Sncro

Live browser debugging for AI assistants — DOM, console, network via MCP.

Quick Install

Automated & IDE Setup

Copy the AI prompt to install this server into Claude Code, Cursor, or another agent — or use 1-click editor setup below.

Add to Cursor Add to VS Code

Manual Client & Custom JSON ConfigExpand JSON ▾

Client Config & Setup

Choose your client or environment

Target File:~/Library/Application Support/Claude/claude_desktop_config.json

claude_desktop_config.json

{
  "mcpServers": {
    "sncro": {
      "command": "npx",
      "args": [
        "-y",
        "sncro"
      ]
    }
  }
}

💡 Paste the JSON block into your client's configuration file under mcpServers, then restart the application.

Documentation Overview

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

Code

 ┌──────────────┐    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:

server.ts

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:

server.ts

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.

Read the full README →View source on GitHub →

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Frequently Asked Questions about Sncro

Add the following block to your claude_desktop_config.json under mcpServers: "mcpServers": { "sncro": { "command": "npx", "args": ["-y", "sncro"] } }

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Technical Specs & Signals

CategoryDeveloper Tools

27Quality signal: Emerging · 27/100How this signal is calculated ▾

Server availabilityNot measured

Not scored for repo-hosted servers — we can't reach the running server, only its GitHub page. Hosted MCP endpoints are health-checked live.

Verified ownership8/20

Documentation & tools11/30

Adoption & activity1/15

Community engagement0/10

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More in 💻 Developer Tools →Best MCP servers for Developers →Alternatives to Sncro →Install in Claude Desktop Install in Cursor Install in VS Code

Documentation Overview

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

Code

 ┌──────────────┐    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:

server.ts

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:

server.ts

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

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.