# MCP Test Client — Validate Any MCP Client Against Hosted Mock Servers

> Validate an MCP client implementation against six hosted mock servers in the browser — echo, Bearer auth, deliberate errors, complex nested schemas, MCP Apps `ui://` resources, and the stateless 2026-07-28 revision. No install, no API key.

**Source:** https://mcpplaygroundonline.com/mcp-test-client

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## The inverse problem

Most MCP tooling helps you test a *server*. This helps you test a *client* — the thing doing the connecting. To do that you need servers that behave in known ways, including behaving badly on purpose.

## What gets exercised

- **Connection lifecycle** — `initialize`, capability negotiation, shutdown
- **Capability negotiation** — does your client correctly honour what the server declares?
- **Tool discovery** — listing and pagination
- **Error handling** — the part that is almost never tested until production
- **Authentication** — 401 handling and Bearer-token retry

## The mock servers behind it

- **Echo server** (`https://mcpplaygroundonline.com/mcp-echo-server`) — A single `echo` tool, no authentication. The simplest possible connectivity check. (Temporarily offline.)
- **Auth server** (`https://mcpplaygroundonline.com/mcp-auth-server`) — Returns 401 with a `WWW-Authenticate` challenge until a Bearer token is supplied. Use it to exercise a client's auth flow.
- **Error server** (`https://mcpplaygroundonline.com/mcp-error-server`) — Returns validation, not_found, rate_limit, timeout and five more error types on demand, so you can test how a client surfaces failures.
- **Complex server** (`https://mcpplaygroundonline.com/mcp-complex-server`) — Four deeply-nested tool schemas. Use it to test tool selection and argument construction under realistic schema pressure.
- **MCP Apps server** (`https://mcpplaygroundonline.com/mcp-app-server`) — Returns a `ui://` MCP Apps resource, for clients that render interactive tool output.
- **Stateless 2026-07-28 server** (`https://mcpplaygroundonline.com/mcp-stateless-server`) — Multi Round-Trip Requests and signed `requestState` from the 2026-07-28 revision.

Every endpoint serves both the stateless 2026-07-28 revision and the 2025-era `initialize` handshake. Append `?rev=2026-07-28` or `?rev=2025-11-25` to pin one and test your rejection paths.

These are public URLs — see [Mock MCP Servers](https://mcpplaygroundonline.com/mock-mcp-servers) to use them directly from your own test suite, curl, or any other MCP client.

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_Canonical page: https://mcpplaygroundonline.com/mcp-test-client_

_MCP Playground (mcpplaygroundonline.com) is a free, browser-based toolkit for the Model Context Protocol: test any remote MCP server, run evals that prove an AI agent can use its tools, build reusable agents across 70+ models, and audit servers for security issues. No install, no local Node, free to start._