--- title: MockServer description: Open-source HTTP/HTTPS mock server, recording proxy, and MCP server for AI assistants. Mock any API, record and replay traffic, generate from OpenAPI. layout: default ---

Getting Started

To get started see the guide for mocking, proxying, connecting an AI assistant via MCP, or the code examples.

What is MockServer

For any system you integrate with via HTTP or HTTPS MockServer can be used as:

a mock configured to return specific responses for different requests
a proxy recording and optionally modifying requests and responses
both a proxy for some requests and a mock for other requests at the same time
an MCP server that lets AI coding assistants drive the mock and proxy in natural language

MockServer is also AI-native: every instance ships a built-in Model Context Protocol (MCP) server at /mockserver/mcp with no extra configuration, and the documentation is published in machine-readable form for LLMs. In practice this means:

AI assistants (Claude Code, Cursor, Windsurf, Cline, OpenCode) can connect over MCP to create expectations, verify requests, retrieve recorded traffic, and debug mismatches — see the MCP tools reference
tools that don't yet speak MCP can fall back to MockServer's OpenAPI spec
LLMs can retrieve and cite the docs via llms.txt, llms-full.txt, and ai.txt

See AI & MCP Integration below for the full list of AI capabilities.

When MockServer receives a request it matches the request against active expectations that have been configured. Then, if no matches are found, it proxies the request if appropriate; otherwise a 404 is returned.

For each request received the following steps happen:

find matching expectation and perform action
if no matching expectation proxy request
if not a proxy request return 404

An expectation defines the action that is taken, for example, a response could be returned.

MockServer supports the follow actions:

return a "mock" response when a request matches an expectation
forward a request when the request matches an expectation (i.e. a dynamic port forwarding proxy)
execute a callback when a request matches an expectation, allowing the response to be created dynamically
return an invalid response or close the connection when a request matches an expectation
verify requests have been sent (i.e. as a test assertion)
retrieve logs, requests or expectations to help debug

Proxying with MockServer

MockServer can:

proxy all requests using any of the following proxying methods:
- Port Forwarding
- Web Proxying (i.e. HTTP proxy)
- HTTPS Tunneling Proxying (using HTTP CONNECT)
- SOCKS Proxying (i.e. dynamic port forwarding)
verify proxied requests have been sent (i.e. in a test assertion)
record proxied requests and responses to analyse how a system behaves

MockServer supports the following proxying techniques:

Port Forwarding
- all requests are forwarded to a single hostname or IP and port
- to proxy requests the HTTP client should use the hostname and port of MockServer
Web Proxying (i.e. HTTP proxy)
- each request is forwarded dynamically using its Host header
- to proxy requests the HTTP client should be configured to use an HTTP Proxy
Secure Web Proxying (i.e. HTTPS tunneling proxying)
- requests are forwarded using a CONNECT request that sets up an HTTP tunnel
- an SSL certificate is auto-generated allowing encrypted HTTPS traffic to be recorded transparently
- to proxy requests the HTTP client should be configured to use an HTTP Proxy
SOCKS Proxying (i.e. dynamic port forwarding)
- requests are forwarded using a SOCK CONNECT CMD request that established a socket tunnel
- if the traffic is encrypted an SSL certificate is auto-generated allowing SSL traffic to be recorded transparently
- to proxy requests the operating system (or JVM) should be configured to use an HTTP Proxy
SSL & Certificates
- all SSL traffic is handled transparently by auto-generating an appropriate SSL certificate
- generated certificates use a single MockServer root CA certificate enabling the root certificate to be easily imported
Port Unification
- to simplify configuration all protocols (i.e. HTTP, HTTPS / TLS, SOCKS, etc) are supported on the same port
- the protocol is dynamically detected by both MockServer
Simultaneous Proxying & Mocking
- if MockServer is being used as a proxy expectations can also be created
- when a request is received it is first matched against active expectations that have been configured
- if an expectations is matched its action will be performed instead of proxying the request

AI & MCP Integration

MockServer is built to be AI-native. A built-in Model Context Protocol (MCP) server lets AI coding assistants drive the mock server in natural language, and an OpenAPI spec provides a fallback for tools that don't yet speak MCP. All documentation, including llms.txt, llms-full.txt, and ai.txt, is structured for retrieval and citation by LLMs.

MCP Setup — connect Claude Code, Cursor, Windsurf, Cline, or OpenCode to a running MockServer at http://localhost:1080/mockserver/mcp over the MCP Streamable HTTP transport
MCP Tools Reference — every tool the AI assistant can invoke: create_expectation, verify_request, retrieve_recorded_requests, retrieve_logs, debug_request_mismatch, and more
AI-Assisted Debugging — diagnose request mismatches and inspect recorded request/response pairs through an AI assistant
AI-Assisted Verification — build test assertions using natural language
AI Traffic Inspection — record outbound traffic and let an AI assistant explain what happened
AI Protocol Mocking — generate mocks for REST, gRPC, and OAuth2 flows from prompts
OpenAPI Fallback for AI — use MockServer's OpenAPI spec with AI tools that don't yet support MCP

The MCP endpoint requires no extra configuration — start MockServer and the AI surface is on the same port:

docker run -d --rm -p 1080:1080 mockserver/mockserver
# MCP endpoint immediately available at http://localhost:1080/mockserver/mcp

Why use MockServer

MockServer allows you to mock any server or service via HTTP or HTTPS, such as a REST or RPC service.

This is useful in the following scenarios:

testing
- easily recreate all types of responses for HTTP dependencies such as REST or RPC services to test applications easily and affectively
- isolate the system-under-test to ensure tests run reliably and only fail when there is a genuine bug. It is important only the system-under-test is tested and not its dependencies to avoid tests failing due to irrelevant external changes such as network failure or a server being rebooted / redeployed.
- easily set up mock responses independently for each test to ensure test data is encapsulated with each test. Avoid sharing data between tests that is difficult to manage and maintain and risks tests infecting each other
- create test assertions that verify the requests the system-under-test has sent
de-coupling development
- start working against a service API before the service is available. If an API or service is not yet fully developed MockServer can mock the API allowing any team who is using the service to start work without being delayed
- isolate development teams during the initial development phases when the APIs / services may be extremely unstable and volatile. Using MockServer allows development work to continue even when an external service fails
isolate single service
- during deployment and debugging it is helpful to run a single application or service or handle a sub-set of requests on on a local machine in debug mode. Using MockServer it is easy to selectively forward requests to a local process running in debug mode, all other request can be forwarded to the real services for example running in a QA or UAT environment

Mocking Dependencies & Verifying Request

Given a system with service dependencies, as follows:

System In Production

MockServer could be used to mock the service dependencies, as follows:

Mocking service dependencies with MockServer

Isolating Single Service / Application

A single page application may load static resources such as HTML, CSS and JavaScript from a web server and also make AJAX calls to one or more separate services, as follows:

Single Page Application

To isolate a single AJAX service, for development or debugging, the MockServer can selectively forward specific requests to local instance of the service:

Isolating Single Service

Using MockServer as a content routing load balancer is described in more detail in the section called Isolate Single Service.

Why use MockServer as a proxy

MockServer allows you to record request from the system-under-test or two analysis an existing system by recording outbound requests.

This is useful in the following scenarios:

testing
- create test assertions that verify the requests the system-under-test has been sent, without needing to mock any requests
analyse existing system
- record all outbound requests so it is possible to analyse and understand what outbound requests an existing system is making
debug HTTP interactions
- log all outbound requests so it is possible to visualise all interactions (for example from a browser) to external services. This is particularly important as network analysis tools in browsers such as Chrome do not accurately show all network interactions, such as, favicon.ico requests. In addition, many proxies do not handle encrypted HTTPS traffic, however, MockServer auto-generates certificates using a single MockServer root CA certificate enabling the root certificate to be easily imported
record & replay
- all recorded requests can be converted into Java code or JSON expectations to simplify the creation of mocks for complex test scenarios

Recording Requests & Analysing Behaviour

MockServer can record all proxied requests, as follows:

Recording service requests with MockServer Proxy

Verifying Request

MockServer can verify proxied service requests, as follows:

Verifying service requests with MockServer Proxy

Developer Documentation

For detailed information about MockServer's internal architecture, infrastructure, build system, and operations, see the Contributing page, which links to comprehensive technical documentation on GitHub.

MockServer