WebSockets

MCP Hangar provides a WebSocket endpoint for real-time streaming of domain events.

Endpoint

Connecting

Use any WebSocket client. Example with websocat:

# Stream all domain events
websocat ws://localhost:8000/api/ws/events

Event Stream (/api/ws/events)

Streams all domain events as they occur. Each message is a JSON object:

{
  "event_type": "McpServerStarted",
  "mcp_server_id": "math",
  "mode": "subprocess",
  "tools_count": 5,
  "startup_duration_ms": 120,
  "timestamp": "2026-03-23T10:15:30.123456"
}

Event Types

All events from domain/events.py are published:

• McpServerStateChanged -- State transition with old/new state.
• McpServerStarted -- MCP Server initialization complete.
• McpServerStopped -- MCP Server shut down.
• McpServerDegraded -- MCP Server marked as degraded.
• HealthCheckPassed / HealthCheckFailed -- Health check results.
• ToolInvocationCompleted / ToolInvocationFailed -- Tool call results.
• ProviderDiscovered / ProviderRegistered / ProviderDeregistered -- Discovery events.
• CircuitBreakerStateChanged -- Circuit breaker transitions.

Filtering

After connecting, send a JSON subscribe message to filter events:

{
  "type": "subscribe",
  "event_types": ["McpServerStateChanged"],
  "mcp_server_ids": ["math"]
}

The server acknowledges with:

{
  "type": "subscribed",
  "event_types": ["McpServerStateChanged"],
  "mcp_server_ids": ["math"]
}

You can update filters at any time by sending another subscribe message. Omit a key to not filter on that dimension (e.g., omit mcp_server_ids to receive events from all MCP servers).

Queue and Backpressure

Each WebSocket connection has an internal message queue (EventStreamQueue). If a client falls behind:

1. Messages queue up to the buffer limit.
2. Beyond the limit, oldest messages are dropped.
3. The client receives a warning frame indicating dropped messages.

Authentication

When auth is enabled, pass credentials via the initial HTTP upgrade headers:

websocat "ws://localhost:8000/api/ws/events" -H "X-API-Key: mcp_your_key_here"

Connection Lifecycle

1. Client sends WebSocket upgrade to /api/ws/events.
2. Server accepts the connection.
3. (Optional) Client sends a subscribe message to set filters.
4. Server streams matching events as JSON messages.
5. Client can send updated subscribe messages at any time.
6. On disconnect, the subscription is cleaned up automatically.