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vivarium.dev/layer1-wasm/cpython-137205

WASM

VIVARIUM · LAYER 1 · PYODIDE · SQLITE3

Reproducing python/cpython#137205

PRAGMA foreign_keys silently dropped under autocommit=False.

✕ UNREPRODUCED|fk_off ≠ fk_on

# repro.pyimport sqlite3def test_fk():conn = sqlite3.connect(':memory:')fk_on = conn.execute('PRAGMA foreign_keys').fetchone()[0]assert fk_on == 1

ElementsConsoleNetworkSources1 error

›await pyodide.runPythonAsync(repro)

✕AssertionError: 0 != 1

at test_fk (repro.py:5)

vivarium.dev/layer2-docker/postgres-lost-update

DOCKER

VIVARIUM · LAYER 2 · DOCKER · POSTGRESQL

Reproducing PostgreSQL lost-update under READ COMMITTED

Concurrent UPDATEs lose writes when a SELECT-then-UPDATE pattern omits row-level locks.

RUNNING — tx-1 ⨯ tx-2

# compose-up.shdocker compose up -dpsql -c "SET TRANSACTION ISOLATION LEVEL READ COMMITTED"parallel ./tx.sh ::= 1 2

container.logtx-1.logtx-2.logrunning

›[docker] Pulling postgres:15-alpine

✓postgres ready in 74ms

›tx-1: UPDATE accounts SET balance = balance + 100

›tx-2: UPDATE accounts SET balance = balance - 50

waiting for both transactions to commit…

vivarium.dev/layer1-wasm/ruby-21709

WASM

VIVARIUM · LAYER 1 · RUBY.WASM · ENCODING

Reproducing ruby/ruby#21709

Regexp interpolation rejects the mixed encodings that String interpolation silently upgrades.

✓ REPRODUCED — bug reproduced

# repro.rbprefix = '\p{In_Arabic}'suffix = prefix.encode('US-ASCII')re = /#{prefix}#{suffix}/str = "#{prefix}#{suffix}"

ElementsConsoleNetworkSourcesverified

›await rubyVm.evalAsync(repro)

✓Regexp raised RegexpError; String built UTF-8

verdict: REPRODUCED — issue#21709 reproducible in ruby.wasm

// SHIPPED · 2026

reproductions catalogued

MCP tools

phases closed

public contract

// THREE-LAYER ARCHITECTURE

Pick the layer that fits your bug.

You never choose by hand — each recipe declares its own layer. The layers exist because no single runtime fits every bug.

Browser-native, instant.

WebAssembly runtimes inside the visitor’s tab. Algorithms, parsers, in-memory database operations. Startup in milliseconds to seconds.

Pyodide · sqlite-wasm · wasm32-wasip1 · Ruby.wasm · PHP.wasm

Container fidelity.

Real filesystem, real processes, real network. Catalogue model: pinned Dockerfile + GHCR image. The visitor reproduces locally with one `docker run`.

Docker · Firecracker · gVisor

Whatever L1 and L2 cannot reach.

Heisenbugs, races, time-travel debugging. No recipes ship here yet — a runtime earns a place only once a reader on an ordinary machine can reproduce and check the result.

Antithesis · CRIU · WASI Preview 3+

// WHERE TO START