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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+rtcstats-server is the server-side component of the rtcstats ecosystem that collects and processes WebRTC statistics from clients (browsers, Electron apps, React Native) and backend services (JVB, JIGASI, JICOFO). It receives real-time statistics over WebSocket connections, processes them through worker threads for feature extraction, and stores the results in configurable backends (AWS S3/DynamoDB or MongoDB/GridFS).
+
+## Development Commands
+
+### Running the server
+```bash
+npm start                    # Production mode
+npm run debug               # Debug mode
+npm run watch:dev           # Development mode with auto-reload
+```
+
+### Testing
+```bash
+npm test                    # Run all Jest tests
+npm run integration         # Run integration test client
+npm run test:fix            # Run extract tests with fix flag
+```
+
+### Linting
+```bash
+npm run lint                # Check for linting errors
+npm run lint:fix            # Auto-fix linting errors
+```
+
+## Configuration
+
+The server uses the [config](https://www.npmjs.com/package/config) module with YAML files in the `config/` directory:
+- `default.yaml` - Base configuration with all available options
+- `debug.yaml`, `production.yaml`, `test.yaml` - Environment-specific overrides
+- `custom-environment-variables.yaml` - Maps environment variables to config values
+
+Configuration is selected via the `NODE_ENV` environment variable (debug, production, test).
+
+### Key Configuration Options
+
+- `server.serviceType`: Either "AWS" or "MongoDB" - determines which backend services to initialize
+- `server.tempPath`: Directory for temporary dump files (default: "temp")
+- `server.port`: WebSocket server port (default: 3000)
+- `server.metrics`: Prometheus metrics endpoint port (default: 8095)
+- `features.disableFeatExtraction`: Disable feature extraction for testing
+
+## Architecture
+
+### Request Flow
+
+1. **WebSocket Connection** (`RTCStatsServer.js:wsConnectionHandler`)
+   - Client connects via WebSocket with protocol version in `Sec-WebSocket-Protocol` header
+   - Protocol format: `{version}_{type}` (e.g., "3.0_LEGACY", "3.0_STANDARD", "3.0_JVB")
+   - Connection metadata extracted by `ConnectionInformation` class
+
+2. **Stream Processing** (`demux.js`)
+   - Incoming JSON messages parsed and routed through `DemuxSink` stream
+   - Each `statsSessionId` maps to a separate file sink (one dump file per session)
+   - Handles four request types: `identity`, `stats-entry`, `close`, `keepalive`
+   - 60-second timeout closes inactive connections/sessions
+   - Dump files stored in `server.tempPath` directory
+
+3. **Feature Extraction** (`worker-pool/`)
+   - On session close, dump file queued for processing via `WorkerPool`
+   - Worker threads execute `ExtractWorker.js` to extract features from dump files
+   - `StandardFeatureExtractor` processes RTCSTATS client dumps
+   - `BackendFeatureExtractor` processes backend service (JVB, JIGASI, JICOFO) dumps
+   - Worker pool maintains configurable number of workers with automatic regeneration on crash
+
+4. **Persistence** (`store/`, `database/`)
+   - Features published to configured services (Amplitude, Firehose)
+   - Metadata saved via `MetadataStorageHandler` (DynamoDB or MongoDB)
+   - Dump files uploaded to storage (S3 or GridFS)
+   - Webhooks sent for JaaS tenants when configured
+
+### Service Initialization (`services/index.js`)
+
+The `setupServices()` function initializes different services based on `server.serviceType`:
+
+**AWS mode:**
+- `FeaturesPublisher` - Publishes to Amplitude and/or Firehose
+- `MetadataStorageHandler` - DynamoDB storage
+- `WebhookSender` - Sends webhooks with JWT auth from AWS Secrets Manager
+- `DumpStorage` - S3 storage
+
+**MongoDB mode:**
+- `MetadataStorageHandler` - MongoDB storage
+- `DumpStorage` - GridFS storage
+
+### Key Components
+
+- **DemuxSink** (`demux.js`) - Stream that multiplexes multiple stat sessions over a single WebSocket, writing each session to a separate dump file
+- **WorkerPool** (`worker-pool/WorkerPool.js`) - Manages worker threads for CPU-intensive feature extraction, keeps pool at target size even on worker crashes
+- **QualityStatsCollector** (`features/quality-stats/`) - Extracts WebRTC quality metrics from getstats data
+- **PromCollector** (`metrics/PromCollector.js`) - Prometheus metrics collection
+
+### WebSocket Protocol (v3.0)
+
+See `API.md` for full protocol details. Key points:
+
+- All messages are JSON with required fields: `statsSessionId`, `type`, and optional `data`
+- `identity` request establishes session metadata (conference, user, app)
+- `stats-entry` contains WebRTC statistics (data must be stringified)
+- `close` triggers feature extraction and persistence
+- `keepalive` prevents 60-second timeout
+
+### Reconnect Handling
+
+Files with duplicate `statsSessionId` are saved with incremental suffix: `{id}_{i}` (e.g., `abc123_1`, `abc123_2`). The `MetadataStorageHandler.saveEntryAssureUnique()` method handles conflicts at the storage level using the same convention.
+
+### Orphan Dump Processing
+
+On startup, `setupWorkDirectory()` processes any dump files left in `server.tempPath` from previous runs, queuing them for feature extraction.
+
+## Testing Patterns
+
+Tests use Jest with mocks in `src/test/mock/` and `src/store/test/mock/`:
+- Unit tests typically mock AWS SDK, MongoDB, and logging modules
+- `test-utils.js` provides common test utilities
+- Integration test client available via `npm run integration`
+
+When implementing tests, place them in the appropriate directory:
+- Feature-specific tests: `src/test/jest/`
+- Storage tests: `src/store/test/jest/`
diff --git a/src/demux.js b/src/demux.js
index ba129a0..d619125 100644
--- a/src/demux.js
+++ b/src/demux.js
@@ -238,6 +238,11 @@ class DemuxSink extends Writable {
             throw new Error('[Demux] statsSessionId missing from request!');
         }
 
+        if (typeof statsSessionId !== 'string' || !/^[a-zA-Z0-9_-]+$/.test(statsSessionId)) {
+            throw new Error('[Demux] statsSessionId contains invalid characters.'
+                + ' Only alphanumerics, hyphens, and underscores are allowed.');
+        }
+
         if (!type) {
             throw new Error('[Demux] type missing from request!');
         }