diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..bd5590b
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,21 @@
+node_modules/
+.npmrc
+.env
+.env.*
+__tests__/
+coverage/
+.nyc_output/
+dist/
+build/
+.cache/
+*.log
+.DS_Store
+tmp/
+.tmp/
+__pycache__/
+*.pyc
+.venv/
+venv/
+*.egg-info/
+.pytest_cache/
+READY_TO_PUBLISH
diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 0000000..b05a9b2
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,24 @@
+**Architecture**
+
+MercuriusMesh (Phase 0) — a deterministic, offline-first sandbox for cross-venue experiments.
+
+Tech stack
+- Language: Python 3.9+
+- Runtime packages: pydantic (data contracts), pytest (tests)
+
+Project layout
+- mercuriusmesh/: core package with contracts, toy adapters, and a minimal deterministic simulator
+- tests/: pytest test-suite
+- test.sh: runs tests and builds the package
+
+Guiding principles
+- Keep core contracts minimal and canonical: LocalBook, OrderEvent, QuoteSnapshot, SharedSignals, PlanDelta
+- Deterministic replay: append-only event log; replay reinitialises adapters and replays events
+- Small, well-tested chunks: this agent implements Phase 0 core contracts + toy two-venue demo
+
+Testing
+- Run `bash test.sh` to run the full verification: pytest and `python3 -m build`.
+
+Contribution rules
+- Follow the existing contracts module when adding fields; prefer additive, backward-compatible changes
+- Add adapter conformance tests to `tests/` and update AGENTS.md when adding new adapters
diff --git a/README.md b/README.md
index 43904d7..54c2469 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,16 @@
-# idea83-mercuriusmesh-cross-venue
+# MercuriusMesh — Cross-Venue Market Microstructure Sandbox (Phase 0)
 
-Source logic for Idea #83
\ No newline at end of file
+This repository contains a Phase 0 implementation of MercuriusMesh: a deterministic,
+offline-first sandbox for cross-venue market microstructure experiments.
+
+What is included
+- Core pydantic contracts: LocalBook, OrderEvent, QuoteSnapshot, SharedSignals, PlanDelta
+- A tiny deterministic two-venue simulator and toy adapters
+- Tests validating model serialization and deterministic replay
+
+Run tests and build
+1. Install dev deps: `pip install -r requirements.txt` or `pip install pydantic pytest build`
+2. Run `bash test.sh` which runs `pytest` and `python3 -m build`
+
+This is a focused chunk (Phase 0). Future work: latency/partition modeling, ADMM-lite coordinator,
+delta-sync, governance logs, conformance harness and adapters marketplace.
diff --git a/mercuriusmesh/__init__.py b/mercuriusmesh/__init__.py
new file mode 100644
index 0000000..ce85eab
--- /dev/null
+++ b/mercuriusmesh/__init__.py
@@ -0,0 +1,16 @@
+"""MercuriusMesh core package (Phase 0).
+
+Contains canonical contracts and a tiny deterministic two-venue simulator used in tests.
+"""
+
+from .contracts import LocalBook, OrderEvent, QuoteSnapshot, SharedSignals, PlanDelta
+from .simulator import DeterministicSimulator
+
+__all__ = [
+    "LocalBook",
+    "OrderEvent",
+    "QuoteSnapshot",
+    "SharedSignals",
+    "PlanDelta",
+    "DeterministicSimulator",
+]
diff --git a/mercuriusmesh/adapters.py b/mercuriusmesh/adapters.py
new file mode 100644
index 0000000..fd6fe9c
--- /dev/null
+++ b/mercuriusmesh/adapters.py
@@ -0,0 +1,44 @@
+from typing import Callable, List
+from .contracts import QuoteSnapshot, OrderEvent
+
+
+class ToyVenueAdapter:
+    """A tiny in-process adapter that exposes a simple order acceptance model.
+
+    It accepts OrderEvent objects and updates an internal top-of-book. Deterministic
+    behaviour is achieved by not using random jitter and relying on caller ordering.
+    """
+
+    def __init__(self, venue_id: str, initial_bid: float = 100.0, initial_ask: float = 100.5):
+        self.venue_id = venue_id
+        self.best_bid = initial_bid
+        self.best_ask = initial_ask
+        self.listeners: List[Callable[[QuoteSnapshot], None]] = []
+
+    def on_snapshot(self, cb: Callable[[QuoteSnapshot], None]):
+        self.listeners.append(cb)
+
+    def publish_snapshot(self):
+        snap = QuoteSnapshot(
+            venue_id=self.venue_id,
+            asset="TEST/USD",
+            bids=[[self.best_bid, 100.0]],
+            asks=[[self.best_ask, 100.0]],
+        )
+        for cb in self.listeners:
+            cb(snap)
+
+    def handle_order(self, order: OrderEvent) -> OrderEvent:
+        # Very small deterministic matcher: marketable at top-of-book executes
+        if order.side == "buy" and order.price is None:
+            order.status = "filled"
+            # naive mid update
+            self.best_bid = max(self.best_bid, order.price or self.best_bid)
+        elif order.side == "sell" and order.price is None:
+            order.status = "filled"
+            self.best_ask = min(self.best_ask, order.price or self.best_ask)
+        else:
+            order.status = "accepted"
+        # publish new snapshot deterministically
+        self.publish_snapshot()
+        return order
diff --git a/mercuriusmesh/contracts.py b/mercuriusmesh/contracts.py
new file mode 100644
index 0000000..d170567
--- /dev/null
+++ b/mercuriusmesh/contracts.py
@@ -0,0 +1,57 @@
+from typing import List, Optional
+from pydantic import BaseModel, Field
+from datetime import datetime
+
+
+class LocalBook(BaseModel):
+    """A minimal per-venue order book snapshot.
+
+    This is intentionally tiny for Phase 0: top-of-book bids/asks with sizes and timestamp.
+    """
+    venue_id: str
+    asset: str
+    best_bid: Optional[float]
+    best_bid_size: Optional[float]
+    best_ask: Optional[float]
+    best_ask_size: Optional[float]
+    timestamp: datetime = Field(default_factory=datetime.utcnow)
+
+
+class OrderEvent(BaseModel):
+    """A canonical order lifecycle event used between adapters and simulator."""
+    order_id: str
+    venue_id: str
+    asset: str
+    side: str  # 'buy' or 'sell'
+    price: Optional[float]
+    size: float
+    created_at: datetime = Field(default_factory=datetime.utcnow)
+    status: str = "submitted"  # submitted, accepted, filled, cancelled
+
+
+class QuoteSnapshot(BaseModel):
+    venue_id: str
+    asset: str
+    bids: List[List[float]] = []  # [[price, size], ...]
+    asks: List[List[float]] = []
+    timestamp: datetime = Field(default_factory=datetime.utcnow)
+
+
+class SharedSignals(BaseModel):
+    """Lightweight aggregated signals used for cross-venue coordination."""
+    asset: str
+    mid_price: Optional[float]
+    liquidity: Optional[float]
+    timestamp: datetime = Field(default_factory=datetime.utcnow)
+
+
+class PlanDelta(BaseModel):
+    """An incremental plan update: proposed hedge or quote update."""
+    plan_id: str
+    author: str
+    asset: str
+    venue_id: str
+    side: str
+    price: Optional[float]
+    size: float
+    created_at: datetime = Field(default_factory=datetime.utcnow)
diff --git a/mercuriusmesh/simulator.py b/mercuriusmesh/simulator.py
new file mode 100644
index 0000000..9198528
--- /dev/null
+++ b/mercuriusmesh/simulator.py
@@ -0,0 +1,70 @@
+from typing import Dict, List
+from .adapters import ToyVenueAdapter
+from .contracts import OrderEvent, LocalBook, SharedSignals
+
+
+class DeterministicSimulator:
+    """A tiny deterministic simulator that runs a sequence of events and supports replay.
+
+    The simulator stores a simple event log (append-only). Replaying the log deterministically
+    rebuilds the same state. This is a minimal skeleton intended for Phase 0 testing and demos.
+    """
+
+    def __init__(self, seed: int = 0):
+        self.seed = seed
+        self.adapters: Dict[str, ToyVenueAdapter] = {}
+        self.event_log: List[OrderEvent] = []
+        self.state: Dict[str, LocalBook] = {}
+        # store adapter initial params so replay can re-create identical adapters
+        self._adapter_initials: Dict[str, Dict[str, float]] = {}
+
+    def add_adapter(self, adapter: ToyVenueAdapter):
+        self.adapters[adapter.venue_id] = adapter
+        # capture initial topology for deterministic replay
+        self._adapter_initials[adapter.venue_id] = {"best_bid": adapter.best_bid, "best_ask": adapter.best_ask}
+
+        def snapshot_listener(snap):
+            # update simple LocalBook
+            self.state[adapter.venue_id] = LocalBook(
+                venue_id=snap.venue_id,
+                asset=snap.asset,
+                best_bid=snap.bids[0][0] if snap.bids else None,
+                best_bid_size=snap.bids[0][1] if snap.bids else None,
+                best_ask=snap.asks[0][0] if snap.asks else None,
+                best_ask_size=snap.asks[0][1] if snap.asks else None,
+            )
+
+        adapter.on_snapshot(snapshot_listener)
+
+    def submit_order(self, order: OrderEvent) -> OrderEvent:
+        # append to log and route to adapter deterministically by venue_id
+        self.event_log.append(order)
+        adapter = self.adapters.get(order.venue_id)
+        if adapter is None:
+            order.status = "rejected"
+            return order
+        return adapter.handle_order(order)
+
+    def replay(self) -> None:
+        # clear state and re-apply events in order
+        self.state = {}
+        # re-create adapters to ensure deterministic initial conditions
+        adapters_copy = {}
+        for vid, initials in self._adapter_initials.items():
+            adapters_copy[vid] = ToyVenueAdapter(vid, initial_bid=initials.get("best_bid", 100.0), initial_ask=initials.get("best_ask", 100.5))
+        self.adapters = adapters_copy
+        for vid in list(self.adapters.keys()):
+            # re-register listeners and initial state
+            self.add_adapter(self.adapters[vid])
+        for e in list(self.event_log):
+            self.submit_order(e)
+
+    def compute_shared_signals(self, asset: str = "TEST/USD") -> SharedSignals:
+        # simple average mid from known LocalBook entries
+        mids = []
+        for lb in self.state.values():
+            if lb.best_bid is not None and lb.best_ask is not None:
+                mids.append((lb.best_bid + lb.best_ask) / 2)
+        mid = sum(mids) / len(mids) if mids else None
+        liquidity = sum((lb.best_bid_size or 0) + (lb.best_ask_size or 0) for lb in self.state.values())
+        return SharedSignals(asset=asset, mid_price=mid, liquidity=liquidity)
diff --git a/pyproject.toml b/pyproject.toml
new file mode 100644
index 0000000..dc7e728
--- /dev/null
+++ b/pyproject.toml
@@ -0,0 +1,14 @@
+[build-system]
+requires = ["setuptools>=61.0", "wheel"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "mercuriusmesh"
+version = "0.0.1"
+readme = "README.md"
+requires-python = ">=3.9"
+dependencies = ["pydantic>=1.10"]
+
+[tool.setuptools]
+packages = ["mercuriusmesh"]
+include-package-data = true
diff --git a/setup.cfg b/setup.cfg
new file mode 100644
index 0000000..7562639
--- /dev/null
+++ b/setup.cfg
@@ -0,0 +1,14 @@
+[metadata]
+name = mercuriusmesh
+version = 0.0.1
+description = MercuriusMesh: Cross-venue market microstructure sandbox (toy phase0)
+long_description = file: README.md
+long_description_content_type = text/markdown
+author = OpenCode Agent
+license = MIT
+
+[options]
+packages = find:
+install_requires =
+    pydantic>=1.10
+    pytest>=7.0
diff --git a/test.sh b/test.sh
new file mode 100644
index 0000000..6cd7761
--- /dev/null
+++ b/test.sh
@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+echo "Running tests..."
+# ensure package dir is importable for pytest
+export PYTHONPATH="$(pwd)"
+echo "Installing test dependencies (pydantic, pytest, build)..."
+python3 -m pip install --quiet --upgrade pip
+python3 -m pip install --quiet pydantic pytest build
+pytest -q
+
+echo "Building source distribution and wheel..."
+python3 -m build
+
+echo "All checks passed."
diff --git a/tests/test_contracts.py b/tests/test_contracts.py
new file mode 100644
index 0000000..3682110
--- /dev/null
+++ b/tests/test_contracts.py
@@ -0,0 +1,14 @@
+from mercuriusmesh.contracts import LocalBook, OrderEvent, QuoteSnapshot, SharedSignals, PlanDelta
+
+
+def test_contract_models():
+    lb = LocalBook(venue_id="V1", asset="TEST/USD", best_bid=100.0, best_bid_size=10, best_ask=100.5, best_ask_size=12)
+    assert lb.venue_id == "V1"
+    oe = OrderEvent(order_id="o1", venue_id="V1", asset="TEST/USD", side="buy", price=None, size=1)
+    assert oe.status == "submitted"
+    qs = QuoteSnapshot(venue_id="V1", asset="TEST/USD", bids=[[100.0, 10]], asks=[[100.5, 12]])
+    assert qs.bids[0][0] == 100.0
+    ss = SharedSignals(asset="TEST/USD", mid_price=100.25, liquidity=22)
+    assert ss.asset == "TEST/USD"
+    pd = PlanDelta(plan_id="p1", author="a1", asset="TEST/USD", venue_id="V1", side="buy", price=100.1, size=2)
+    assert pd.plan_id == "p1"
diff --git a/tests/test_simulation_replay.py b/tests/test_simulation_replay.py
new file mode 100644
index 0000000..1e051a1
--- /dev/null
+++ b/tests/test_simulation_replay.py
@@ -0,0 +1,27 @@
+from mercuriusmesh.simulator import DeterministicSimulator
+from mercuriusmesh.adapters import ToyVenueAdapter
+from mercuriusmesh.contracts import OrderEvent
+
+
+def test_deterministic_replay():
+    sim = DeterministicSimulator()
+    a = ToyVenueAdapter("V1", initial_bid=100.0, initial_ask=100.5)
+    b = ToyVenueAdapter("V2", initial_bid=99.8, initial_ask=100.3)
+    sim.add_adapter(a)
+    sim.add_adapter(b)
+
+    o1 = OrderEvent(order_id="o1", venue_id="V1", asset="TEST/USD", side="buy", price=None, size=1)
+    o2 = OrderEvent(order_id="o2", venue_id="V2", asset="TEST/USD", side="sell", price=None, size=2)
+
+    sim.submit_order(o1)
+    sim.submit_order(o2)
+
+    # snapshot state
+    s1 = sim.compute_shared_signals()
+
+    # replay and compare
+    sim.replay()
+    s2 = sim.compute_shared_signals()
+
+    assert s1.mid_price == s2.mid_price
+    assert s1.liquidity == s2.liquidity