MCP App Inspector
The sunpeak inspector replicates ChatGPT and Claude runtimes on localhost. Inspect any MCP server across hosts, themes, display modes, and device types without deploying or paying for host accounts.
npx sunpeak inspect --server URL Definition
The sunpeak inspector is a local development tool that replicates the ChatGPT and Claude MCP App runtimes on localhost. It renders your resources exactly as the real hosts would, with full control over host, theme, display mode, and device type. It also powers the sunpeak testing framework's E2E tests.
Why Use the sunpeak Inspector?
MCP Apps run inside AI hosts like ChatGPT and Claude. Testing them against real hosts means paid subscriptions, burned AI credits, and non-deterministic LLM behavior on every code change.
The sunpeak inspector replicates those host runtimes on localhost. It renders your resources exactly as ChatGPT and Claude would — same display modes, themes, safe areas, and conversation chrome — so you can develop and test without deploying.
The inspector works with any MCP server. Run npx sunpeak inspect --server URL to inspect any running server, or pnpm dev in a sunpeak project where the inspector is built into the dev server.
Inspect Manually. Test Automatically.
Manual Inspection
Develop your MCP App with instant feedback. Switch between ChatGPT and Claude hosts, toggle themes and display modes, edit tool data live in the sidebar, and see changes instantly with HMR.
- Toggle ChatGPT/Claude, light/dark, mobile/tablet/desktop
- Edit tool input and output live in the sidebar
- HMR — changes appear instantly, no reload needed
- Works with any MCP server, not just sunpeak projects
Automated Testing
The inspector also powers automated Playwright E2E tests. Define states with simulation files and assert against the rendered resource across hosts, themes, and display modes in CI/CD.
sunpeak's testing framework adds unit tests, visual regression, live host tests, and multi-model evals on top of the inspector.
Try the Inspector
Toggle hosts, themes, and display modes from the sidebar.
How the Inspector Works
Point at Any MCP Server
Run npx sunpeak inspect --server URL to inspect any running MCP server, or pnpm dev in a sunpeak project where the inspector is built into the dev server.
Switch Hosts, Themes, and Modes
Toggle between ChatGPT and Claude runtimes, light and dark themes, mobile and desktop widths, and inline/pip/fullscreen display modes from the sidebar.
Load Simulations
Select simulation files from the sidebar to render your resource with specific mock data. Each simulation defines a reproducible tool state: tool input, tool result, and server tool mocks.
Develop with HMR
Edit your resource components and see changes instantly. The inspector supports hot module replacement so your development loop stays fast.
Inspector Features
- Multi-Host Runtime Replication
Replicated ChatGPT and Claude runtimes with accurate display modes, themes, safe areas, and conversation chrome.
- Interactive Sidebar Controls
Configure host, theme, device width, display mode, tool input/output, and app context from the sidebar. All settings reflected in the URL.
- Simulation Fixtures
JSON files that define reproducible tool states. Load them from the sidebar to render your resource with specific mock data without calling real tool handlers.
- Hot Module Replacement
Edit your resource components and see changes instantly in the inspector. No page reload, no re-triggering tool calls.
- Powers Automated Testing
The inspector doubles as the test runtime for the sunpeak testing framework: E2E tests, visual regression, and more.
- Works with Any MCP Server
Inspect any MCP server with
npx sunpeak inspect --server URL. No sunpeak project required.
Who the inspector is for
Coding Agents
Agents like Claude Code, Codex, and Cursor can run the inspector, execute Playwright tests, and iterate on MCP Apps without manual testing in a real host.
MCP Server Authors
Inspect any MCP server — Python, TypeScript, Go, anything. No sunpeak project needed. Just point the inspector at your server URL or startup command.
QA & Testing Teams
Automated Playwright tests against the inspector catch rendering regressions across hosts, themes, and devices. Run in CI/CD with zero external dependencies.
Getting Started
Inspect any MCP server — no sunpeak project required:
npx sunpeak inspect --server URL Or in a sunpeak project, the inspector is built into the dev server:
pnpm dev
For automated testing with the inspector, see the testing framework.
Frequently Asked Questions
What is the sunpeak inspector?
The sunpeak inspector replicates MCP App host runtimes (ChatGPT, Claude) on your local machine. It renders your MCP App resources exactly as they appear inside the real hosts, with full control over host, theme, display mode, and device type. Use it for manual development and automated testing.
Do I need a sunpeak project to use the inspector?
No. The inspector works with any MCP server. Run "npx sunpeak inspect --server http://localhost:8000/mcp" to inspect any running MCP server, or pass a command like "npx sunpeak inspect --server python my_server.py" to start and inspect it in one step.
How do I test MCP Apps across ChatGPT and Claude?
The inspector ships with both ChatGPT and Claude host runtimes built in. Switch between them from the sidebar dropdown, or pass ?host=claude in the URL. For automated testing, see the <a href="/testing-framework">testing framework</a>.
What are simulation files?
Simulation files are JSON fixtures in tests/simulations/ that define reproducible tool states — tool input, tool result, and server tool mocks. The inspector loads them automatically. Select a simulation from the sidebar to render your resource with that mock data, or select "None" to call the real tool handler.
Can I use the inspector for automated testing?
Yes. The inspector doubles as the test runtime for Playwright E2E tests. sunpeak provides a full <a href="/testing-framework">testing framework</a> with unit tests, E2E tests, visual regression, live host tests, and multi-model evals.
Is the sunpeak inspector free?
Yes. sunpeak is MIT licensed and open source. The inspector and all tooling are free to use. No paid accounts needed for local development.
Open Source & MIT Licensed
sunpeak is free to use, modify, and distribute.
Want automated tests? See the Testing Framework page. Building MCP Apps? See the MCP App Framework page. Building for ChatGPT? See the ChatGPT App Framework page.
