should fall back through vars."""
+    from config_overlay import detect_env
+    monkeypatch.delenv("TIMMY_ENV", raising=False)
+    monkeypatch.delenv("HERMES_ENV", raising=False)
+    monkeypatch.setenv("ENVIRONMENT", "cron")
+    assert detect_env() == "cron"
+
+
+def test_real_config_overlay():
+    """Test against actual config files in the repo."""
+    import sys
+    sys.path.insert(0, os.path.dirname(os.path.dirname(__file__)))
+    from config_overlay import load_config
+
+    base_path = os.path.join(os.path.dirname(__file__), '..', 'config.yaml')
+    if os.path.exists(base_path):
+        config = load_config(base_path, env='dev')
+        assert config['model']['default'] == 'qwen3:30b'  # dev overrides
+        assert config['agent']['max_turns'] == 50  # dev overrides
+        assert 'terminal' in config  # base keys preserved