MCP Tool Tester
Connect to MCP servers and test tools via JSON-RPC
Public servers are auto-proxied through our SSRF-protected endpoint. Local/private URLs need a local CORS proxy (see diagnostic below if it fails).
Frequently Asked Questions
What is the MCP tool tester?
MCP (Model Context Protocol) is Anthropic's open protocol that lets AI apps (Claude Desktop, Claude Code, etc.) call external tools and data sources in a uniform way. This tester lets you connect to an MCP server (HTTP or SSE), list its tools, fill in parameters, fire JSON-RPC calls, and inspect responses — entirely in the browser.
Who is this for?
- Engineers building MCP servers who need to validate independently of any AI client - Teams integrating MCP into enterprise apps, diagnosing protocol-layer issues - MCP server users who want to preview capabilities before wiring up a client - AI agent developers debugging the model ↔ tool interaction loop
Which MCP transports are supported?
HTTP and SSE (Server-Sent Events) remote transports are supported today. The local stdio transport requires inter-process communication, which a browser cannot do natively — use Claude Desktop or the official CLI for that.
What if I can't connect to my MCP server?
Common causes: - CORS: the server must allow your browser's origin (`Access-Control-Allow-Origin`) - Auth: some servers require an Authorization header or token - Mixed content: HTTPS pages usually block plain HTTP backends - The server has not implemented `initialize` / `tools/list` The tester surfaces the raw JSON-RPC error so you can trace the problem.
Are requests and responses uploaded?
Traffic flows directly between your browser and the target MCP server — DevToolkit does not proxy it. Be aware that the target server may log payloads, so confirm its policies before sending sensitive data.
How is this better than curl or Postman?
With curl or Postman you hand-build the JSON-RPC envelope (`id`, `method`, `params`) and perform every handshake step (initialize → tools/list → tools/call). The MCP tool tester handles that orchestration so you just pick a tool, fill in parameters, and view the result — one of the few dedicated debuggers in the MCP space.