diff --git a/.scratch/distributed-inference-network/prd.json b/.scratch/distributed-inference-network/prd.json
index 2840388..800e160 100644
--- a/.scratch/distributed-inference-network/prd.json
+++ b/.scratch/distributed-inference-network/prd.json
@@ -93,11 +93,12 @@
         "Commit only this story's code changes with a focused conventional commit message"
       ],
       "priority": 4,
-      "passes": false,
+      "passes": true,
       "notes": "Source issue: .scratch/distributed-inference-network/issues/04-node-client-startup.md",
       "dependsOn": [
         "US-003"
-      ]
+      ],
+      "completionNotes": "Completed by Ralph iteration 86510a10; verified by pytest, compileall, editable installs, CLI help, and final review."
     },
     {
       "id": "US-005",
diff --git a/packages/node/meshnet_node/cli.py b/packages/node/meshnet_node/cli.py
index 67d2ab5..8aa889a 100644
--- a/packages/node/meshnet_node/cli.py
+++ b/packages/node/meshnet_node/cli.py
@@ -2,6 +2,7 @@
 
 import argparse
 import sys
+import time
 
 
 def main() -> None:
@@ -12,20 +13,34 @@ def main() -> None:
     subparsers = parser.add_subparsers(dest="command")
 
     start_cmd = subparsers.add_parser("start", help="Start the node server")
-    start_cmd.add_argument("--tracker", default="http://localhost:8080", help="Tracker URL")
+    start_cmd.add_argument(
+        "--tracker", default="http://localhost:8080", help="Tracker URL"
+    )
     start_cmd.add_argument("--port", type=int, default=7000, help="Port to listen on")
+    start_cmd.add_argument(
+        "--model", default="stub-model", help="Model preset to request from tracker"
+    )
+    start_cmd.add_argument(
+        "--host", default="0.0.0.0", help="Interface to bind to"
+    )
+    start_cmd.add_argument(
+        "--advertise-host",
+        help="Reachable host/IP to advertise to the tracker (defaults to FQDN when binding 0.0.0.0)",
+    )
 
     args = parser.parse_args()
 
     if args.command == "start":
-        from meshnet_node.server import StubNodeServer
+        from meshnet_node.startup import run_startup
 
-        node = StubNodeServer(port=args.port)
-        port = node.start()
-        print(f"meshnet-node listening on port {port}")
-        print(f"Tracker URL: {args.tracker}")
+        node = run_startup(
+            tracker_url=args.tracker,
+            port=args.port,
+            model=args.model,
+            host=args.host,
+            advertise_host=args.advertise_host,
+        )
         try:
-            import time
             while True:
                 time.sleep(1)
         except KeyboardInterrupt:
diff --git a/packages/node/meshnet_node/downloader.py b/packages/node/meshnet_node/downloader.py
new file mode 100644
index 0000000..5716d49
--- /dev/null
+++ b/packages/node/meshnet_node/downloader.py
@@ -0,0 +1,62 @@
+"""Shard downloader — fetches or stubs a model shard from HuggingFace Hub.
+
+Cache layout: ~/.cache/meshnet/shards/<model>/layers_<start>-<end>/
+
+For "stub-model" (no HF repo), a placeholder JSON file is written so the
+test suite never touches the network.
+"""
+
+import json
+from pathlib import Path
+
+_DEFAULT_CACHE = Path.home() / ".cache" / "meshnet" / "shards"
+
+
+def download_shard(
+    model: str,
+    shard_start: int,
+    shard_end: int,
+    cache_dir: Path = _DEFAULT_CACHE,
+    hf_repo: str | None = None,
+    progress: bool = True,
+) -> Path:
+    """Ensure the shard is present in *cache_dir* and return its local path.
+
+    When *hf_repo* is None (or *model* is ``"stub-model"``), a placeholder
+    weights file is created locally — no network access required.  This keeps
+    the test suite hermetic while the real download path is exercised by
+    passing a non-stub *hf_repo*.
+    """
+    shard_dir = cache_dir / model / f"layers_{shard_start}-{shard_end}"
+    shard_dir.mkdir(parents=True, exist_ok=True)
+
+    if hf_repo is None or model == "stub-model":
+        stub_file = shard_dir / "weights.json"
+        if not stub_file.exists():
+            stub_file.write_text(json.dumps({
+                "model": model,
+                "shard_start": shard_start,
+                "shard_end": shard_end,
+                "stub": True,
+            }))
+            if progress:
+                print(f"  [stub] shard placeholder written to {stub_file}", flush=True)
+        else:
+            if progress:
+                print(f"  [stub] shard already cached at {shard_dir}", flush=True)
+        return shard_dir
+
+    from huggingface_hub import snapshot_download  # type: ignore[import]
+
+    if progress:
+        print(
+            f"  Downloading layers {shard_start}-{shard_end} from {hf_repo} ...",
+            flush=True,
+        )
+
+    local_dir = snapshot_download(
+        repo_id=hf_repo,
+        cache_dir=str(cache_dir),
+        local_dir=str(shard_dir),
+    )
+    return Path(local_dir)
diff --git a/packages/node/meshnet_node/hardware.py b/packages/node/meshnet_node/hardware.py
new file mode 100644
index 0000000..33724dd
--- /dev/null
+++ b/packages/node/meshnet_node/hardware.py
@@ -0,0 +1,33 @@
+"""GPU hardware detection with graceful CPU fallback."""
+
+import subprocess
+
+
+def detect_hardware() -> dict:
+    """Detect GPU model and available VRAM. Returns hardware profile dict."""
+    try:
+        import torch  # type: ignore[import]
+        if torch.cuda.is_available():
+            idx = torch.cuda.current_device()
+            name = torch.cuda.get_device_name(idx)
+            props = torch.cuda.get_device_properties(idx)
+            vram_mb = props.total_memory // (1024 * 1024)
+            return {"device": "cuda", "gpu_name": name, "vram_mb": vram_mb}
+    except ImportError:
+        pass
+
+    try:
+        result = subprocess.run(
+            ["nvidia-smi", "--query-gpu=name,memory.total", "--format=csv,noheader,nounits"],
+            capture_output=True, text=True, timeout=5,
+        )
+        if result.returncode == 0 and result.stdout.strip():
+            line = result.stdout.strip().splitlines()[0]
+            parts = line.split(",", 1)
+            gpu_name = parts[0].strip()
+            vram_mb = int(parts[1].strip()) if len(parts) > 1 else 0
+            return {"device": "cuda", "gpu_name": gpu_name, "vram_mb": vram_mb}
+    except (FileNotFoundError, subprocess.TimeoutExpired, ValueError, IndexError):
+        pass
+
+    return {"device": "cpu", "gpu_name": None, "vram_mb": 0}
diff --git a/packages/node/meshnet_node/startup.py b/packages/node/meshnet_node/startup.py
new file mode 100644
index 0000000..c7bc959
--- /dev/null
+++ b/packages/node/meshnet_node/startup.py
@@ -0,0 +1,154 @@
+"""Full node startup sequence — self-configuring, non-interactive."""
+
+from __future__ import annotations
+
+import json
+import socket
+import sys
+import urllib.error
+import urllib.parse
+import urllib.request
+from pathlib import Path
+
+from .downloader import download_shard
+from .hardware import detect_hardware
+from .server import StubNodeServer
+from .wallet import load_or_create_wallet
+
+
+def _post_json(url: str, payload: dict, timeout: float = 10.0) -> dict:
+    data = json.dumps(payload).encode()
+    req = urllib.request.Request(
+        url, data=data, headers={"Content-Type": "application/json"}, method="POST"
+    )
+    with urllib.request.urlopen(req, timeout=timeout) as r:
+        return json.loads(r.read())
+
+
+def _get_json(url: str, timeout: float = 10.0) -> dict:
+    with urllib.request.urlopen(url, timeout=timeout) as r:
+        return json.loads(r.read())
+
+
+def run_startup(
+    tracker_url: str,
+    port: int = 0,
+    model: str = "stub-model",
+    wallet_path: Path | None = None,
+    cache_dir: Path | None = None,
+    host: str = "127.0.0.1",
+    advertise_host: str | None = None,
+) -> StubNodeServer:
+    """Execute the full startup sequence and return a running node server.
+
+    Steps (all non-interactive):
+      1. Detect GPU / hardware profile
+      2. Load or generate Solana wallet keypair
+      3. Query tracker for optimal shard assignment
+      4. Download (or stub) the assigned shard
+      5. Start local HTTP server
+      6. Register with tracker
+
+    Prints a compact status summary on completion.
+    """
+
+    tracker_url = tracker_url.rstrip("/")
+
+    # 1. Hardware detection
+    print("Detecting hardware...", flush=True)
+    hw = detect_hardware()
+    device: str = hw["device"]
+    gpu_name: str | None = hw.get("gpu_name")
+    vram_mb: int = hw.get("vram_mb", 0)
+
+    if device == "cpu":
+        print("  WARNING: No CUDA GPU detected — running in CPU mode", flush=True)
+    else:
+        print(f"  GPU: {gpu_name} ({vram_mb} MB VRAM)", flush=True)
+
+    # 2. Wallet
+    print("Loading wallet...", flush=True)
+    wallet_kwargs: dict = {}
+    if wallet_path is not None:
+        wallet_kwargs["path"] = wallet_path
+    _, _, address = load_or_create_wallet(**wallet_kwargs)
+    print(f"  Wallet: {address}", flush=True)
+
+    # 3. Shard assignment from tracker
+    print("Querying tracker for shard assignment...", flush=True)
+    assign_qs = urllib.parse.urlencode({
+        "model": model,
+        "device": device,
+        "vram_mb": vram_mb,
+    })
+    try:
+        assignment = _get_json(f"{tracker_url}/v1/nodes/assign?{assign_qs}")
+    except urllib.error.URLError as exc:
+        print(f"  ERROR: Cannot reach tracker at {tracker_url}: {exc}", file=sys.stderr, flush=True)
+        raise
+
+    shard_start: int = assignment["shard_start"]
+    shard_end: int = assignment["shard_end"]
+    assigned_model: str = assignment.get("model", model)
+    hf_repo: str | None = assignment.get("hf_repo")
+    print(f"  Shard: layers {shard_start}-{shard_end} of {assigned_model}", flush=True)
+
+    # 4. Download shard
+    print("Downloading shard...", flush=True)
+    dl_kwargs: dict = {}
+    if cache_dir is not None:
+        dl_kwargs["cache_dir"] = cache_dir
+    if hf_repo is not None:
+        dl_kwargs["hf_repo"] = hf_repo
+    shard_path = download_shard(assigned_model, shard_start, shard_end, **dl_kwargs)
+    print(f"  Cached at: {shard_path}", flush=True)
+
+    # 5. Start HTTP server
+    is_last = shard_end >= assignment.get("model_layers_end", shard_end)
+    node = StubNodeServer(
+        host=host,
+        port=port,
+        shard_start=shard_start,
+        shard_end=shard_end,
+        is_last_shard=is_last,
+    )
+    actual_port = node.start()
+    public_host = advertise_host or (socket.getfqdn() if host == "0.0.0.0" else host)
+    endpoint = f"http://{public_host}:{actual_port}"
+
+    # 6. Register with tracker
+    print("Registering with tracker...", flush=True)
+    try:
+        reg_resp = _post_json(
+            f"{tracker_url}/v1/nodes/register",
+            {
+                "endpoint": endpoint,
+                "shard_start": shard_start,
+                "shard_end": shard_end,
+                "hardware_profile": hw,
+                "wallet_address": address,
+                "score": 1.0,
+            },
+        )
+        node_id: str = reg_resp["node_id"]
+    except Exception:
+        node.stop()
+        raise
+
+    # Status summary
+    hw_str = device.upper()
+    if gpu_name:
+        hw_str += f" ({gpu_name}, {vram_mb} MB)"
+    print(
+        f"\n{'=' * 32}\n"
+        f"meshnet-node ready\n"
+        f"  Wallet:   {address}\n"
+        f"  Shard:    layers {shard_start}-{shard_end} ({assigned_model})\n"
+        f"  Endpoint: {endpoint}\n"
+        f"  Node ID:  {node_id}\n"
+        f"  Hardware: {hw_str}\n"
+        f"{'=' * 32}",
+        flush=True,
+    )
+
+    return node
diff --git a/packages/node/meshnet_node/wallet.py b/packages/node/meshnet_node/wallet.py
new file mode 100644
index 0000000..3a4477c
--- /dev/null
+++ b/packages/node/meshnet_node/wallet.py
@@ -0,0 +1,64 @@
+"""Solana wallet management — load or generate an Ed25519 keypair.
+
+Solana keypair format: 64-byte array stored as JSON list of ints.
+  bytes 0-31: private scalar (seed)
+  bytes 32-63: public key (on-curve point)
+Wallet address: base58-encoded 32-byte public key.
+"""
+
+import json
+from pathlib import Path
+
+_DEFAULT_WALLET_PATH = Path.home() / ".config" / "meshnet" / "wallet.json"
+
+_B58_ALPHABET = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"
+
+
+def _b58encode(data: bytes) -> str:
+    n = int.from_bytes(data, "big")
+    chars: list[str] = []
+    while n:
+        n, rem = divmod(n, 58)
+        chars.append(_B58_ALPHABET[rem])
+    leading = 0
+    for b in data:
+        if b == 0:
+            leading += 1
+        else:
+            break
+    return "1" * leading + "".join(reversed(chars))
+
+
+def load_or_create_wallet(
+    path: Path = _DEFAULT_WALLET_PATH,
+) -> tuple[bytes, bytes, str]:
+    """Return (secret_32, public_32, address_base58).
+
+    Loads from *path* if it exists; otherwise generates a new keypair and
+    saves it.  The parent directory is created if needed.
+    """
+    if path.exists():
+        mode = path.stat().st_mode & 0o777
+        if mode & 0o077:
+            path.chmod(0o600)
+        raw = bytes(json.loads(path.read_text()))
+        if len(raw) != 64:
+            raise ValueError(f"Wallet at {path} has unexpected length {len(raw)}, expected 64")
+        secret, public = raw[:32], raw[32:]
+        return secret, public, _b58encode(public)
+
+    from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
+    from cryptography.hazmat.primitives.serialization import (
+        Encoding, NoEncryption, PrivateFormat, PublicFormat,
+    )
+
+    priv = Ed25519PrivateKey.generate()
+    secret = priv.private_bytes(Encoding.Raw, PrivateFormat.Raw, NoEncryption())
+    public = priv.public_key().public_bytes(Encoding.Raw, PublicFormat.Raw)
+    keypair = list(secret) + list(public)
+
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(json.dumps(keypair))
+    path.chmod(0o600)
+
+    return secret, public, _b58encode(public)
diff --git a/packages/node/pyproject.toml b/packages/node/pyproject.toml
index 68b4cf3..46225c8 100644
--- a/packages/node/pyproject.toml
+++ b/packages/node/pyproject.toml
@@ -8,6 +8,11 @@ version = "0.1.0"
 description = "Distributed Inference Network node client"
 requires-python = ">=3.10"
 
+dependencies = [
+    "cryptography>=41",
+    "huggingface-hub>=0.20",
+]
+
 [project.scripts]
 meshnet-node = "meshnet_node.cli:main"
 
diff --git a/packages/tracker/meshnet_tracker/server.py b/packages/tracker/meshnet_tracker/server.py
index 785e3da..dd66e8e 100644
--- a/packages/tracker/meshnet_tracker/server.py
+++ b/packages/tracker/meshnet_tracker/server.py
@@ -4,7 +4,8 @@ HTTP API contract:
 - POST /v1/nodes/register
   Request JSON: {
     "endpoint": "http://host:port", "shard_start": int, "shard_end": int,
-    "hardware_profile": object, "score": number optional
+    "hardware_profile": object, "wallet_address": str optional,
+    "score": number optional
   }
   Response 200: {"node_id": str}
   Response 400: {"error": str}
@@ -32,7 +33,7 @@ DEFAULT_MODEL_PRESETS: dict[str, dict] = {
 class _NodeEntry:
     __slots__ = (
         "node_id", "endpoint", "shard_start", "shard_end",
-        "hardware_profile", "score", "last_heartbeat",
+        "hardware_profile", "wallet_address", "score", "last_heartbeat",
     )
 
     def __init__(
@@ -42,6 +43,7 @@ class _NodeEntry:
         shard_start: int,
         shard_end: int,
         hardware_profile: dict,
+        wallet_address: str | None,
         score: float,
     ) -> None:
         self.node_id = node_id
@@ -49,6 +51,7 @@ class _NodeEntry:
         self.shard_start = shard_start
         self.shard_end = shard_end
         self.hardware_profile = hardware_profile
+        self.wallet_address = wallet_address
         self.score = score
         self.last_heartbeat: float = time.monotonic()
 
@@ -146,6 +149,8 @@ class _TrackerHandler(http.server.BaseHTTPRequestHandler):
         parsed = urllib.parse.urlparse(self.path)
         if parsed.path == "/v1/route":
             self._handle_route(parsed)
+        elif parsed.path == "/v1/nodes/assign":
+            self._handle_assign(parsed)
         else:
             self.send_response(404)
             self.end_headers()
@@ -180,6 +185,10 @@ class _TrackerHandler(http.server.BaseHTTPRequestHandler):
         if not isinstance(hardware_profile, dict):
             self._send_json(400, {"error": "hardware_profile must be an object"})
             return
+        wallet_address = body.get("wallet_address")
+        if wallet_address is not None and not isinstance(wallet_address, str):
+            self._send_json(400, {"error": "wallet_address must be a string"})
+            return
 
         node_id = str(uuid.uuid4())
         entry = _NodeEntry(
@@ -188,6 +197,7 @@ class _TrackerHandler(http.server.BaseHTTPRequestHandler):
             shard_start=shard_start,
             shard_end=shard_end,
             hardware_profile=hardware_profile,
+            wallet_address=wallet_address,
             score=score,
         )
         with server.lock:
@@ -207,6 +217,81 @@ class _TrackerHandler(http.server.BaseHTTPRequestHandler):
             entry.last_heartbeat = time.monotonic()
         self._send_json(200, {})
 
+    def _handle_assign(self, parsed: urllib.parse.ParseResult):
+        """Return an optimal shard assignment for a node given its hardware profile.
+
+        Query params:
+          model   — model preset name (default: first preset)
+          device  — "cuda" | "cpu"
+          vram_mb — integer VRAM in MB (0 for CPU)
+
+        The greedy strategy: find the first gap in current layer coverage
+        and assign it.  If no gap exists, assign the full model range so the
+        node provides redundant coverage.
+        """
+        server: _TrackerHTTPServer = self.server  # type: ignore[assignment]
+        params = urllib.parse.parse_qs(parsed.query)
+
+        model_list = params.get("model")
+        if not model_list:
+            model = next(iter(server.model_presets), None)
+            if model is None:
+                self._send_json(503, {"error": "no model presets configured"})
+                return
+        else:
+            model = model_list[0]
+
+        preset = server.model_presets.get(model)
+        if preset is None:
+            self._send_json(404, {"error": f"unknown model preset: {model!r}"})
+            return
+
+        required_start: int = preset["layers_start"]
+        required_end: int = preset["layers_end"]
+
+        with server.lock:
+            self._purge_expired_nodes()
+            alive = list(server.registry.values())
+
+        device = params.get("device", ["cpu"])[0]
+        try:
+            vram_mb = int(params.get("vram_mb", ["0"])[0])
+        except ValueError:
+            vram_mb = 0
+        max_layers = required_end - required_start + 1
+        if device != "cuda" or vram_mb < 8192:
+            max_layers = min(max_layers, 16)
+
+        # Collect covered intervals sorted by start layer.
+        covered = sorted(
+            [(n.shard_start, n.shard_end) for n in alive],
+            key=lambda t: t[0],
+        )
+
+        # Walk from required_start to find the first uncovered layer.
+        gap_start = required_start
+        for s, e in covered:
+            if s <= gap_start:
+                gap_start = max(gap_start, e + 1)
+            else:
+                break  # gap found before this node
+
+        if gap_start > required_end:
+            # Full coverage exists — assign the full range for redundancy.
+            shard_start = required_start
+            shard_end = min(required_end, shard_start + max_layers - 1)
+        else:
+            shard_start = gap_start
+            shard_end = min(required_end, shard_start + max_layers - 1)
+
+        self._send_json(200, {
+            "shard_start": shard_start,
+            "shard_end": shard_end,
+            "model": model,
+            "model_layers_end": required_end,
+            **({"hf_repo": preset["hf_repo"]} if "hf_repo" in preset else {}),
+        })
+
     def _handle_route(self, parsed: urllib.parse.ParseResult):
         server: _TrackerHTTPServer = self.server  # type: ignore[assignment]
         params = urllib.parse.parse_qs(parsed.query)
diff --git a/tests/test_node_startup.py b/tests/test_node_startup.py
new file mode 100644
index 0000000..9eefc73
--- /dev/null
+++ b/tests/test_node_startup.py
@@ -0,0 +1,297 @@
+"""US-004 integration tests: node self-configuring startup sequence."""
+
+import json
+import sys
+import types
+import urllib.request
+from pathlib import Path
+
+import pytest
+
+from meshnet_node.downloader import download_shard
+from meshnet_node.hardware import detect_hardware
+from meshnet_node.startup import run_startup
+from meshnet_node.wallet import _b58encode, load_or_create_wallet
+from meshnet_tracker.server import TrackerServer
+
+
+# ---------------------------------------------------------------------------
+# Unit tests — hardware, wallet, downloader
+# ---------------------------------------------------------------------------
+
+
+def test_detect_hardware_returns_valid_profile():
+    """Hardware detection always returns a dict with required keys."""
+    hw = detect_hardware()
+    assert hw["device"] in {"cuda", "cpu"}
+    assert isinstance(hw.get("vram_mb"), int)
+    if hw["device"] == "cpu":
+        assert hw["gpu_name"] is None
+        assert hw["vram_mb"] == 0
+    else:
+        assert isinstance(hw["gpu_name"], str) and hw["gpu_name"]
+        assert hw["vram_mb"] > 0
+
+
+def test_wallet_generates_new_keypair(tmp_path):
+    """A new wallet is created when none exists, saved to disk."""
+    wallet_file = tmp_path / "wallet.json"
+    assert not wallet_file.exists()
+
+    secret, public, address = load_or_create_wallet(path=wallet_file)
+
+    assert wallet_file.exists()
+    assert wallet_file.stat().st_mode & 0o777 == 0o600
+    assert len(secret) == 32
+    assert len(public) == 32
+    assert len(address) > 20  # base58-encoded 32 bytes is 43-44 chars typically
+    # Solana addresses are base58 — no '0', 'O', 'I', 'l'
+    for ch in "0OIl":
+        assert ch not in address, f"Invalid base58 char {ch!r} in address {address!r}"
+
+
+def test_wallet_loads_existing_keypair(tmp_path):
+    """Loading the same wallet file twice returns identical keys and address."""
+    wallet_file = tmp_path / "wallet.json"
+
+    secret1, public1, address1 = load_or_create_wallet(path=wallet_file)
+    secret2, public2, address2 = load_or_create_wallet(path=wallet_file)
+
+    assert secret1 == secret2
+    assert public1 == public2
+    assert address1 == address2
+
+
+def test_wallet_load_repairs_insecure_permissions(tmp_path):
+    """Existing private key files are tightened to owner-only permissions."""
+    wallet_file = tmp_path / "wallet.json"
+    load_or_create_wallet(path=wallet_file)
+    wallet_file.chmod(0o644)
+
+    load_or_create_wallet(path=wallet_file)
+
+    assert wallet_file.stat().st_mode & 0o777 == 0o600
+
+
+def test_base58_counts_only_leading_zero_bytes():
+    """Zero bytes inside the public key do not become extra base58 leading ones."""
+    assert _b58encode(bytes([0, 1, 0])) == "15R"
+
+
+def test_download_shard_stub_creates_cache(tmp_path):
+    """Stub-model shard creates a local cache file without network access."""
+    shard_dir = download_shard("stub-model", 0, 31, cache_dir=tmp_path)
+    assert shard_dir.exists()
+    weights = shard_dir / "weights.json"
+    assert weights.exists()
+    data = json.loads(weights.read_text())
+    assert data["stub"] is True
+    assert data["shard_start"] == 0
+    assert data["shard_end"] == 31
+
+
+def test_download_shard_uses_huggingface_when_repo_is_assigned(tmp_path, monkeypatch):
+    """Non-stub shards use the HuggingFace snapshot_download path."""
+    calls = []
+
+    def fake_snapshot_download(repo_id, cache_dir, local_dir):
+        calls.append({"repo_id": repo_id, "cache_dir": cache_dir, "local_dir": local_dir})
+        Path(local_dir).mkdir(parents=True, exist_ok=True)
+        return local_dir
+
+    monkeypatch.setitem(
+        sys.modules,
+        "huggingface_hub",
+        types.SimpleNamespace(snapshot_download=fake_snapshot_download),
+    )
+
+    shard_dir = download_shard(
+        "tiny-llama",
+        0,
+        3,
+        cache_dir=tmp_path,
+        hf_repo="org/tiny-llama-shards",
+        progress=False,
+    )
+
+    assert shard_dir == tmp_path / "tiny-llama" / "layers_0-3"
+    assert calls == [{
+        "repo_id": "org/tiny-llama-shards",
+        "cache_dir": str(tmp_path),
+        "local_dir": str(shard_dir),
+    }]
+
+
+def test_download_shard_stub_idempotent(tmp_path):
+    """Calling download_shard twice does not error — file already exists."""
+    download_shard("stub-model", 0, 31, cache_dir=tmp_path, progress=False)
+    shard_dir = download_shard("stub-model", 0, 31, cache_dir=tmp_path, progress=False)
+    assert shard_dir.exists()
+
+
+# ---------------------------------------------------------------------------
+# Tracker assign endpoint
+# ---------------------------------------------------------------------------
+
+
+def _get_json(url: str) -> dict:
+    with urllib.request.urlopen(url, timeout=5) as r:
+        return json.loads(r.read())
+
+
+def test_tracker_assign_returns_shard_for_empty_registry():
+    """Tracker assigns the full layer range when no nodes are registered."""
+    tracker = TrackerServer()
+    port = tracker.start()
+    try:
+        resp = _get_json(
+            f"http://127.0.0.1:{port}/v1/nodes/assign?model=stub-model&device=cpu&vram_mb=0"
+        )
+        assert resp["shard_start"] == 0
+        assert resp["shard_end"] == 15
+        assert resp["model"] == "stub-model"
+        assert resp["model_layers_end"] == 31
+    finally:
+        tracker.stop()
+
+
+def test_tracker_assign_fills_gap():
+    """Tracker assigns the first uncovered layer range when a node is already registered."""
+    import json as _json
+    import urllib.request as _ur
+
+    tracker = TrackerServer()
+    port = tracker.start()
+    try:
+        # Register a node covering layers 0-15
+        data = _json.dumps({
+            "endpoint": "http://127.0.0.1:9100",
+            "shard_start": 0,
+            "shard_end": 15,
+            "hardware_profile": {},
+            "score": 1.0,
+        }).encode()
+        req = _ur.Request(
+            f"http://127.0.0.1:{port}/v1/nodes/register",
+            data=data,
+            headers={"Content-Type": "application/json"},
+            method="POST",
+        )
+        with _ur.urlopen(req) as r:
+            r.read()
+
+        # Assignment should fill the gap: layers 16-31
+        resp = _get_json(
+            f"http://127.0.0.1:{port}/v1/nodes/assign?model=stub-model&device=cpu&vram_mb=0"
+        )
+        assert resp["shard_start"] == 16
+        assert resp["shard_end"] == 31
+    finally:
+        tracker.stop()
+
+
+def test_tracker_assign_returns_huggingface_repo_when_configured():
+    """Tracker includes the HuggingFace repo identifier in shard assignments."""
+    tracker = TrackerServer(model_presets={
+        "tiny-llama": {"layers_start": 0, "layers_end": 7, "hf_repo": "org/tiny-llama-shards"}
+    })
+    port = tracker.start()
+    try:
+        resp = _get_json(
+            f"http://127.0.0.1:{port}/v1/nodes/assign?model=tiny-llama&device=cuda&vram_mb=24576"
+        )
+        assert resp["model"] == "tiny-llama"
+        assert resp["hf_repo"] == "org/tiny-llama-shards"
+        assert resp["shard_start"] == 0
+        assert resp["shard_end"] == 7
+    finally:
+        tracker.stop()
+
+
+# ---------------------------------------------------------------------------
+# Full startup integration test
+# ---------------------------------------------------------------------------
+
+
+def test_full_startup_sequence(tmp_path):
+    """Full startup: hardware → wallet → assign → download → start → register."""
+    tracker = TrackerServer(model_presets={"stub-model": {"layers_start": 0, "layers_end": 15}})
+    tracker_port = tracker.start()
+    tracker_url = f"http://127.0.0.1:{tracker_port}"
+
+    wallet_path = tmp_path / "wallet.json"
+    cache_dir = tmp_path / "shards"
+
+    try:
+        node = run_startup(
+            tracker_url=tracker_url,
+            model="stub-model",
+            wallet_path=wallet_path,
+            cache_dir=cache_dir,
+        )
+        try:
+            # Wallet was created on disk
+            assert wallet_path.exists()
+            wallet_data = json.loads(wallet_path.read_text())
+            assert len(wallet_data) == 64
+
+            # Shard was cached
+            assert any(cache_dir.rglob("weights.json"))
+
+            # Node appears in tracker registry (route resolves successfully)
+            route_resp = _get_json(f"{tracker_url}/v1/route?model=stub-model")
+            assert len(route_resp["route"]) >= 1
+            assert node.port is not None
+            assert any(str(node.port) in ep for ep in route_resp["route"])
+
+            # Node accepts an inference request
+            payload = json.dumps({
+                "messages": [{"role": "user", "content": "hello"}],
+            }).encode()
+            req = urllib.request.Request(
+                f"http://127.0.0.1:{node.port}/v1/infer",
+                data=payload,
+                headers={"Content-Type": "application/json"},
+                method="POST",
+            )
+            with urllib.request.urlopen(req, timeout=5) as resp:
+                assert resp.status == 200
+                body = json.loads(resp.read())
+            # Last-shard node returns text; first-shard returns activations
+            assert "text" in body or "activations" in body
+        finally:
+            node.stop()
+    finally:
+        tracker.stop()
+
+
+def test_startup_cpu_fallback(tmp_path, monkeypatch):
+    """Node starts with CPU warning when no GPU is detected."""
+    import meshnet_node.startup as startup_mod
+
+    monkeypatch.setattr(
+        startup_mod,
+        "detect_hardware",
+        lambda: {"device": "cpu", "gpu_name": None, "vram_mb": 0},
+    )
+
+    tracker = TrackerServer(model_presets={"stub-model": {"layers_start": 0, "layers_end": 15}})
+    tracker_port = tracker.start()
+    tracker_url = f"http://127.0.0.1:{tracker_port}"
+
+    try:
+        node = run_startup(
+            tracker_url=tracker_url,
+            model="stub-model",
+            wallet_path=tmp_path / "wallet.json",
+            cache_dir=tmp_path / "shards",
+        )
+        try:
+            # Node is running even on CPU
+            assert node.port is not None
+            route_resp = _get_json(f"{tracker_url}/v1/route?model=stub-model")
+            assert len(route_resp["route"]) >= 1
+        finally:
+            node.stop()
+    finally:
+        tracker.stop()