mockserver-client

An idiomatic Rust client for MockServer's control-plane REST API.

Installation

Add to your Cargo.toml:

[dev-dependencies]
mockserver-client = "7.0"

Quick Start

use mockserver_client::{ClientBuilder, HttpRequest, HttpResponse, VerificationTimes};

fn main() -> mockserver_client::Result<()> {
    let client = ClientBuilder::new("localhost", 1080).build()?;

    // Create an expectation
    client.when(HttpRequest::new().method("GET").path("/hello"))
        .respond(HttpResponse::new().status_code(200).body("world"))?;

    // Verify the request was received
    client.verify(
        HttpRequest::new().path("/hello"),
        VerificationTimes::at_least(1),
    )?;

    // Reset all expectations
    client.reset()?;
    Ok(())
}

LLM and MCP mocking

The llm and mcp modules provide fluent builders that produce the same expectation wire JSON as the Java, Node, and Python clients, so a mock scripted from Rust behaves identically to one scripted from any other client.

LLM completions, embeddings, conversations, and failover

use mockserver_client::llm::{
    completion, conversation, embedding, header, llm_failover, llm_mock, turn, usage, Provider,
};

// A single chat-completion mock (action carried in `httpLlmResponse`).
llm_mock("/v1/chat/completions")
    .with_provider(Provider::OPENAI)
    .with_model("gpt-4o")
    .responding_with(completion().with_text("Hello!").with_usage(usage().with_output_tokens(3)))
    .apply_to(&client)?;

// An embedding mock (clears any completion).
llm_mock("/v1/embeddings")
    .with_provider(Provider::OPENAI)
    .responding_with(embedding().with_dimensions(1536).with_deterministic_from_input(true))
    .apply_to(&client)?;

// A multi-turn conversation using MockServer scenario-state advancement.
conversation()
    .with_path("/v1/chat/completions")
    .with_provider(Provider::ANTHROPIC)
    .isolate_by(header("x-session-id")) // optional per-session isolation
    .turn(turn().responding_with(completion().with_text("Hi, how can I help?")))
    .turn(
        turn()
            .when_latest_message_contains("weather")
            .responding_with(completion().with_text("It's sunny.")),
    )
    .apply_to(&client)?;

// Fail N times (with default provider-shaped error bodies), then succeed.
llm_failover()
    .with_path("/v1/chat/completions")
    .with_provider(Provider::OPENAI)
    .fail_with_count(429, 2) // coalesced into one expectation with times = 2
    .fail_with(500)
    .then_respond_with(completion().with_text("Recovered"))
    .apply_to(&client)?;

provider is serialised UPPERCASE (OPENAI, ANTHROPIC, ...). Conversation turns advance through scenario states Started -> turn_1 -> ... -> __done. Each builder also has a build() method returning the raw serde_json::Value(s) if you want to inspect or submit them yourself via client.upsert_raw(...).

MCP (Model Context Protocol) servers

mcp_mock builds the set of expectations needed to emulate a Streamable-HTTP MCP server speaking JSON-RPC 2.0 (initialize, ping, notifications/initialized, tools/list, tools/call, resources/list, resources/read, prompts/list, prompts/get). Responses use Velocity templates that echo the inbound JSON-RPC id.

use mockserver_client::mcp::mcp_mock;
use mockserver_client::llm::Role;

mcp_mock("/mcp")
    .with_server_name("MyServer")
    .with_tool("get_weather")
        .with_description("Get the weather for a city")
        .with_input_schema(r#"{"type":"object","properties":{"city":{"type":"string"}}}"#)
        .responding_with("72F and sunny", false)
        .and()
    .with_resource("file:///config.json")
        .with_name("config")
        .with_content(r#"{"debug":true}"#)
        .and()
    .with_prompt("greeting")
        .with_argument("name", Some("who to greet".to_string()), true)
        .responding_with(Role::ASSISTANT, "Hello there")
        .and()
    .apply_to(&client)?;

Use mcp_mock_default() for the default /mcp path. build() returns the ordered Vec<serde_json::Value> of expectations.

Features

• Fluent builder API — client.when(request).respond(response)
• Response, Forward, and Error actions — full MVP control-plane coverage
• Verification — verify (count-based) and verify_sequence (order-based)
• Retrieve — recorded requests, active expectations, recorded expectations, logs
• Clear / Reset — by request matcher, by expectation ID, or full reset
• Status / Bind — query ports, bind additional ports
• LLM and MCP builders — fluent llm / mcp mock builders, wire-identical to the other clients
• Blocking (synchronous) — uses reqwest blocking client; no async runtime needed
• TLS support — optional HTTPS with configurable certificate verification
• Secured control plane — control_plane_bearer_token(..) (JWT auth), ca_cert_pem_path(..) / ca_cert_pem(..) (trust a server CA), and client_cert_pem(cert, key) (mTLS)

API Overview

use mockserver_client::*;

let client = ClientBuilder::new("localhost", 1080).build().unwrap();

// Fluent expectation creation
client.when(HttpRequest::new().method("POST").path("/api/users"))
    .times(Times::exactly(3))
    .respond(HttpResponse::new()
        .status_code(201)
        .header("Location", "/api/users/1")
        .body(r#"{"id": 1}"#))?;

// Forward action
client.when(HttpRequest::new().path("/proxy"))
    .forward(HttpForward::new("backend.local", 8080).scheme("HTTP"))?;

// Verify
client.verify(
    HttpRequest::new().method("POST").path("/api/users"),
    VerificationTimes::between(1, 3),
)?;

// Verify sequence
client.verify_sequence(vec![
    HttpRequest::new().path("/first"),
    HttpRequest::new().path("/second"),
])?;

// Clear by request matcher
client.clear(
    Some(&HttpRequest::new().path("/api/users")),
    Some(ClearType::Expectations),
)?;

// Clear by ID
client.clear_by_id("my-expectation-id", None)?;

// Retrieve recorded requests
let requests = client.retrieve_recorded_requests(None)?;

// Retrieve active expectations
let expectations = client.retrieve_active_expectations(None)?;

// Server status
let ports = client.status()?;
println!("Listening on: {:?}", ports.ports);

// Reset everything
client.reset()?;

Interactive Breakpoints

Register breakpoint matchers to pause forwarded/proxied traffic at REQUEST, RESPONSE, RESPONSE_STREAM, or INBOUND_STREAM phases. A callback WebSocket connection is opened automatically.

use mockserver_client::*;

let client = ClientBuilder::new("localhost", 1080).build()?;

// REQUEST-only breakpoint
let id = client.add_request_breakpoint(
    HttpRequest::new().path("/api/.*"),
    Box::new(|req| Some(req)),  // continue with original
)?;

// REQUEST + RESPONSE breakpoint
let id2 = client.add_request_response_breakpoint(
    HttpRequest::new().path("/api/.*"),
    Box::new(|req| Some(req)),
    Box::new(|_req, resp| Some(resp)),
)?;

// Streaming breakpoint
let id3 = client.add_stream_breakpoint(
    HttpRequest::new().path("/stream/.*"),
    &[phase::RESPONSE_STREAM],
    Box::new(|frame| {
        Some(StreamFrameDecision::continue_frame(&frame.correlation_id))
    }),
)?;

// Manage matchers
let list = client.list_breakpoint_matchers()?;
client.remove_breakpoint_matcher(&id)?;
client.clear_breakpoint_matchers()?;
client.close_breakpoint_websocket();

Stream frame decisions: StreamFrameDecision::continue_frame, ::modify, ::drop_frame, ::inject, ::close.

Start / Launch MockServer

The Rust client can download and launch a local MockServer instance directly -- no Java installation and no Docker required. The launcher downloads a self-contained platform bundle (mockserver-<version>-<os>-<arch>) from the GitHub Release, verifies its SHA-256, caches it per-user, and starts it.

Quick start

use mockserver_client::launcher;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut handle = launcher::start(1080)?;
    println!("MockServer running on port {}", handle.port());

    // ... use MockServer ...

    handle.stop()?;
    Ok(())
}

Just ensure the binary is present

let launcher_path = launcher::ensure_launcher()?;
println!("Launcher at: {}", launcher_path.display());

Specify a version

let mut handle = launcher::start_with_version(
    "7.1.0", 1080, &launcher::EnsureOptions::default()
)?;

API reference

Supported platforms

Environment variables

Version

By default the launcher downloads the MockServer version matching this crate (derived from Cargo.toml at compile time via env!("CARGO_PKG_VERSION")). Pass an explicit version to override.

Building

cargo build
cargo test
cargo clippy

Integration Tests

Integration tests require a running MockServer and are skipped by default:

# Start MockServer (e.g., via Docker)
docker run -d -p 1080:1080 mockserver/mockserver

# Run integration tests
MOCKSERVER_URL=http://localhost:1080 cargo test -- --ignored

License

Apache-2.0