Log Streaming

MCP Hangar captures stderr output from subprocess and Docker MCP servers and makes it available via REST API.

Architecture

MCP Server Process (stderr)
        |
   stderr-reader thread
        |
   ProviderLogBuffer (ring buffer, 1000 lines)
        |
   GET /api/mcp_servers/{id}/logs
        |
     REST API

Each MCP server gets a dedicated ProviderLogBuffer -- a thread-safe ring buffer holding the most recent 1000 log lines. A background reader thread continuously reads the MCP server's stderr and appends lines to the buffer.

Log Line Format

Each log line is stored as a LogLine value object:

{
  "timestamp": "2026-03-23T10:15:30.123456",
  "line": "INFO: Server started on port 8080",
  "mcp_server_id": "math",
  "stream": "stderr"
}

REST API

Get buffered logs

GET /api/mcp_servers/{mcp_server_id}/logs?lines=100

Returns the most recent lines entries from the ring buffer (default 100, max 1000).

Response:

{
  "logs": [
    {"timestamp": "...", "line": "...", "mcp_server_id": "math", "stream": "stderr"},
    {"timestamp": "...", "line": "...", "mcp_server_id": "math", "stream": "stderr"}
  ],
  "mcp_server_id": "math",
  "count": 2
}

Returns an empty list if the MCP server exists but has no log buffer yet (MCP server not started). Returns 404 if the MCP server is not registered.

Configuration

Log capture is automatic for subprocess and Docker MCP servers. No configuration is required.