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MCP Test Client — Validate Your MCP Client Against Hosted Mock Servers

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Validate your MCP client against hosted mock servers — test echo responses, authentication flows, and error handling. Then connect to real AI: see how Claude, GPT, Gemini and 60+ models interact with your implementation before you ship.

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Connect your server to AI — test how Claude, GPT & Gemini call your tools, compare models, or build a reusable agent from a template.

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Select Demo Server

Choose a demo server to test your MCP client implementation

Connection Details

no ?rev= sent

The server works out which version you speak from the request itself.

/mcp-complex-server

Available Test Servers

Six hosted MCP servers, each targeting a different part of your client implementation. Every URL below is public and free — paste it into this tool, into your own client, or into curl:

  • Echo Server (/mcp-echo-server): Returns whatever you send — basic connectivity and message handling. Temporarily offline.
  • Auth Server (/mcp-auth-server): Answers 401 with a WWW-Authenticate challenge until you present a Bearer token — test your auth flow
  • Error Server (/mcp-error-server): Deliberately returns validation, rate-limit, timeout and five more error types on demand — validate your error handling
  • Complex Server (/mcp-complex-server): Four tools with deeply nested, mutually confusable schemas — test parameter validation and tool selection
  • MCP Apps Server (/mcp-app-server): Returns a ui:// resource that should render as live UI, not raw JSON
  • Stateless 2026-07-28 Server (/mcp-stateless-server): Multi Round-Trip Requests and signed requestState from the new revision

Full endpoint URLs, tool names, curl examples, and the ?rev= pinning options are on the mock MCP servers page.

What to Test in Your MCP Client

A robust MCP client should handle these scenarios correctly:

  • Connection lifecycle: Initialize, maintain, and gracefully close connections
  • Capability negotiation: Correctly advertise and respect server capabilities
  • Tool discovery: List and cache available tools from the server
  • Tool execution: Call tools with proper arguments and handle responses
  • Error handling: Gracefully handle network errors, timeouts, and protocol errors
  • Authentication: Properly send and refresh credentials when required

Building an MCP Client?

If you're implementing an MCP client from scratch, the official MCP SDK provides client libraries for:

  • TypeScript/JavaScript (Node.js)
  • Python

Once you've built your client, test it against our servers here, then try connecting to real servers from the MCP servers directory. To validate the other side — your own MCP server — use the MCP Server Tester or audit it for security with the MCP Security Scanner.

Frequently Asked Questions

What is the MCP Test Client?
The MCP Test Client is a free browser-based tool for validating any MCP (Model Context Protocol) client implementation against six hosted mock servers — echo, auth, error, complex, MCP Apps, and stateless 2026-07-28. Use it to verify your client handles connections, capability negotiation, tool discovery, tool execution, authentication, and error paths correctly before pointing it at production servers.
Is the MCP Test Client free?
Yes, completely free. No sign-up, no API key, no install, no rate limit. Run as many test sessions as you need, against any of the six hosted mock servers.
What mock MCP servers are available?
Six hosted mock servers: Echo (returns whatever you send — basic connectivity and message-handling tests, temporarily offline), Auth (requires Bearer-token auth — exercises your auth flow), Error (deliberately returns protocol errors, timeouts, and edge-case responses — validates your error handling), Complex (deeply nested schemas — tests parameter validation and tool selection), MCP Apps (returns a ui:// resource rendered as live UI), and 2026 Spec Features (Multi Round-Trip Requests and signed requestState — the machinery the stateless revision introduced).
What does my MCP client need to handle?
A robust MCP client should handle the connection lifecycle, capability negotiation, tool discovery and caching, tool execution with proper argument shapes, error handling (network, timeout, protocol), and authentication including credential refresh. On the 2026-07-28 revision it also needs the Multi Round-Trip Request retry loop: when a server answers with an input_required result, the client gathers the answers and re-issues the original request with inputResponses plus the echoed requestState. The mock servers exercise each of these areas explicitly.
Which MCP protocol revisions do the mock servers support?
Both. Every mock serves the stateless 2026-07-28 revision and the 2025-era initialize handshake, detecting which one you are speaking from the request itself — so you can point an old or new client at the same URL. You can also pin a revision with the Protocol Revision switch to test rejection paths deliberately: pinning 2026-07-28 and sending a 2025 handshake returns -32022 UnsupportedProtocolVersion.
Which transports does the MCP Test Client support?
The hosted mock servers support Streamable HTTP, the transport both current spec revisions use. HTTP+SSE still works for legacy clients but was reclassified as Deprecated in the 2026-07-28 revision. Stdio is not exposed by the hosted mocks since stdio is a local-only transport.
How is this different from the MCP Server Tester?
The MCP Test Client validates that your client code speaks MCP correctly — it gives you mock servers to talk to. The MCP Server Tester does the opposite: it acts as a client so you can test your own MCP server. Use both during development: Test Client for client implementations, Server Tester for server implementations.
Can I test my MCP client with real AI models after?
Yes. After validating against the mock servers, point MCP Agent Studio at any MCP server URL to see how Claude, GPT-5, Gemini, GLM, Qwen, and Grok actually call your tools — useful for catching tool-selection drift before shipping.
MCP Test Client — Validate MCP Clients Against Hosted Mock Servers | MCP Playground | MCP Playground