Web Debugger

Web App Debugger

Runtime debugging for web applications via the web-debugger-mcp server. Provides structured logs and on-demand state snapshots from both browser and server — all saved to files for context-efficient consumption.

Architecture

┌─────────────┐  WebSocket   ┌──────────────────┐  stdio   ┌───────────┐
│  Web App     │────────────→│  web-debugger-mcp │←────────→│  Claude   │
│  (SDK)       │  port 7600  │  (MCP server)     │          │  (agent)  │
└─────────────┘              └──────────────────┘          └───────────┘

The web app uses the client SDK to send logs and register state providers. The MCP server receives these over WebSocket, writes them to .web-debugger/, and exposes three MCP tools for the agent.

MCP Tools

Setup

The MCP server is bundled with the ai-resources plugin. When the plugin is installed, the web-debugger MCP server starts automatically — no manual claude mcp add needed.

The server listens on WebSocket port 7600 (override with WEB_DEBUGGER_PORT env var). Output directory defaults to .web-debugger/ (override with WEB_DEBUGGER_DIR env var).

After building (cd web-debugger-mcp && bun run build), the binary is placed in the plugin's servers/ directory.

Client SDK Integration

The web app must integrate the client SDK. This is done by the app developer, not the agent.

import { createDebugger } from "web-debugger-mcp/client";

const dbg = createDebugger();

// Log events
dbg.log("info", "User logged in", { userId: "abc123" });

// Register state providers for on-demand snapshots
dbg.registerProvider("app-state", () => store.getState());
dbg.registerProvider("react-query", () => queryClient.getQueryCache().getAll());

The SDK auto-detects browser vs server environment, silently degrades when the MCP server isn't running, and is a no-op in production (NODE_ENV explicitly set to non-development).

Debugging Workflow

1. Check Available Providers

Always start by discovering what state the app exposes:

list_providers → see what's available

2. Read Logs

Get the log file path and read it to understand what happened:

get_logs → { logFile: ".web-debugger/logs/session-20260330-143022-a1b2.jsonl" }

Then read the file. Each line is a JSON object:

{"timestamp":"2026-03-30T14:30:22.000Z","level":"error","source":"server","message":"Failed to fetch user","context":{"userId":"abc","statusCode":500}}

Log fields: timestamp, level (info/warn/error/debug), source (browser/server), message, context (optional structured data).

3. Take Snapshots

Request a snapshot from a specific provider to inspect runtime state:

get_snapshot → provider: "app-state"
→ { snapshotFile: ".web-debugger/snapshots/app-state-20260330-143045-c3d4.json" }

Then read the snapshot file. Snapshots are pretty-printed JSON.

4. Combine with Playwright

For full debugging, combine with browser automation:

1. Use Playwright to reproduce the issue (navigate, click, fill forms)
2. Use get_logs to check what errors occurred during the interaction
3. Use get_snapshot to inspect application state at the point of failure
4. Use Playwright to verify the fix

See /browser-automation for Playwright tool usage.

Serialization Format

Snapshots automatically serialize non-JSON-native types. When reading snapshot files, recognize these tagged objects:

File Locations

All output is under .web-debugger/ (relative to the project root):

.web-debugger/
├── logs/
│   └── session-YYYYMMDD-HHmmss-XXXX.jsonl    # one per server connect
└── snapshots/
    └── <provider>-YYYYMMDD-HHmmss-XXXX.json  # one per snapshot request

Troubleshooting

• get_logs returns "No active session" — The web app hasn't connected yet. Start the dev server and ensure the SDK is initialized.
• list_providers returns empty — No providers registered. The app must call debugger.registerProvider().
• get_snapshot returns "provider_not_found" — The provider name doesn't match. Use list_providers to check exact names.
• get_snapshot returns "snapshot_timeout" — The provider took >10s to respond. The app may be frozen or the provider callback is hanging.
• No WebSocket connection — Check that the MCP server is running and the port matches (default 7600).