pip install mockworld-mcp
mockworld run mock:payments      # a stateful fake Stripe as an MCP server

Status: released — mockworld-mcp on PyPI. Deterministic engine, 6 built-in mocks (5 services + a hello example), MCP stdio + HTTP, fault injection, control plane, registry, world composition, record-mode, snapshots, and a stampede Target. Companion to stampede.

Contents

• Why
• Install
• Quickstart
• Use it in your tests
• Built-in mocks
• Author & share mocks
• Compose, record, and simulate
• Project structure
• Documentation
• Contributing
• License
• Part of the Swarm Proof toolkit

Why

Agents need to do things — charge a card, send an email, place a trade, update a record — but you can't point a half-finished, non-deterministic agent at real Stripe/Gmail/an exchange during development. So teams hand-build throwaway mocks for every project, or test against nothing and find failures in production.

Recent agent sandboxes (Veris Sandbox, AWS ToolSimulator) put an LLM in the response path, so the mock does not behave the same way twice. That rules out a CI run that is green for the same reason twice, a byte-identical bug repro, and offline or air-gapped testing.

mockworld is deterministic, MCP-native, and open:

• Deterministic & LLM-free. A seed determines state, IDs, timing, and every injected fault. reset --seed 42 produces the same decline across 50 parallel CI workers. No LLM in the response path.
• MCP-native. Services are MCP servers with tool descriptions, stateful behavior, and business-logic fault semantics (declines, insufficient funds, rate limits, disputes). Postman and WireMock target human-driven HTTP testing; they don't speak MCP and don't model business state.
• Open & self-hostable. pip install, runs locally, offline, Apache-2.0 — not a hosted SaaS.

Install

pip install mockworld-mcp

Requires Python 3.11+. The distribution is named mockworld-mcp; the import package and CLI are simply mockworld.

Quickstart

mockworld list                        # the built-in mocks
mockworld run mock:payments           # a stateful fake Stripe over stdio (MCP)
mockworld run mock:payments --transport http --port 8931   # Streamable HTTP + control plane
mockworld run mock:payments --seed 42 --faults hostile     # deterministic + adversarial
mockworld inspect mock:crm            # tools, faults, and state shape without running
mockworld demo mock:payments          # prove determinism: same seed → identical transcript

Point any MCP client (or a stampede swarm) at it. mockworld reset --seed 42 returns a running server to a byte-identical world, every time.

Use it in your tests

Installing mockworld gives every pytest run a mockworld fixture — a deterministic fake Stripe in two lines:

def test_agent_handles_a_decline(mockworld):
    pay = mockworld.start("mock:payments", seed=7, faults="hostile")
    cust = pay.call("create_customer", {"name": "Ada", "balance": 10_000}).data
    result = my_agent.charge(pay, cust["id"], 2_500)   # your agent, against a fake Stripe
    assert result.retried_sanely

Seeded and in-memory: each test is deterministic and isolated, and 50 parallel workers don't collide.

Built-in mocks

Each enforces stateful invariants and injects seeded, business-shaped faults. A declarative mock.yaml plus an optional Python handler defines a mock.

Author & share mocks

mockworld new mystripe                # scaffold a runnable, clean-linting mock to grow from
mockworld validate ./mystripe         # schema, handler ABI, determinism smells, description quality
mockworld pack ./mystripe             # print a registry entry (checksum + metadata) to publish

mockworld search weather              # the public registry
mockworld add mock:weather            # install a community mock — checksum-verified + safety-gated

The registry (swarmproof/mockworld-registry) is an index-as-repo: contribute a mock by opening a PR with a folder and an index entry. See docs/AUTHORING.md and mock:hello.

Compose, record, and simulate

# Compose several mocks into one world with a shared customer namespace:
mockworld run world:examples/worlds/ecommerce.yaml --seed 42
#   → payments + crm + email share the same 50 customers: charge → update CRM → email, consistently.

# Scaffold a runnable mock from an OpenAPI spec — or from captured traffic (HAR):
mockworld record --openapi ./petstore.yaml --out ./petstore_mock
mockworld record --har ./session.har --name orders --out ./orders_mock

# Run a scripted-persona swarm → an Agent Readiness Report (the misuse map):
mockworld swarm mock:crm --agents 200 --goal hide --seed 42
#   ⚠ 32.5% of agents destroyed data they meant to hide (delete vs archive) — reproducible.
mockworld swarm mock:crm --agents 200 --seed 42 --descriptions ambiguous
#   ⚠ 45.5% — same swarm, vaguer tool descriptions.

# Save a dirtied world as a portable artifact, then boot a server from it to reproduce a bug:
mockworld snapshot save mock:payments bug123.mw.json --seed 7
mockworld run mock:payments --snapshot bug123.mw.json

# Govern fidelity drift against a real provider's OpenAPI contract:
mockworld verify mock:payments --against ./stripe-openapi.yaml

# Export target-side traces (OTel GenAI profile) to any OTLP collector:
mockworld run mock:payments --otlp http://localhost:4318

The joint chaos demo — a transport interruption and a business decline at once, with the side-effect firing exactly once — runs standalone:

python examples/demos/exactly_once_under_chaos.py

Project structure

mockworld/
├── src/mockworld/
│   ├── determinism.py       # the seeded entropy funnel (clock/ids/rng/fault-dice)
│   ├── state.py             # copy-on-write state store (memory / sqlite)
│   ├── session.py           # per-session isolation
│   ├── schema.py            # the mock.yaml pydantic models
│   ├── faults.py            # business-logic fault injector
│   ├── dispatch.py          # declarative CRUD + Python handler ABI
│   ├── engine.py            # the transport-free call path (start here)
│   ├── server.py            # MCP exposure: stdio + Streamable HTTP + resources
│   ├── control.py           # control plane + stampede Target protocol
│   ├── trace.py             # OTel-GenAI-profile spans + NDJSON + OTLP export
│   ├── registry.py          # add / search / pack (index-as-repo)
│   ├── world.py             # compose mocks with a shared identity namespace
│   ├── record.py            # scaffold a mock from OpenAPI / HAR
│   ├── snapshot.py          # portable scenario snapshots (+ migration)
│   ├── swarm.py             # persona swarm → Agent Readiness Report
│   ├── verify.py            # contract-drift check vs OpenAPI
│   ├── cli.py               # the mockworld command
│   └── mocks/               # payments · crm · exchange · email · files · hello
├── tests/                   # 94 tests mapping to the TEST-PLAN gates
├── docs/                    # ARCHITECTURE · PRD · AUTHORING · RELEASING · TEST-PLAN · …
├── examples/                # worlds/ · demos/ · registry/
└── .github/workflows/       # ci.yml · release.yml

The engine has no MCP dependency; server.py, control.py, and cli.py are adapters over it. This keeps the determinism and isolation tests dependency-free.

Documentation

Contributing

Contributions welcome — bug reports, new mocks, and features. See CONTRIBUTING.md. Three rules: determinism is required (all entropy comes from the seeded ctx; the validator enforces it), faults are business-logic only, and every mock ships a fidelity.md.

git clone https://github.com/swarmproof/mockworld && cd mockworld
uv venv && uv pip install -e ".[dev]"
python -m pytest -q

License

Apache-2.0. Citable via CITATION.cff.

Part of the Swarm Proof toolkit

mockworld is one of the Swarm Proof projects for building and testing reliable agents: