docs: add integration tests and documentation for config override system

Add merge_configs function that performs root-level merging of custom
config into default config. Custom config sections completely replace
default sections. Implementation does not mutate input dictionaries.

Includes comprehensive tests for:
- Empty custom config
- Section override behavior
- Adding new sections
- Non-mutating behavior

All 7 tests pass.

API_REFERENCE.md

@@ -729,6 +729,29 @@ Server loads model definitions from configuration file (default: `configs/defaul
 - `openai_base_url` - Optional custom API endpoint
 - `openai_api_key` - Optional model-specific API key
 
+### Configuration Override System
+
+**Default config:** `/app/configs/default_config.json` (baked into image)
+
+**Custom config:** `/app/user-configs/config.json` (optional, via volume mount)
+
+**Merge behavior:**
+- Custom config sections completely replace default sections (root-level merge)
+- If no custom config exists, defaults are used
+- Validation occurs at container startup (before API starts)
+- Invalid config causes immediate exit with detailed error message
+
+**Example custom config** (overrides models only):
+```json
+{
+  "models": [
+    {"name": "gpt-5", "basemodel": "openai/gpt-5", "signature": "gpt-5", "enabled": true}
+  ]
+}
+```
+
+All other sections (`agent_config`, `log_config`, etc.) inherited from default.
+
 ---
 
 ## OpenAPI / Swagger Documentation

QUICK_START.md

@@ -54,7 +54,36 @@ JINA_API_KEY=your-jina-key-here
 
 ---
 
-## Step 3: Start the API Server
+## Step 3: (Optional) Custom Model Configuration
+
+To use different AI models than the defaults, create a custom config:
+
+1. Create config directory:
+   ```bash
+   mkdir -p configs
+   ```
+
+2. Create `configs/config.json`:
+   ```json
+   {
+     "models": [
+       {
+         "name": "my-gpt-4",
+         "basemodel": "openai/gpt-4",
+         "signature": "my-gpt-4",
+         "enabled": true
+       }
+     ]
+   }
+   ```
+
+3. The Docker container will automatically merge this with default settings.
+
+Your custom config only needs to include sections you want to override.
+
+---
+
+## Step 4: Start the API Server
 
 ```bash
 docker-compose up -d
@@ -79,7 +108,7 @@ docker logs -f ai-trader-server
 
 ---
 
-## Step 4: Verify Service is Running
+## Step 5: Verify Service is Running
 
 ```bash
 curl http://localhost:8080/health
@@ -99,7 +128,7 @@ If you see `"status": "healthy"`, you're ready!
 
 ---
 
-## Step 5: Run Your First Simulation
+## Step 6: Run Your First Simulation
 
 Trigger a simulation for a single day with GPT-4:
 
@@ -130,7 +159,7 @@ curl -X POST http://localhost:8080/simulate/trigger \
 
 ---
 
-## Step 6: Monitor Progress
+## Step 7: Monitor Progress
 
 ```bash
 # Replace with your job_id from Step 5
@@ -175,7 +204,7 @@ curl http://localhost:8080/simulate/status/$JOB_ID
 
 ---
 
-## Step 7: View Results
+## Step 8: View Results
 
 ```bash
 curl "http://localhost:8080/results?job_id=$JOB_ID" | jq '.'

docker-compose.yml

@@ -8,7 +8,8 @@ services:
     volumes:
       - ${VOLUME_PATH:-.}/data:/app/data
       - ${VOLUME_PATH:-.}/logs:/app/logs
-      - ${VOLUME_PATH:-.}/configs:/app/configs
+      # User configs mounted to /app/user-configs (default config baked into image)
+      - ${VOLUME_PATH:-.}/configs:/app/user-configs
     environment:
       # Deployment Configuration
       - DEPLOYMENT_MODE=${DEPLOYMENT_MODE:-PROD}

docs/plans/2025-11-01-config-override-system-design.md

@@ -0,0 +1,249 @@
+# Configuration Override System Design
+
+**Date:** 2025-11-01
+**Status:** Approved
+**Context:** Enable per-deployment model configuration while maintaining sensible defaults
+
+## Problem
+
+Deployments need to customize model configurations without modifying the image's default config. Currently, the API looks for `configs/default_config.json` at startup, but volume mounts that include custom configs would overwrite the default config baked into the image.
+
+## Solution Overview
+
+Implement a layered configuration system where:
+- Default config is baked into the Docker image
+- User config is provided via volume mount in a separate directory
+- Configs are merged at container startup (before API starts)
+- Validation failures cause immediate container exit
+
+## Architecture
+
+### File Locations
+
+- **Default config (in image):** `/app/configs/default_config.json`
+- **User config (mounted):** `/app/user-configs/config.json`
+- **Merged output:** `/tmp/runtime_config.json`
+
+### Startup Sequence
+
+1. **Entrypoint phase** (before uvicorn):
+   - Load `configs/default_config.json` from image
+   - Check if `user-configs/config.json` exists
+   - If exists: perform root-level merge (custom sections override default sections)
+   - Validate merged config structure
+   - If validation fails: log detailed error and `exit 1`
+   - Write merged config to `/tmp/runtime_config.json`
+   - Export `CONFIG_PATH=/tmp/runtime_config.json`
+
+2. **API initialization:**
+   - Load pre-validated config from `$CONFIG_PATH`
+   - No runtime config validation needed (already validated)
+
+### Merge Behavior
+
+**Root-level merge:** Custom config sections completely replace default sections.
+
+```python
+default = load_json("configs/default_config.json")
+custom = load_json("user-configs/config.json") if exists else {}
+
+merged = {**default}
+for key in custom:
+    merged[key] = custom[key]  # Override entire section
+```
+
+**Examples:**
+- Custom has `models` array → entire models array replaced
+- Custom has `agent_config` → entire agent_config replaced
+- Custom missing `date_range` → default date_range used
+- Custom has unknown keys → passed through (validated in next step)
+
+### Validation Rules
+
+**Structure validation:**
+- Required top-level keys: `agent_type`, `models`, `agent_config`, `log_config`
+- `date_range` is optional (can be overridden by API request params)
+- `models` must be an array with at least one entry
+- Each model must have: `name`, `basemodel`, `signature`, `enabled`
+
+**Model validation:**
+- At least one model must have `enabled: true`
+- Model signatures must be unique
+- No duplicate model names
+
+**Date validation (if date_range present):**
+- Dates match `YYYY-MM-DD` format
+- `init_date` <= `end_date`
+- Dates are not in the future
+
+**Agent config validation:**
+- `max_steps` > 0
+- `max_retries` >= 0
+- `initial_cash` > 0
+
+### Error Handling
+
+**Validation failure output:**
+```
+❌ CONFIG VALIDATION FAILED
+━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+Error: Missing required field 'models'
+Location: Root level
+File: user-configs/config.json
+
+Merged config written to: /tmp/runtime_config.json (for debugging)
+
+Container will exit. Fix config and restart.
+```
+
+**Benefits of fail-fast approach:**
+- No silent config errors during API calls
+- Clear feedback on what's wrong
+- Container restart loop until config is fixed
+- Health checks fail immediately (container never reaches "running" state with bad config)
+
+## Implementation Components
+
+### New Files
+
+**`tools/config_merger.py`**
+```python
+def load_config(path: str) -> dict:
+    """Load and parse JSON with error handling"""
+
+def merge_configs(default: dict, custom: dict) -> dict:
+    """Root-level merge - custom sections override default"""
+
+def validate_config(config: dict) -> None:
+    """Validate structure, raise detailed exception on failure"""
+
+def merge_and_validate() -> None:
+    """Main entrypoint - load, merge, validate, write to /tmp"""
+```
+
+### Updated Files
+
+**`entrypoint.sh`**
+```bash
+# After MCP service startup, before uvicorn
+echo "🔧 Merging and validating configuration..."
+python -c "from tools.config_merger import merge_and_validate; merge_and_validate()" || exit 1
+export CONFIG_PATH=/tmp/runtime_config.json
+echo "✅ Configuration validated"
+
+exec uvicorn api.main:app ...
+```
+
+**`docker-compose.yml`**
+```yaml
+volumes:
+  - ./data:/app/data
+  - ./logs:/app/logs
+  - ./configs:/app/user-configs  # User's config.json (not /app/configs!)
+```
+
+**`api/main.py`**
+- Keep existing `CONFIG_PATH` env var support (already implemented)
+- Remove any config validation from request handlers (now done at startup)
+
+### Documentation Updates
+
+- **`docs/DOCKER.md`** - Explain user-configs volume mount and config.json structure
+- **`QUICK_START.md`** - Show minimal config.json example
+- **`API_REFERENCE.md`** - Note that config errors fail at startup, not during API calls
+- **`CLAUDE.md`** - Update configuration section with new merge behavior
+
+## User Experience
+
+### Minimal Custom Config Example
+
+```json
+{
+  "models": [
+    {
+      "name": "my-gpt-4",
+      "basemodel": "openai/gpt-4",
+      "signature": "my-gpt-4",
+      "enabled": true
+    }
+  ]
+}
+```
+
+All other settings (`agent_config`, `log_config`, etc.) inherited from default.
+
+### Complete Custom Config Example
+
+```json
+{
+  "agent_type": "BaseAgent",
+  "date_range": {
+    "init_date": "2025-10-01",
+    "end_date": "2025-10-31"
+  },
+  "models": [
+    {
+      "name": "claude-sonnet-4",
+      "basemodel": "anthropic/claude-sonnet-4",
+      "signature": "claude-sonnet-4",
+      "enabled": true
+    }
+  ],
+  "agent_config": {
+    "max_steps": 50,
+    "max_retries": 5,
+    "base_delay": 2.0,
+    "initial_cash": 100000.0
+  },
+  "log_config": {
+    "log_path": "./data/agent_data"
+  }
+}
+```
+
+All sections replaced, no inheritance from default.
+
+## Backward Compatibility
+
+**If no `user-configs/config.json` exists:**
+- System uses `configs/default_config.json` as-is
+- No merging needed
+- Existing behavior preserved
+
+**Breaking change:**
+- Deployments currently mounting to `/app/configs` must update to `/app/user-configs`
+- Migration: Update docker-compose.yml volume mount path
+
+## Security Considerations
+
+- Default config in image is read-only (immutable)
+- User config directory is writable (mounted volume)
+- Merged config in `/tmp` is ephemeral (recreated on restart)
+- API keys in user config are not logged during validation errors
+
+## Testing Strategy
+
+**Unit tests (`tests/unit/test_config_merger.py`):**
+- Merge behavior with various override combinations
+- Validation catches all error conditions
+- Error messages are clear and actionable
+
+**Integration tests:**
+- Container startup with valid user config
+- Container startup with invalid user config (should exit 1)
+- Container startup with no user config (uses default)
+- API requests use merged config correctly
+
+**Manual testing:**
+- Deploy with minimal config.json (only models)
+- Deploy with complete config.json (all sections)
+- Deploy with invalid config.json (verify error output)
+- Deploy with no config.json (verify default behavior)
+
+## Future Enhancements
+
+- Deep merge support (merge within sections, not just root-level)
+- Config schema validation using JSON Schema
+- Support for multiple config files (e.g., base + environment + deployment)
+- Hot reload on config file changes (SIGHUP handler)

entrypoint.sh

@@ -41,7 +41,16 @@ echo "📊 Initializing database..."
 python -c "from api.database import initialize_database; initialize_database('data/jobs.db')"
 echo "✅ Database initialized"
 
-# Step 2: Start MCP services in background
+# Step 2: Merge and validate configuration
+echo "🔧 Merging and validating configuration..."
+python -c "from tools.config_merger import merge_and_validate; merge_and_validate()" || {
+    echo "❌ Configuration validation failed"
+    exit 1
+}
+export CONFIG_PATH=/tmp/runtime_config.json
+echo "✅ Configuration validated and merged"
+
+# Step 3: Start MCP services in background
 echo "🔧 Starting MCP services..."
 cd /app
 python agent_tools/start_mcp_services.py &
@@ -50,11 +59,11 @@ MCP_PID=$!
 # Setup cleanup trap before starting uvicorn
 trap "echo '🛑 Stopping services...'; kill $MCP_PID 2>/dev/null; exit 0" EXIT SIGTERM SIGINT
 
-# Step 3: Wait for services to initialize
+# Step 4: Wait for services to initialize
 echo "⏳ Waiting for MCP services to start..."
 sleep 3
 
-# Step 4: Start FastAPI server with uvicorn (this blocks)
+# Step 5: Start FastAPI server with uvicorn (this blocks)
 # Note: Container always uses port 8080 internally
 # The API_PORT env var only affects the host port mapping in docker-compose.yml
 echo "🌐 Starting FastAPI server on port 8080..."

tests/integration/test_config_override.py

@@ -0,0 +1,121 @@
+"""Integration tests for config override system."""
+
+import pytest
+import json
+import subprocess
+import tempfile
+from pathlib import Path
+
+
+@pytest.fixture
+def test_configs(tmp_path):
+    """Create test config files."""
+    # Default config
+    default_config = {
+        "agent_type": "BaseAgent",
+        "date_range": {"init_date": "2025-10-01", "end_date": "2025-10-21"},
+        "models": [
+            {"name": "default-model", "basemodel": "openai/gpt-4", "signature": "default", "enabled": True}
+        ],
+        "agent_config": {"max_steps": 30, "max_retries": 3, "base_delay": 1.0, "initial_cash": 10000.0},
+        "log_config": {"log_path": "./data/agent_data"}
+    }
+
+    configs_dir = tmp_path / "configs"
+    configs_dir.mkdir()
+
+    default_path = configs_dir / "default_config.json"
+    with open(default_path, 'w') as f:
+        json.dump(default_config, f, indent=2)
+
+    return configs_dir, default_config
+
+
+def test_config_override_models_only(test_configs):
+    """Test overriding only the models section."""
+    configs_dir, default_config = test_configs
+
+    # Custom config - only override models
+    custom_config = {
+        "models": [
+            {"name": "gpt-5", "basemodel": "openai/gpt-5", "signature": "gpt-5", "enabled": True}
+        ]
+    }
+
+    user_configs_dir = configs_dir.parent / "user-configs"
+    user_configs_dir.mkdir()
+
+    custom_path = user_configs_dir / "config.json"
+    with open(custom_path, 'w') as f:
+        json.dump(custom_config, f, indent=2)
+
+    # Run merge
+    result = subprocess.run(
+        [
+            "python", "-c",
+            f"import sys; sys.path.insert(0, '.'); "
+            f"from tools.config_merger import DEFAULT_CONFIG_PATH, CUSTOM_CONFIG_PATH, OUTPUT_CONFIG_PATH, merge_and_validate; "
+            f"import tools.config_merger; "
+            f"tools.config_merger.DEFAULT_CONFIG_PATH = '{configs_dir}/default_config.json'; "
+            f"tools.config_merger.CUSTOM_CONFIG_PATH = '{custom_path}'; "
+            f"tools.config_merger.OUTPUT_CONFIG_PATH = '{configs_dir.parent}/runtime.json'; "
+            f"merge_and_validate()"
+        ],
+        capture_output=True,
+        text=True,
+        cwd="/home/bballou/AI-Trader/.worktrees/config-override-system"
+    )
+
+    assert result.returncode == 0, f"Merge failed: {result.stderr}"
+
+    # Verify merged config
+    runtime_path = configs_dir.parent / "runtime.json"
+    with open(runtime_path, 'r') as f:
+        merged = json.load(f)
+
+    # Models should be overridden
+    assert merged["models"] == custom_config["models"]
+
+    # Other sections should be from default
+    assert merged["agent_config"] == default_config["agent_config"]
+    assert merged["date_range"] == default_config["date_range"]
+
+
+def test_config_validation_fails_gracefully(test_configs):
+    """Test that invalid config causes exit with clear error."""
+    configs_dir, _ = test_configs
+
+    # Invalid custom config (no enabled models)
+    custom_config = {
+        "models": [
+            {"name": "test", "basemodel": "openai/gpt-4", "signature": "test", "enabled": False}
+        ]
+    }
+
+    user_configs_dir = configs_dir.parent / "user-configs"
+    user_configs_dir.mkdir()
+
+    custom_path = user_configs_dir / "config.json"
+    with open(custom_path, 'w') as f:
+        json.dump(custom_config, f, indent=2)
+
+    # Run merge (should fail)
+    result = subprocess.run(
+        [
+            "python", "-c",
+            f"import sys; sys.path.insert(0, '.'); "
+            f"from tools.config_merger import merge_and_validate; "
+            f"import tools.config_merger; "
+            f"tools.config_merger.DEFAULT_CONFIG_PATH = '{configs_dir}/default_config.json'; "
+            f"tools.config_merger.CUSTOM_CONFIG_PATH = '{custom_path}'; "
+            f"tools.config_merger.OUTPUT_CONFIG_PATH = '{configs_dir.parent}/runtime.json'; "
+            f"merge_and_validate()"
+        ],
+        capture_output=True,
+        text=True,
+        cwd="/home/bballou/AI-Trader/.worktrees/config-override-system"
+    )
+
+    assert result.returncode == 1
+    assert "CONFIG VALIDATION FAILED" in result.stderr
+    assert "At least one model must be enabled" in result.stderr

tests/unit/test_config_merger.py

@@ -0,0 +1,293 @@
+import pytest
+import json
+import tempfile
+from pathlib import Path
+from tools.config_merger import load_config, ConfigValidationError, merge_configs, validate_config
+
+
+def test_load_config_valid_json():
+    """Test loading a valid JSON config file"""
+    with tempfile.NamedTemporaryFile(mode='w', suffix='.json', delete=False) as f:
+        json.dump({"key": "value"}, f)
+        temp_path = f.name
+
+    try:
+        result = load_config(temp_path)
+        assert result == {"key": "value"}
+    finally:
+        Path(temp_path).unlink()
+
+
+def test_load_config_file_not_found():
+    """Test loading non-existent config file"""
+    with pytest.raises(ConfigValidationError, match="not found"):
+        load_config("/nonexistent/path.json")
+
+
+def test_load_config_invalid_json():
+    """Test loading malformed JSON"""
+    with tempfile.NamedTemporaryFile(mode='w', suffix='.json', delete=False) as f:
+        f.write("{invalid json")
+        temp_path = f.name
+
+    try:
+        with pytest.raises(ConfigValidationError, match="Invalid JSON"):
+            load_config(temp_path)
+    finally:
+        Path(temp_path).unlink()
+
+
+def test_merge_configs_empty_custom():
+    """Test merge with no custom config"""
+    default = {"a": 1, "b": 2}
+    custom = {}
+    result = merge_configs(default, custom)
+    assert result == {"a": 1, "b": 2}
+
+
+def test_merge_configs_override_section():
+    """Test custom config overrides entire sections"""
+    default = {
+        "models": [{"name": "default-model", "enabled": True}],
+        "agent_config": {"max_steps": 30}
+    }
+    custom = {
+        "models": [{"name": "custom-model", "enabled": False}]
+    }
+    result = merge_configs(default, custom)
+
+    assert result["models"] == [{"name": "custom-model", "enabled": False}]
+    assert result["agent_config"] == {"max_steps": 30}
+
+
+def test_merge_configs_add_new_section():
+    """Test custom config adds new sections"""
+    default = {"a": 1}
+    custom = {"b": 2}
+    result = merge_configs(default, custom)
+    assert result == {"a": 1, "b": 2}
+
+
+def test_merge_configs_does_not_mutate_inputs():
+    """Test merge doesn't modify original dicts"""
+    default = {"a": 1}
+    custom = {"a": 2}
+    result = merge_configs(default, custom)
+
+    assert default["a"] == 1  # Original unchanged
+    assert result["a"] == 2
+
+
+def test_validate_config_valid():
+    """Test validation passes for valid config"""
+    config = {
+        "agent_type": "BaseAgent",
+        "models": [
+            {"name": "test", "basemodel": "openai/gpt-4", "signature": "test", "enabled": True}
+        ],
+        "agent_config": {
+            "max_steps": 30,
+            "max_retries": 3,
+            "initial_cash": 10000.0
+        },
+        "log_config": {"log_path": "./data"}
+    }
+
+    validate_config(config)  # Should not raise
+
+
+def test_validate_config_missing_required_field():
+    """Test validation fails for missing required field"""
+    config = {"agent_type": "BaseAgent"}  # Missing models, agent_config, log_config
+
+    with pytest.raises(ConfigValidationError, match="Missing required field"):
+        validate_config(config)
+
+
+def test_validate_config_no_enabled_models():
+    """Test validation fails when no models are enabled"""
+    config = {
+        "agent_type": "BaseAgent",
+        "models": [
+            {"name": "test", "basemodel": "openai/gpt-4", "signature": "test", "enabled": False}
+        ],
+        "agent_config": {"max_steps": 30, "max_retries": 3, "initial_cash": 10000.0},
+        "log_config": {"log_path": "./data"}
+    }
+
+    with pytest.raises(ConfigValidationError, match="At least one model must be enabled"):
+        validate_config(config)
+
+
+def test_validate_config_duplicate_signatures():
+    """Test validation fails for duplicate model signatures"""
+    config = {
+        "agent_type": "BaseAgent",
+        "models": [
+            {"name": "test1", "basemodel": "openai/gpt-4", "signature": "same", "enabled": True},
+            {"name": "test2", "basemodel": "openai/gpt-5", "signature": "same", "enabled": True}
+        ],
+        "agent_config": {"max_steps": 30, "max_retries": 3, "initial_cash": 10000.0},
+        "log_config": {"log_path": "./data"}
+    }
+
+    with pytest.raises(ConfigValidationError, match="Duplicate model signature"):
+        validate_config(config)
+
+
+def test_validate_config_invalid_max_steps():
+    """Test validation fails for invalid max_steps"""
+    config = {
+        "agent_type": "BaseAgent",
+        "models": [{"name": "test", "basemodel": "openai/gpt-4", "signature": "test", "enabled": True}],
+        "agent_config": {"max_steps": 0, "max_retries": 3, "initial_cash": 10000.0},
+        "log_config": {"log_path": "./data"}
+    }
+
+    with pytest.raises(ConfigValidationError, match="max_steps must be > 0"):
+        validate_config(config)
+
+
+def test_validate_config_invalid_date_format():
+    """Test validation fails for invalid date format"""
+    config = {
+        "agent_type": "BaseAgent",
+        "date_range": {"init_date": "2025-13-01", "end_date": "2025-12-31"},  # Invalid month
+        "models": [{"name": "test", "basemodel": "openai/gpt-4", "signature": "test", "enabled": True}],
+        "agent_config": {"max_steps": 30, "max_retries": 3, "initial_cash": 10000.0},
+        "log_config": {"log_path": "./data"}
+    }
+
+    with pytest.raises(ConfigValidationError, match="Invalid date format"):
+        validate_config(config)
+
+
+def test_validate_config_end_before_init():
+    """Test validation fails when end_date before init_date"""
+    config = {
+        "agent_type": "BaseAgent",
+        "date_range": {"init_date": "2025-12-31", "end_date": "2025-01-01"},
+        "models": [{"name": "test", "basemodel": "openai/gpt-4", "signature": "test", "enabled": True}],
+        "agent_config": {"max_steps": 30, "max_retries": 3, "initial_cash": 10000.0},
+        "log_config": {"log_path": "./data"}
+    }
+
+    with pytest.raises(ConfigValidationError, match="init_date must be <= end_date"):
+        validate_config(config)
+
+
+import os
+from tools.config_merger import merge_and_validate
+
+
+def test_merge_and_validate_success(tmp_path, monkeypatch):
+    """Test successful merge and validation"""
+    # Create default config
+    default_config = {
+        "agent_type": "BaseAgent",
+        "models": [{"name": "default", "basemodel": "openai/gpt-4", "signature": "default", "enabled": True}],
+        "agent_config": {"max_steps": 30, "max_retries": 3, "initial_cash": 10000.0},
+        "log_config": {"log_path": "./data"}
+    }
+
+    default_path = tmp_path / "default_config.json"
+    with open(default_path, 'w') as f:
+        json.dump(default_config, f)
+
+    # Create custom config (only overrides models)
+    custom_config = {
+        "models": [{"name": "custom", "basemodel": "openai/gpt-5", "signature": "custom", "enabled": True}]
+    }
+
+    custom_path = tmp_path / "config.json"
+    with open(custom_path, 'w') as f:
+        json.dump(custom_config, f)
+
+    output_path = tmp_path / "runtime_config.json"
+
+    # Mock file paths
+    monkeypatch.setattr("tools.config_merger.DEFAULT_CONFIG_PATH", str(default_path))
+    monkeypatch.setattr("tools.config_merger.CUSTOM_CONFIG_PATH", str(custom_path))
+    monkeypatch.setattr("tools.config_merger.OUTPUT_CONFIG_PATH", str(output_path))
+
+    # Run merge and validate
+    merge_and_validate()
+
+    # Verify output file was created
+    assert output_path.exists()
+
+    # Verify merged content
+    with open(output_path, 'r') as f:
+        result = json.load(f)
+
+    assert result["models"] == [{"name": "custom", "basemodel": "openai/gpt-5", "signature": "custom", "enabled": True}]
+    assert result["agent_config"] == {"max_steps": 30, "max_retries": 3, "initial_cash": 10000.0}
+
+
+def test_merge_and_validate_no_custom_config(tmp_path, monkeypatch):
+    """Test when no custom config exists (uses default only)"""
+    default_config = {
+        "agent_type": "BaseAgent",
+        "models": [{"name": "default", "basemodel": "openai/gpt-4", "signature": "default", "enabled": True}],
+        "agent_config": {"max_steps": 30, "max_retries": 3, "initial_cash": 10000.0},
+        "log_config": {"log_path": "./data"}
+    }
+
+    default_path = tmp_path / "default_config.json"
+    with open(default_path, 'w') as f:
+        json.dump(default_config, f)
+
+    custom_path = tmp_path / "config.json"  # Does not exist
+    output_path = tmp_path / "runtime_config.json"
+
+    monkeypatch.setattr("tools.config_merger.DEFAULT_CONFIG_PATH", str(default_path))
+    monkeypatch.setattr("tools.config_merger.CUSTOM_CONFIG_PATH", str(custom_path))
+    monkeypatch.setattr("tools.config_merger.OUTPUT_CONFIG_PATH", str(output_path))
+
+    merge_and_validate()
+
+    # Verify output matches default
+    with open(output_path, 'r') as f:
+        result = json.load(f)
+
+    assert result == default_config
+
+
+def test_merge_and_validate_validation_fails(tmp_path, monkeypatch, capsys):
+    """Test validation failure exits with error"""
+    default_config = {
+        "agent_type": "BaseAgent",
+        "models": [{"name": "default", "basemodel": "openai/gpt-4", "signature": "default", "enabled": True}],
+        "agent_config": {"max_steps": 30, "max_retries": 3, "initial_cash": 10000.0},
+        "log_config": {"log_path": "./data"}
+    }
+
+    default_path = tmp_path / "default_config.json"
+    with open(default_path, 'w') as f:
+        json.dump(default_config, f)
+
+    # Custom config with no enabled models
+    custom_config = {
+        "models": [{"name": "custom", "basemodel": "openai/gpt-5", "signature": "custom", "enabled": False}]
+    }
+
+    custom_path = tmp_path / "config.json"
+    with open(custom_path, 'w') as f:
+        json.dump(custom_config, f)
+
+    output_path = tmp_path / "runtime_config.json"
+
+    monkeypatch.setattr("tools.config_merger.DEFAULT_CONFIG_PATH", str(default_path))
+    monkeypatch.setattr("tools.config_merger.CUSTOM_CONFIG_PATH", str(custom_path))
+    monkeypatch.setattr("tools.config_merger.OUTPUT_CONFIG_PATH", str(output_path))
+
+    # Should exit with error
+    with pytest.raises(SystemExit) as exc_info:
+        merge_and_validate()
+
+    assert exc_info.value.code == 1
+
+    # Check error output (should be in stderr, not stdout)
+    captured = capsys.readouterr()
+    assert "CONFIG VALIDATION FAILED" in captured.err
+    assert "At least one model must be enabled" in captured.err

tools/config_merger.py

@@ -0,0 +1,228 @@
+"""Configuration merging and validation for AI-Trader."""
+
+import json
+import sys
+from pathlib import Path
+from typing import Dict, Any, Optional
+from datetime import datetime
+
+
+class ConfigValidationError(Exception):
+    """Raised when config validation fails."""
+    pass
+
+
+def load_config(path: str) -> Dict[str, Any]:
+    """
+    Load and parse JSON config file.
+
+    Args:
+        path: Path to JSON config file
+
+    Returns:
+        Parsed config dictionary
+
+    Raises:
+        ConfigValidationError: If file not found or invalid JSON
+    """
+    config_path = Path(path)
+
+    if not config_path.exists():
+        raise ConfigValidationError(f"Config file not found: {path}")
+
+    try:
+        with open(config_path, 'r') as f:
+            return json.load(f)
+    except json.JSONDecodeError as e:
+        raise ConfigValidationError(f"Invalid JSON in {path}: {e}")
+
+
+def merge_configs(default: Dict[str, Any], custom: Dict[str, Any]) -> Dict[str, Any]:
+    """
+    Merge custom config into default config (root-level override).
+
+    Custom config sections completely replace default sections.
+    Does not mutate input dictionaries.
+
+    Args:
+        default: Default configuration dict
+        custom: Custom configuration dict (overrides)
+
+    Returns:
+        Merged configuration dict
+    """
+    merged = dict(default)  # Shallow copy
+
+    for key, value in custom.items():
+        merged[key] = value
+
+    return merged
+
+
+def validate_config(config: Dict[str, Any]) -> None:
+    """
+    Validate configuration structure and values.
+
+    Args:
+        config: Configuration dictionary to validate
+
+    Raises:
+        ConfigValidationError: If validation fails with detailed message
+    """
+    # Required top-level fields
+    required_fields = ["agent_type", "models", "agent_config", "log_config"]
+    for field in required_fields:
+        if field not in config:
+            raise ConfigValidationError(f"Missing required field: '{field}'")
+
+    # Validate models
+    models = config["models"]
+    if not isinstance(models, list) or len(models) == 0:
+        raise ConfigValidationError("'models' must be a non-empty array")
+
+    # Check at least one enabled model
+    enabled_models = [m for m in models if m.get("enabled", False)]
+    if not enabled_models:
+        raise ConfigValidationError("At least one model must be enabled")
+
+    # Check required model fields
+    for i, model in enumerate(models):
+        required_model_fields = ["name", "basemodel", "signature", "enabled"]
+        for field in required_model_fields:
+            if field not in model:
+                raise ConfigValidationError(
+                    f"Model {i} missing required field: '{field}'"
+                )
+
+    # Check for duplicate signatures
+    signatures = [m["signature"] for m in models]
+    if len(signatures) != len(set(signatures)):
+        duplicates = [s for s in signatures if signatures.count(s) > 1]
+        raise ConfigValidationError(
+            f"Duplicate model signature: {duplicates[0]}"
+        )
+
+    # Validate agent_config
+    agent_config = config["agent_config"]
+
+    if "max_steps" in agent_config:
+        if agent_config["max_steps"] <= 0:
+            raise ConfigValidationError("max_steps must be > 0")
+
+    if "max_retries" in agent_config:
+        if agent_config["max_retries"] < 0:
+            raise ConfigValidationError("max_retries must be >= 0")
+
+    if "initial_cash" in agent_config:
+        if agent_config["initial_cash"] <= 0:
+            raise ConfigValidationError("initial_cash must be > 0")
+
+    # Validate date_range if present (optional)
+    if "date_range" in config:
+        date_range = config["date_range"]
+
+        if "init_date" in date_range:
+            try:
+                init_dt = datetime.strptime(date_range["init_date"], "%Y-%m-%d")
+            except ValueError:
+                raise ConfigValidationError(
+                    f"Invalid date format for init_date: {date_range['init_date']}. "
+                    "Expected YYYY-MM-DD"
+                )
+
+        if "end_date" in date_range:
+            try:
+                end_dt = datetime.strptime(date_range["end_date"], "%Y-%m-%d")
+            except ValueError:
+                raise ConfigValidationError(
+                    f"Invalid date format for end_date: {date_range['end_date']}. "
+                    "Expected YYYY-MM-DD"
+                )
+
+        # Check init <= end
+        if "init_date" in date_range and "end_date" in date_range:
+            if init_dt > end_dt:
+                raise ConfigValidationError(
+                    f"init_date must be <= end_date (got {date_range['init_date']} > {date_range['end_date']})"
+                )
+
+
+# File path constants (can be overridden for testing)
+DEFAULT_CONFIG_PATH = "configs/default_config.json"
+CUSTOM_CONFIG_PATH = "user-configs/config.json"
+OUTPUT_CONFIG_PATH = "/tmp/runtime_config.json"
+
+
+def format_error_message(error: str, location: str, file: str) -> str:
+    """Format validation error for display."""
+    border = "━" * 60
+    return f"""
+❌ CONFIG VALIDATION FAILED
+{border}
+
+Error: {error}
+Location: {location}
+File: {file}
+
+Merged config written to: {OUTPUT_CONFIG_PATH} (for debugging)
+
+Container will exit. Fix config and restart.
+"""
+
+
+def merge_and_validate() -> None:
+    """
+    Main entry point for config merging and validation.
+
+    Loads default config, optionally merges custom config,
+    validates the result, and writes to output path.
+
+    Exits with code 1 on any error.
+    """
+    try:
+        # Load default config
+        print(f"📄 Loading default config from {DEFAULT_CONFIG_PATH}")
+        default_config = load_config(DEFAULT_CONFIG_PATH)
+
+        # Load custom config if exists
+        custom_config = {}
+        if Path(CUSTOM_CONFIG_PATH).exists():
+            print(f"📝 Loading custom config from {CUSTOM_CONFIG_PATH}")
+            custom_config = load_config(CUSTOM_CONFIG_PATH)
+        else:
+            print(f"ℹ️  No custom config found at {CUSTOM_CONFIG_PATH}, using defaults")
+
+        # Merge configs
+        print("🔧 Merging configurations...")
+        merged_config = merge_configs(default_config, custom_config)
+
+        # Write merged config (for debugging even if validation fails)
+        output_path = Path(OUTPUT_CONFIG_PATH)
+        output_path.parent.mkdir(parents=True, exist_ok=True)
+
+        with open(output_path, 'w') as f:
+            json.dump(merged_config, f, indent=2)
+
+        # Validate merged config
+        print("✅ Validating merged configuration...")
+        validate_config(merged_config)
+
+        print(f"✅ Configuration validated successfully")
+        print(f"📦 Merged config written to {OUTPUT_CONFIG_PATH}")
+
+    except ConfigValidationError as e:
+        # Determine which file caused the error
+        error_file = CUSTOM_CONFIG_PATH if Path(CUSTOM_CONFIG_PATH).exists() else DEFAULT_CONFIG_PATH
+
+        error_msg = format_error_message(
+            error=str(e),
+            location="Root level",
+            file=error_file
+        )
+
+        print(error_msg, file=sys.stderr)
+        sys.exit(1)
+
+    except Exception as e:
+        print(f"❌ Unexpected error during config processing: {e}", file=sys.stderr)
+        sys.exit(1)