sncro
Live browser debugging for AI assistants — DOM, console, network via MCP.
Install
Add this to your client's MCP configuration (Claude Desktop: claude_desktop_config.json; Cursor: .cursor/mcp.json; VS Code and others use the same "mcpServers" shape).
claude mcp add --transport http sncro 'https://relay.sncro.net/tools/mcp'{
"mcpServers": {
"sncro": {
"type": "http",
"url": "https://relay.sncro.net/tools/mcp"
}
}
}Documentation
From the project's README on GitHub, refreshed weekly. Links open on GitHub.
sncro-relay
Open-source components of sncro — the MCP relay, the browser-side agent, and the framework plugins that let AI coding assistants inspect a live browser.
What this repo contains
| Path | What |
|---|---|
relay/ | FastAPI app that exposes an MCP server plus long-poll endpoints for agent.js |
relay/static/agent.js | Browser-side script injected by the middleware; pushes console + DOM data to the relay |
middleware/sncro_middleware.py | FastAPI / Starlette plugin — drop-in middleware for FastAPI apps |
middleware/sncro_flask.py | Flask plugin — drop-in middleware for Flask apps |
trysncro/ | try.sncro.net — a deliberately-broken demo app for exercising sncro end-to-end |
How it works
┌──────────────┐ MCP ┌──────────┐ long-poll ┌─────────────┐
│ Claude Code │──tools────▶│ relay │◀──────────────│ agent.js │
│ (or other │ │ (relay/) │ snapshots │ (injected │
│ MCP client) │◀──results──│ │──────────────▶│ by plugin) │
└──────────────┘ └──────────┘ └─────────────┘
▲
│ same-origin
│ cookies
┌─────────────┐
│ your app │
│ (plugin is │
│ installed) │
└─────────────┘- Claude calls
create_session(MCP tool) → relay returns a 9-digit session key + URL - User visits the URL in the browser where their app is running → confirms via "Allow sncro debugging?"
- The plugin drops a cookie, agent.js is injected into subsequent HTML responses on that origin
- agent.js pushes baseline data (console, errors) and long-polls for on-demand queries (
query_element,get_page_snapshot, etc.)
Using sncro
Most users don't need to run the relay yourself — the hosted version at relay.sncro.net is free-tier friendly. Register your project at sncro.net and grab your project key.
FastAPI: drop middleware/sncro_middleware.py into your project, then:
from middleware.sncro_middleware import SncroMiddleware, sncro_routes
app = FastAPI(debug=True) # sncro only loads when debug=True
if app.debug:
app.include_router(sncro_routes)
app.add_middleware(SncroMiddleware, relay_url="https://relay.sncro.net")Flask: drop middleware/sncro_flask.py into your project, then:
from sncro_flask import init_sncro
app = Flask(__name__)
if app.debug:
init_sncro(app, relay_url="https://relay.sncro.net")Both middlewares only activate in debug mode — zero overhead in production.
Contributing
We love new framework plugins. CONTRIBUTING.md has the full spec for what a plugin must do — cookies, routes, security headers — plus the test template. Django, Rails, Express, Next.js, ASP.NET, Go — all welcome.
Bug reports and security issues: see SECURITY.md.
License
MIT. See LICENSE.
The dashboard at sncro.net (project management, billing, admin) lives in a separate proprietary repo.
Frequently asked questions
What is sncro?
Live browser debugging for AI assistants — DOM, console, network via MCP.
How do I install sncro?
It runs as a remote server. In Claude Code: claude mcp add --transport http sncro 'https://relay.sncro.net/tools/mcp'. For other clients, paste the JSON configuration from this page into their MCP settings.
Does sncro need an API key?
Its registry entry lists no required key or secret. Some features may still need an account with the service it connects to.
Is sncro open source?
Yes. Its code is on GitHub under the MIT license.
Who maintains sncro?
It is published by scottconfusedgorilla. AI.tn lists it but does not maintain or endorse it.
From the official MCP Registry. Listing is not an endorsement: review a server's code and permissions before giving it access to your data.